Interpreted Languages: Perl, PHP, Python, Ruby (Sheet One)
2014-02-24 10:52
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文章来自于：http://hyperpolyglot.org/scripting
a side-by-side reference sheet
sheet one: grammar and invocation | variables
and expressions | arithmetic and logic | strings | regexes | dates
and time| arrays | dictionaries | functions | execution
control | exceptions | concurrency
sheet two: file handles | files | file
formats | directories | processes and
environment | option parsing | libraries
and namespaces | objects | polymorphism | reflection | net
and web | unit tests | debugging and profiling | java
interop
sheet two: file handles | files | directories | processes
and environment | option parsing | libraries
and namespaces |objects | polymorphism | reflection | net
and web | unit tests | debugging and profiling | deployment
The versions used for testing code in the reference sheet.
How to get the version.
perl:
Also available in the predefined variable $], or in a different format in $^V and $PERL_VERSION.
php:
The function phpversion() will return the version number as a string.
python:
The following function will return the version number as a string:
[/code]
ruby:
Also available in the global constant RUBY_VERSION.
Code which examples in the sheet assume to have already been executed.
perl:
We adopt the convention that if an example uses a variable without declaring it, it should be taken to have been previously declared with my.
python:
We assume that os, re,
and sys are always imported.
The customary name of the interpreter and how to invoke it.
php:
php -f will only execute portions of the source file within a <?php php code ?>
tag as php code. Portions of the source file outside of such tags is not treated as executable code and is echoed to standard out.
If short tags are enabled, then php code can also be placed inside <? php code ?> and <?= php code ?> tags.
<?= php code ?> is identical to <?php echo php code ?>.
The customary name of the repl.
perl:
The Perl REPL perl -de 0 does not save or display the result of an expression. perl
-d is the Perl debugger andperl -e runs code provided on the command line.
perl -de 0 does not by default have readline, but it can be added:
[/code]
php:
The php -a REPL does not save or display the result of an expression.
python:
The python repl saves the result of the last statement in _.
ruby:
irb saves the result of the last statement in _.
How to pass the code to be executed to the interpreter as a command line argument.
How blocks are delimited.
perl:
Curly brackets {} delimit blocks. They are also used for:
hash literal syntax which returns a reference to the hash: $rh = { 'true' => 1, 'false' => 0 }
hash value lookup: $h{'true'}, $rh->{'true'}
variable name delimiter: $s = "hello"; print "${s}goodbye";
python:
Python blocks begin with a line that ends in a colon. The block ends with the first line that is not indented further than the initial line. Python raises an IndentationError if the statements in the block that are not in a nested block
are not all indented the same. Using tabs in Python source code is unrecommended and many editors replace them automatically with spaces. If the Python interpreter encounters a tab, it is treated as 8 spaces.
The python repl switches from a >>> prompt
to a … prompt inside a block. A blank line terminates the block.
Colons are also used to separate keys from values in dictionary literals and in sequence slice notation.
ruby:
Curly brackets {} delimit blocks. A matched curly bracket pair can be replaced by the do and end keywords.
By convention curly brackets are used for one line blocks.
The end keyword also terminates blocks started by def, class,
or module.
Curly brackets are also used for hash literals, and the #{ } notation is used to interpolate expressions into strings.
How the parser determines the end of a statement.
perl:
In a script statements are separated by semicolons and never by newlines. However, when using perl -de 0 a
newline terminates the statement.
php:
Inside braces statements must be terminated by a semicolon. The following causes a parse error:
[/code]
The last statement inside <?= ?> or <?
?> tags does not need to be semicolon terminated, however. The following code is legal:
[/code]
python:
Newline does not terminate a statement when:
inside parens
inside list [] or dictionary {} literals
Python single quote '' and double quote "" strings cannot contain newlines except as the two character escaped form \n. Putting a newline in these strings results in a syntax error. There is however a multi-line string literal which starts
and ends with three single quotes ''' or three double quotes: """.
A newline that would normally terminate a statement can be escaped with a backslash.
ruby:
Newline does not terminate a statement when:
inside single quotes '', double quotes "", backticks ``, or parens ()
after an operator such as + or , that expects another argument
Ruby permits newlines in array [] or hash literals, but only after a comma , or associator =>. Putting a newline before the comma or associator results in a syntax error.
A newline that would normally terminate a statement can be escaped with a backslash.
How to identify the character encoding for a source code file.
Setting the source code encoding makes it possible to safely use non-ASCII characters in string literals and regular expression literals.
How to create a comment that ends at the next newline.
How to comment out multiple lines.
python:
The triple single quote ''' and triple double quote """ syntax is a syntax for string literals.
How to declare variables which are local to the scope defining region which immediately contain them.
perl:
Variables don't need to be declared unless use strict is in effect.
If not initialized, scalars are set to undef, arrays are set to an empty array, and hashes are set to an empty hash.
Perl can also declare variables with local. These replace the value of a global variable with the same name, if any, for the duration of the enclosing scope, after which the old value is restored.
php:
Variables do not need to be declared and there is no syntax for declaring a local variable. If a variable with no previous reference is accessed, its value is NULL.
python:
A variable is created by assignment if one does not already exist. If the variable is inside a function or method, then its scope is the body of the function or method. Otherwise it is a global.
ruby:
Variables are created by assignment. If the variable does not have a dollar sign ($) or ampersand (@) as its first character then its scope is scope defining region which most immediately contains it.
A lower case name can refer to a local variable or method. If both are defined, the local variable takes precedence. To invoke the method make the receiver explicit: e.g. self.name. However, outside of class and modules local
variables hide functions because functions are private methods in the class Object. Assignment to name will create a local variable if one with that name does not exist, even if there is a method name.
A list of regions which define a lexical scope for the local variables they contain.
Local variables defined inside the region are only in scope while code within the region is executing. If the language does not have closures, then code outside the region has no access to local variables defined inside the region. If
the language does have closures, then code inside the region can make local variables accessible to code outside the region by returning a reference.
A region which is top level hides local variables in the scope which contains it from the code it contains. A region can also be top level if the syntax requirements of the language prohibit it from being placed inside another
scope defining region.
A region is nestable if it can be placed inside another scope defining region, and if code in the inner region can access local variables in the outer region.
perl:
A local variable can be defined outside of any function definition or anonymous block, in which case the scope of the variable is the file containing the source code. In this way Perl resembles Ruby and contrasts with PHP and Python.
In PHP and Python, any variable defined outside a function definition is global.
In Perl, when a region which defines a scope is nested inside another, then the inner region has read and write access to local variables defined in the outer region.
Note that the blocks associated with the keywords if, unless, while, until, for, and foreach are anonymous blocks, and thus any my declarations in them create variables local
to the block.
php:
Only function bodies and method bodies define scope. Function definitions can be nested, but when this is done lexical variables in the outer function are not visible to code in the body of the inner function.
Braces can be used to set off blocks of codes in a manner similar to the anonymous blocks of Perl. However, these braces do not define a scope. Local variables created inside the braces will be visible to subsequent code outside of the
braces.
Local variables cannot be created in class bodies.
python:
Only functions and methods define scope. Function definitions can be nested. When this is done, inner scopes have read access to variables defined in outer scopes. Attempting to write (i.e. assign) to a variable defined in an outer scope
will instead result in a variable getting created in the inner scope. Python trivia question: what would happen if the following code were executed?
[/code]
ruby:
The keywords if, unless, case, while, and until each define a block which is terminated by an end keyword, but none of these blocks have their own scope.
Anonymous functions can be created with the lambda keyword. Ruby anonymous blocks can be provided after a function invocation and are bounded by curly brackets { } or the do and end keywords. Both anonymous
functions and anonymous blocks can have parameters which are specified at the start of the block within pipes. Here are some examples:
[/code]
The scope of the parameters of an anonymous block or anonymous function is local to the block or function body.
It is possible to mark variables as local, even when they are already defined in the containing scope. Such variables are listed inside the parameter pipes, separated from the parameters by a semicolon:
[/code]
How to declare and access a variable with global scope.
perl:
Undeclared variables, which are permitted unless use strict is in effect, are global. If use strict is in effect, a global can be declared at the top level of a package (i.e. outside any blocks or functions) with the our keyword.
A variable declared withmy inside a function will hide a global with the same name, if there is one.
php:
A variable is global if it is used at the top level (i.e. outside any function definition) or if it is declared inside a function with the global keyword. A function must use the global keyword to access the global variable.
python:
A variable is global if it is defined at the top level of a file (i.e. outside any function definition). Although the variable is global, it must be imported individually or be prefixed with the module name prefix to be accessed from
another file. To be accessed from inside a function or method it must be declared with the global keyword.
ruby:
A variable is global if it starts with a dollar sign: $.
How to declare a constant.
php:
A constant can be declared inside a class:
[/code]
Refer to a class constant like this:
[/code]
ruby:
Capitalized variables contain constants and class/module names. By convention, constants are all caps and class/module names are camel case. The ruby interpreter does not prevent modification of constants, it only gives a warning. Capitalized
variables are globally visible, but a full or relative namespace name must be used to reach them: e.g. Math::PI.
How to assign a value to a variable.
perl:
Assignment operators have right precedence and evaluate to the right argument, so assignments can be chained:
[/code]
python:
If the variable on the left has not previously been defined in the current scope, then it is created. This may hide a variable in a containing scope.
Assignment does not return a value and cannot be used in an expression. Thus, assignment cannot be used in a conditional test, removing the possibility of using assignment (=) when an equality test (==) was intended. Assignments can nevertheless
be chained to assign a value to multiple variables:
[/code]
ruby:
Assignment operators have right precedence and evaluate to the right argument, so they can be chained. If the variable on the left does not exist, then it is created.
How to assign values to variables in parallel.
python:
The r-value can be a list or tuple:
[/code]
Nested sequences of expression can be assigned to a nested sequences of l-values, provided the nesting matches. This assignment will set a to 1, b to 2, and c to 3:
[/code]
This assignment will raise a TypeError:
[/code]
In Python 3 the splat operator * can be used to collect the remaining right side elements in a list:
[/code]
ruby:
The r-value can be an array:
[/code]
How to swap the values held by two variables.
Compound assignment operators mutate a variable, setting it to the value of an operation which takes the previous value of the variable as an argument.
If <OP> is a binary operator and the language has the compound assignment operator <OP>=,
then the following are equivalent:
[/code]
The compound assignment operators are displayed in this order:
First row: arithmetic operator assignment: addition, subtraction, multiplication, (float) division, integer division, modulus, and exponentiation.
Second row: string concatenation assignment and string replication assignment
Third row: logical operator assignment: and, or, xor
Fourth row: bit operator assignment: left shift, right shift, and, or, xor.
python:
Python compound assignment operators do not return a value and hence cannot be used in expressions.
The C-style increment and decrement operators can be used to increment or decrement values. They return values and thus can be used in expressions. The prefix versions return the value in the variable after mutation, and the postfix version
return the value before mutation.
Incrementing a value two or more times in an expression makes the order of evaluation significant:
[/code]
Python avoids the problem by not having an in-expression increment or decrement.
Ruby mostly avoids the problem by providing a non-mutating increment and decrement. However, here is a Ruby expression which is dependent on order of evaluation:
[/code]
perl:
The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:
[/code]
php:
The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:
[/code]
ruby:
The Integer class defines succ, pred,
and next, which is a synonym for succ.
The String class defines succ, succ!, next,
and next!. succ! and next! mutate
the string.
The null literal.
How to test if a variable contains null.
perl:
$v == undef does not imply that $v is undef. Any comparison between undef and a falsehood will return true. The following comparisons are true:
[/code]
php:
$v == NULL does not imply that $v is NULL, since any comparison between NULL and a falsehood will return true. In particular, the following comparisons are true:
[/code]
The result of attempting to access an undefined variable.
perl:
Perl does not distinguish between unset variables and variables that have been set to undef. In Perl, calling defined($a) does not result in a error if $a is undefined, even with the strict pragma.
php:
PHP does not provide the programmer with a mechanism to distinguish an undefined variable from a variable which has been set to NULL.
A test showing that isset is
the logical negation of is_null.
python:
How to test if a variable is defined:
[/code]
ruby:
How to test if a variable is defined:
[/code]
How to write a conditional expression. A ternary operator is an operator which takes three arguments. Since
condition ? true value : false value
is the only ternary operator in C, it is unambiguous to refer to it as the ternary operator.
python:
The Python conditional expression comes from Algol.
ruby:
The Ruby if statement is also an expression:
[/code]
Literals for the booleans.
These are the return values of the relational operators.
php:
Any identifier which matches TRUE case-insensitive can be used for the TRUE boolean. Similarly for FALSE.
In general, PHP variable names are case-sensitive, but function names are case-insensitive.
When converted to a string for display purposes, TRUE renders as "1" and FALSE as "". The equality tests TRUE
== 1and FALSE == "" evaluate as TRUE but the equality tests TRUE
=== 1 and FALSE === "" evaluate as FALSE.
Values which behave like the false boolean in a conditional context.
Examples of conditional contexts are the conditional clause of an if statement and the test of a while loop.
python:
Whether a object evaluates to True or False in a boolean context can be customized by implementing a __nonzero__(Python 2) or __bool__ (Python 3) instance
method for the class.
Logical and, or, and not.
php, perl, ruby:
&& and || have higher precedence than assignment, compound assignment, and the ternary operator (?:), which have higher precedence than and and or.
Equality, inequality, greater than, less than, greater than or equal, less than or equal.
php:
Most of the relational operators will convert a string to a number if the other operand is a number. Thus 0 == "0" is true. The operators === and !== do not perform this conversion, so 0 === "0" is false.
perl:
The operators: == != > < >= <= convert strings to numbers before performing a comparison. Many string evaluate as zero in a numeric context and are equal according to the == operator. To perform a lexicographic string comparison, use: eq,ne, gt, lt, ge, le.
python:
Relational operators can be chained. The following expressions evaluate to true:
[/code]
In general if Ai are expressions and opi are relational operators, then
A1 op1 A2 op2 A3 …
An opn An+1
is true if and only if each of the following is true
A1 op1 A2
A2 op2 A3
…
An opn An+1
How to compare strings.
How to get the min and max.
Binary comparison operators which return -1, 0, or 1 depending upon whether the left argument is less than, equal to, or greater than the right argument.
The <=> symbol is called the spaceship operator.
The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation.
How to get the integer quotient of two integers.
perl:
The integer pragma makes all arithmetic operations integer operations. Floating point numbers are truncated
before they are used. Hence integer division could be performed with:
[/code]
How to get the quotient and remainder with single function call.
What happens when an integer is divided by zero.
How to perform floating point division, even if the operands might be integers.
What happens when a float is divided by zero.
How to get the value of a number raised to a power.
The square root function.
The result of taking the square root of negative one.
Some mathematical functions. Trigonometric functions are in radians unless otherwise noted. Logarithms are natural unless otherwise noted.
python:
Python also has math.log10. To compute the log of x for base b, use:
[/code]
ruby:
Ruby also has Math.log2, Math.log10. To compute the log of x for base b, use
[/code]
Constants for π and Euler's constant.
How to truncate a float to the nearest integer towards zero; how to round a float to the nearest integer; how to find the nearest integer above a float; how to find the nearest integer below a float; how to take the absolute value.
perl:
The CPAN module Number::Format provides a round function.
The 2nd argument specifies the number of digits to keep to the right of the radix. The default is 2.
[/code]
How to get the absolute value of a number.
What happens when the largest representable integer is exceeded.
What happens when the largest representable float is exceeded.
How to create rational numbers and get the numerator and denominator.
ruby:
Require the library mathn and integer division will yield rationals instead of truncated integers.
python:
Most of the functions in math have analogues in cmath which will work correctly on complex numbers.
How to generate a random integer between 0 and 99, include, float between zero and one in a uniform distribution, or a float in a normal distribution with mean zero and standard deviation one.
How to set the random seed; how to get the current random seed and later restore it.
All the languages in the sheet set the seed automatically to a value that is difficult to predict. The Ruby MRI interpreter uses the current time and process ID, for example. As a result there is usually no need to set the seed.
Setting the seed to a hardcoded value yields a random but repeatable sequence of numbers. This can be used to ensure that unit tests which cover code using random numbers doesn't intermittently fail.
The seed is global state. If multiple functions are generating random numbers then saving and restoring the seed may be necessary to produce a repeatable sequence.
The bit operators for left shift, right shift, and, inclusive or, exclusive or, and negation.
Binary, octal, and hex integer literals
How to convert integers to strings of digits of a given base. How to convert such strings into integers.
perl
Perl has the functions oct and hex which
convert strings encoded in octal and hex and return the corresponding integer. The oct function will handle binary or hex encoded
strings if they have "0b" or "0x" prefixes.
[/code]
python
Python has the functions bin, oct,
and hex which take an integer and return a string encoding the integer in base 2, 8, and 16.
[/code]
The type or types using for strings.
perl:
Perl has supported multibyte characters since version 5.6.
php:
PHP assumes all strings have single byte characters.
python:
In Python 2.7 the str type assumes single byte characters. A separate unicode type
is available for working with Unicode strings.
In Python 3 the str type supports multibtye characters and the unicode type
has been removed.
There is a mutable bytearray type and an immutable bytes type
for working with sequences of bytes.
ruby:
The {String}} type supports multibtye characters. All strings have an explicit Encoding.
The syntax for string literals.
perl:
When use strict is not in effect bareword strings are permitted.
Barewords are strings without quote delimiters. They are a feature of shells. Barewords cannot contain whitespace or any other character used by the tokenizer to distinguish words.
Before Perl 5 subroutines were invoked with an ampersand prefix & or the older do keyword. With Perl 5
neither is required, but this made it impossible to distinguish a bareword string from a subroutine without knowing all the subroutines which are in scope.
The following code illustrates the bareword ambiguity:
[/code]
Whether newlines are permitted in string literals.
python:
Newlines are not permitted in single quote and double quote string literals. A string can continue onto the following line if the last character on the line is a backslash. In this case, neither the backslash nor the newline are taken
to be part of the string.
Triple quote literals, which are string literals terminated by three single quotes or three double quotes, can contain newlines:
[/code]
Backslash escape sequences for inserting special characters into string literals.
perl:
In addition to the character escapes, Perl has the following translation escapes:
When use charnames is in effect the \N escape sequence is available:
[/code]
python:
When string literals have an r or R prefix
there are no backslash escape sequences and any backslashes thus appear in the created string. The delimiter can be inserted into a string if it is preceded by a backslash, but the backslash is also inserted. It is thus not possible to create a string with
an r or R prefix
that ends in a backslash. The r and R prefixes
can be used with single or double quotes:
[/code]
The \uhhhh escapes are also available inside Python 2 Unicode literals. Unicode literals have a u prefiix:
[/code]
This syntax is also available in Python 3.3, but not Python 3.2. In Python 3.3 it creates a string of type str which
has the same features as the unicode type of Python 2.7.
How to specify custom delimiters for single and double quoted strings. These can be used to avoid backslash escaping. If the left delimiter is (, [, or { the right delimiter must be ), ], or }, respectively.
Here documents are strings terminated by a custom identifier. They perform variable substitution and honor the same backslash escapes as double quoted strings.
perl:
Put the custom identifier in single quotes to prevent variable interpolation and backslash escape interpretation:
[/code]
python:
Python lacks variable interpolation in strings. Triple quotes honor the same backslash escape sequences as regular quotes, so triple quotes can otherwise be used like here documents:
[/code]
ruby:
Put the customer identifier in single quotes to prevent variable interpolation and backslash escape interpretation:
[/code]
How to interpolate variables into strings.
python:
str.format will take named or positional parameters. When used with named parameters str.format can
mimic the variable interpolation feature of the other languages.
A selection of variables in scope can be passed explicitly:
[/code]
Python 3 has format_map which accepts a dict as
an argument:
[/code]
How to interpolate the result of evaluating an expression into a string.
How to create a string using a printf style format.
python:
The % operator will interpolate arguments into printf-style format strings.
The str.format with positional parameters provides an alternative format using curly braces {0}, {1},
… for replacement fields.
The curly braces are escaped by doubling:
[/code]
If the replacement fields appear in sequential order and aren't repeated, the numbers can be omitted:
[/code]
Are strings mutable?
How to copy a string such that changes to the original do not modify the copy.
The string concatenation operator.
The string replication operator.
How to put a string into all caps or all lower case letters.
How to capitalize a string and the words in a string.
ruby:
Rails monkey patches the String class with the titleize method
for capitalizing the words in a string.
How to remove whitespace from the ends of a string.
perl:
An example of how to trim a string without installing a library:
[/code]
The return value of the =~ operator is boolean, indicating whether a match occurred. Also the left hand
side of the =~operator must be a scalar variable that can be modified. Using the =~ operator
is necessarily imperative, unlike theText::Trim functions which can be used in expressions.
How to pad the edge of a string with spaces so that it is a prescribed length.
How to convert numeric data to string data.
How to convert string data to numeric data.
php:
PHP converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.
perl:
Perl converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.
python:
float and int raise an error if called on a string and any part of the string is not numeric.
ruby:
to_i and to_f always succeed on a string, returning the numeric value of the digits at the start of the string, or zero if there are no initial digits.
How to concatenate the elements of an array into a string with a separator.
How to split a string containing a separator into an array of substrings.
See also scan.
python:
str.split() takes simple strings as delimiters; use re.split() to
split on a regular expression:
[/code]
How to split a string in two.
python:
Methods for splitting a string into three parts using the first or last occurrence of a substring:
[/code]
How to split a string with the delimiters preserved as separate elements.
How to get the length in characters of a string.
How to find the index of the leftmost occurrence of a substring in a string; how to find the index of the rightmost occurrence.
How to extract a substring from a string by index.
How to extract a character from a string by its index.
Converting characters to ASCII codes and back.
The languages in this reference sheet do not have character literals, so characters are represented by strings of length one.
How to split a string into an array of single character strings.
How to apply a character mapping to a string.
How to remove all specified characters from a string; how to remove all but the specified characters from a string.
How to replace multiple adjacent occurrences of a character with a single occurrence.
PHP PCRE Regexes
perlre and perlreref
Python re library: 2.7, 3.1
Ruby Regexp
Regular expressions or regexes are a way of specifying sets of strings. If a string belongs to the set, the string and regex "match". Regexes can also be used to parse strings.
The modern notation for regexes was introduced by Unix command line tools in the 1970s. POSIX standardized the notation into two types: extended regexes and the more archaic basic regexes. Perl regexes are extended regexes augmented by
new character class abbreviations and a few other features introduced by the Perl interpreter in the 1990s. All the languages in this sheet use Perl regexes.
Any string that doesn't contain regex metacharacters is a regex which matches itself. The regex metacharacters are: [
] . | ( ) * + ? { } ^ $ \
character classes: [ ] .
A character class is a set of characters in brackets: [ ]. When used in a regex it matches any character
it contains.
Character classes have their own set of metacharacters: ^ - \ ]
The ^ is only special when it is the first character in the character class. Such a character class matches
its complement; that is, any character not inside the brackets. When not the first character the ^ refers to itself.
The hyphen is used to specify character ranges: e.g. 0-9 or A-Z.
When the hyphen is first or last inside the brackets it matches itself.
The backslash can be used to escape the above characters or the terminal character class delimiter: ].
It can be used in character class abbreviations or string backslash escapes.
The period . is a character class abbreviation which matches any character except for newline. In all
languages the period can be made to match all characters. PHP and Perl use the m modifier. Python uses the re.M flag.
Ruby uses the s modifier.
character class abbreviations:
alternation and grouping: | ( )
The vertical pipe | is used for alternation and parens () for grouping.
A vertical pipe takes as its arguments everything up to the next vertical pipe, enclosing paren, or end of string.
Parentheses control the scope of alternation and the quantifiers described below. They are also used for capturing groups, which are the substrings which matched parenthesized parts of the regular expression. Each language numbers the
groups and provides a mechanism for extracting them when a match is made. A parenthesized subexpression can be removed from the groups with this syntax: (?:expr)
quantifiers: * + ? { }
As an argument quantifiers take the preceding regular character, character class, or group. The argument can itself be quantified, so that ^a{4}*$ matches
strings with the letter a in multiples of 4.
When there is a choice, greedy quantifiers will match the maximum possible number of occurrences of the argument. Lazy quantifiers match the minimum possible number.
anchors: ^ $
escaping: \
To match a metacharacter, put a backslash in front of it. To match a backslash use two backslashes.
php:
PHP 5.3 still supports the EREG engine, though the functions which use it are deprecated. These include the splitfunction
and functions which start with ereg. The preferred functions are preg_split and
the other functions with apreg prefix.
The literal for a regular expression; the literal for a regular expression with a custom delimiter.
php:
PHP regex literals are strings. The first character is the delimiter and it must also be the last character. If the start delimiter is (, {, or [ the end delimiter must be ), }, or ], respectively.
Here are the signatures from the PHP manual for the preg functions used in this sheet:
[/code]
python:
Python does not have a regex literal, but the re.compile function can be used to create regex objects.
Compiling regexes can always be avoided:
[/code]
The supported character class abbreviations.
Note that \h refers to horizontal whitespace (i.e. a space or tab) in PHP and Perl and a hex digit in
Ruby. Similarly \Hrefers to something that isn't horizontal whitespace in PHP and Perl and isn't a hex digit in Ruby.
The supported anchors.
How to test whether a string matches a regular expression.
python:
The re.match function returns true only if the regular expression matches the beginning of the string. re.searchreturns
true if the regular expression matches any substring of the of string.
ruby:
match is a method of both Regexp and String so
can match with both
[/code]
and
[/code]
When variables are involved it is safer to invoke the Regexp method because string variables are more
likely to containnil.
How to perform a case insensitive match test.
Modifiers that can be used to adjust the behavior of a regular expression.
The lists are not comprehensive. For all languages except Ruby there are additional modifiers.
Python modifiers are bit flags. To use more than one flag at the same time, join them with bit or: |
How to replace all occurrences of a matching pattern in a string with the provided substitution string.
php:
The number of occurrences replaced can be controlled with a 4th argument to preg_replace:
[/code]
If no 4th argument is provided, all occurrences are replaced.
perl:
The =~ operator performs the substitution in place on the string and returns the number of substitutions
performed.
The g modifiers specifies that all occurrences should be replaced. If omitted, only the first occurrence
is replaced.
python:
The 3rd argument to sub controls the number of occurrences which are replaced.
[/code]
If there is no 3rd argument, all occurrences are replaced.
ruby:
The gsub operator returns a copy of the string with the substitution made, if any. The gsub! performs the substitution on the original string and returns the modified string.
The sub and sub! operators only replace the first occurrence of the match pattern.
How to get the substring that matched the regular expression, as well as the part of the string before and after the matching substring.
perl:
The special variables $&, $`,
and $' also contain the match, prematch, and postmatch.
ruby:
The special variables $&, $`,
and $' also contain the match, prematch, and postmatch.
How to get the substrings which matched the parenthesized parts of a regular expression.
ruby:
Ruby has syntax for extracting a group from a match in a single expression. The following evaluates to "1999":
[/code]
How to get the substrings which matched the parenthesized parts of a regular expression and put them into a dictionary.
For reference, we call the (?P<foo>...) notation Python-style and
the (?<foo>...) notation Perl-style.
perl:
Perl 5.10 added support for both Python-style and Perl-style named groups.
php:
PHP originally supported Python-style named groups since that was the style that was added to the PCRE regex engine. Perl-style named groups were added to PHP 5.2.
python:
The Python interpreter was the first to support named groups.
How to return all non-overlapping substrings which match a regular expression as an array.
How to use backreferences in a regex; how to use backreferences in the replacement string of substitution.
Examples of recursive regexes.
The examples match substrings containing balanced parens.
In ISO 8601 terminology, a date specifies a day in the Gregorian calendar and a time does not contain date information; it merely specifies a time of day. A data type which combines both date and time information is
probably more useful than one which contains just date information or just time information; it is unfortunate that ISO 8601 doesn't provide a name for this entity. The word timestamp often gets used to denote a combined date and time. PHP and Python
use the compound noun datetime for combined date and time values.
An useful property of ISO 8601 dates, times, and date/time combinations is that they are correctly ordered by a lexical sort on their string
representations. This is because they are big-endian (the year is the leftmost element) and they used fixed-length fields for each term in the string representation.
The C standard library provides two methods for representing dates. The first is the UNIX epoch, which is the seconds since January 1, 1970 in UTC. If such a time were stored in a 32-bit signed integer, the rollover would happen on January
18, 2038.
The other method of representing dates is the tm struct, a definition of which can be found on Unix systems
in/usr/include/time.h:
[/code]
Perl and Python both use and expose the tm struct of the standard library. In the case of Perl, the first
nine values of the struct (up to the member tm_isdst) are put into an array. Python, meanwhile, has a module called time which
is a thin wrapper to the standard library functions which operate on this struct. Here is how get a tm struct in Python:
[/code]
The tm struct is a low level entity, and interacting with it directly should be avoided. In the case of
Python it is usually sufficient to use the datetime module instead. For Perl, one can use the Time::Piece module
to wrap the tm struct in an object.
The data type used to hold a combined date and time.
perl
Built in Perl functions work with either (1) scalars containing the Unix epoch as an integer or (2) arrays containing the first nine values of the standard C library tm struct. When use
Time::Piece is in effect functions which work with tm arrays are replaced with variant that work with the Time::Piece wrapper.
The modules Time::Local and Date::Parse can
create scalars containing the Unix epoch.
CPAN provides the DateTime module which provides objects with functionality comparable to the DateTime
objects of PHP and Python.
How to get the combined date and time for the present moment in both local time and UTC.
How to convert the native date/time type to the Unix epoch which is the number of seconds since the start of January 1, 1970 UTC.
python:
The Python datetime object created by now() and utcnow() has
no timezone information associated with it. Thestrftime() method assumes a receiver with no timezone information represents
a local time. Thus it is an error to callstrftime() on the return value of utcnow().
Here are two different ways to get the current Unix epoch. The second way is faster:
[/code]
Replacing now() with utcnow() in
the first way, or utcnow() with now() in
the second way produces an incorrect value.
How to get the current time as a Unix epoch timestamp.
How to format a date/time as a string using the format notation of the strftime function from the standard C library. This same format notation is used by the Unix date command.
php:
PHP supports strftime but it also has its own time formatting system used by date, DateTime::format,
andDateTime::createFromFormat. The letters used in the PHP time formatting system are described
here.
Examples of how a date/time object appears when treated as a string such as when it is printed to standard out.
The formats are in all likelihood locale dependent. The provided examples come from a machine running Mac OS X in the Pacific time zone of the USA.
php:
It is a fatal error to treat a DateTime object as a string.
How to parse a date/time using the format notation of the strptime function from the standard C library.
How to parse a date without providing a format string.
The data type that results when subtraction is performed on two combined date and time values.
How to add a time duration to a date/time.
A time duration can easily be added to a date/time value when the value is a Unix epoch value.
ISO 8601 distinguishes between a time interval, which is defined by two date/time endpoints, and a duration, which is the length of a time interval and can be defined by a unit of time such as '10 minutes'. A time interval can also be
defined by date and time representing the start of the interval and a duration.
ISO 8601 defines notation for durations. This notation starts with a 'P' and uses a 'T' to separate the day and larger units from
the hour and smaller units. Observing the location relative to the 'T' is important for interpreting the letter 'M', which is used for both months and minutes.
Do date/time values include timezone information. When a date/time value for the local time is created, how the local timezone is determined.
A date/time value can represent a local time but not have any timezone information associated with it.
On Unix systems processes determine the local timezone by inspecting the file /etc/localtime.
php:
The default timezone can also be set in the php.ini file.
[/code]
Here is the list of timezones supported by PHP.
How to convert a timestamp to the equivalent timestamp in an arbitrary timezone.
How to get time zone information: the name of the timezone, the offset in hours from UTC, and whether the timezone is currently in daylight savings.
Timezones are often identified by three or four letter abbreviations. As can be seen from the list, many of the abbreviations
do not uniquely identify a timezone. Furthermore many of the timezones have been altered in the past. TheOlson database (aka Tz database) decomposes the world
into zones in which the local clocks have all been set to the same time since 1970 and it gives these zones unique names.
perl:
It is not necessary to create a DateTime object to get the local timezone offset:
[/code]
ruby:
The Time class has a zone method
which returns the time zone abbreviation for the object. There is a tzinfo gem which can be used to create timezone objects
using the Olson database name. This can in turn be used to convert between UTC times and local times which are daylight saving aware.
How to get the microseconds component of a combined date and time value. The SI abbreviations for milliseconds and microseconds are ms and μs,
respectively. The C standard library uses the letter u as an abbreviation for micro.
Here is a struct defined in /usr/include/sys/time.h:
[/code]
How to put the process to sleep for a specified number of seconds. In Python and Ruby the default version of sleepsupports
a fractional number of seconds.
php:
PHP provides usleep which takes an argument in microseconds:
[/code]
perl:
The Perl standard library includes a version of sleep which supports fractional seconds:
[/code]
How to cause a process to timeout if it takes too long.
Techniques relying on SIGALRM only work on Unix systems.
What the languages call their basic container types:
php:
PHP uses the same data structure for arrays and dictionaries.
perl:
array refers to a data type. list refers to a context.
python:
Python has the mutable list and the immutable tuple. Both are sequences. To be a sequence, a class must implement __getitem__, __setitem__, __delitem__, __len__, __contains__, __iter__, __add__, __mul__, __radd__,
and __rmul__.
ruby:
Ruby provides an Array datatype. If a class defines an each iterator and a comparison operator <=>, then it can mix in the Enumerable module.
Array literal syntax.
perl:
Square brackets create an array and return a reference to it:
[/code]
The quote words operator, which is a literal for arrays of strings where each string contains a single word.
How to get the number of elements in an array.
How to test whether an array is empty.
How to access a value in an array by index.
perl:
A negative index refers to the length - index element.
python:
A negative index refers to the length - index element.
[/code]
ruby:
A negative index refers to to the length - index element.
How to update the value at an index.
What happens when the value at an out-of-bounds index is referenced.
perl:
Some techniques for getting the index of an array element.
How to slice a subarray from an array by specifying a start index and an end index; how to slice a subarray from an array by specifying an offset index and a length index.
perl:
Perl arrays can take an array of indices as the index value. The range of values selected can be discontinuous and the order of the values can be manipulated:
[/code]
python:
Slices can leave the first or last index unspecified, in which case the first or last index of the sequence is used:
[/code]
Python has notation for taking every nth element:
[/code]
The third argument in the colon-delimited slice argument can be negative, which reverses the order of the result:
[/code]
How to slice to the end of an array.
The examples take all but the first element of the array.
How to add and remove elements from the back or high index end of an array.
These operations can be used to use the array as a stack.
How to add and remove elements from the front or low index end of an array.
These operations can be used to use the array as a stack. They can be used with the operations that manipulate the back of the array to use the array as a queue.
How to create an array by concatenating two arrays; how to modify an array by concatenating another array to the end of it.
How to create an array containing the same value replicated n times.
How to make an address copy, a shallow copy, and a deep copy of an array.
After an address copy is made, modifications to the copy also modify the original array.
After a shallow copy is made, the addition, removal, or replacement of elements in the copy does not modify of the original array. However, if elements in the copy are modified, those elements are also modified in the original array.
A deep copy is a recursive copy. The original array is copied and a deep copy is performed on all elements of the array. No change to the contents of the copy will modify the contents of the original array.
perl:
Taking a reference is customary way to make an address copy in Perl, but the Perl example is not equivalent to the other languages in that different syntax has to be used to access the original array and the address copy: @a and @$a1.
To make @a1 and @a refer to the same array, use typeglobs:
[/code]
python:
The slice operator can be used to make a shallow copy:
[/code]
list(v) always returns a list, but v[:] returns
a value of the same as v. The slice operator can be used in this manner on strings and tuples but there is little incentive
to do so since both are immutable.
copy.copy can be used to make a shallow copy on types that don't support the slice operator such as a
dictionary. Like the slice operator copy.copy returns a value with the same type as the argument.
How arrays are passed as arguments.
How to iterate through the elements of an array.
perl:
for and foreach are
synonyms. Some use for exclusively for C-style for loops and foreach for
array iteration.
How to iterate through the elements of an array while keeping track of the index of each element.
Iterate over a range without instantiating it as a list.
perl:
With Perl 5.005 the for and foreach operators
were optimized to not instantiate a range argument as a list.
How to convert a range to an array.
Python 3 ranges and Ruby ranges implement some of the functionality of arrays without allocating space to hold all the elements.
python:
In Python 2 range() returns a list.
In Python 3 range() returns an object which implements the immutable sequence API.
ruby:
The Range class includes the Enumerable module.
How to create a reversed copy of an array, and how to reverse an array in place.
python:
reversed returns an iterator which can be used in a for/in construct:
[/code]
reversed can be used to create a reversed list:
[/code]
How to create a sorted copy of an array, and how to sort an array in place. Also, how to set the comparison function when sorting.
php:
usort sorts an array in place and accepts a comparison function as a 2nd argument:
[/code]
python:
In Python 2 it is possible to specify a binary comparision function when calling sort:
[/code]
In Python 3 the cmp parameter was removed. One can achieve the same effect by defining cmp method
on the class of the list element.
How to remove extra occurrences of elements from an array.
python:
Python sets support the len, in,
and for operators. It may be more efficient to work with the result of the set constructor directly rather than convert it back
to a list.
How to test for membership in an array.
How to compute an intersection.
python:
Python has literal notation for sets:
[/code]
Use set and list to
convert lists to sets and vice versa:
[/code]
ruby:
The intersect operator & always produces an array with no duplicates.
ruby:
The union operator | always produces an array with no duplicates.
How to compute the relative complement of two arrays or sets; how to compute the symmetric difference.
ruby:
If an element is in the right argument, then it will not be in the return value even if it is contained in the left argument multiple times.
Create an array by applying a function to each element of a source array.
ruby:
The map! method applies the function to the elements of the array in place.
collect and collect! are
synonyms for map and map!.
Create an array containing the elements of a source array which match a predicate.
ruby:
The in place version is select!.
reject returns the complement of select. reject! is
the in place version.
The partition method returns two arrays:
[/code]
Return the result of applying a binary operator to all the elements of the array.
python:
reduce is not needed to sum a list of numbers:
[/code]
ruby:
The code for the reduction step can be provided by name. The name can be a symbol or a string:
[/code]
How to test whether a condition holds for all members of an array; how to test whether a condition holds for at least one member of any array.
A universal test is always true for an empty array. An existential test is always false for an empty array.
A existential test can readily be implemented with a filter. A universal test can also be implemented with a filter, but it is more work: one must set the condition of the filter to the negation of the predicate and test whether the result
is empty.
How to shuffle an array. How to extract a random sample from an array.
php:
The array_rand function returns a random sample of the indices of an array. The result can easily be converted
to a random sample of array values:
[/code]
How to interleave arrays. In the case of two arrays the result is an array of pairs or an associative list.
perl:
zip expects arrays as arguments, which makes it difficult to define the arrays to be zipped on the same
line as the invocation. It can be done like this:
[/code]
perl:
Curly brackets create a hash and return a reference to it:
[/code]
How to get the number of dictionary keys in a dictionary.
How to lookup a dictionary value using a dictionary key.
perl:
Use the ampersand prefix @ to slice a Perl hash. The index is a list of keys.
[/code]
What happens when a lookup is performed on a key that is not in a dictionary.
python:
Use dict.get() to avoid handling KeyError exceptions:
[/code]
How to check for the presence of a key in a dictionary without raising an exception. Distinguishes from the case where the key is present but mapped to null or a value which evaluates to false.
How to remove a key/value pair from a dictionary.
How to create a dictionary from an array of pairs; how to create a dictionary from an even length array.
How to merge the values of two dictionaries.
In the examples, if the dictionaries d1 and d2 share
keys then the values from d2 will be used in the merged dictionary.
How to turn a dictionary into its inverse. If a key 'foo' is mapped to value 'bar' by a dictionary, then its inverse will map the key 'bar' to the value 'foo'. However, if multiple keys are mapped to the same value in the original dictionary,
then some of the keys will be discarded in the inverse.
How to iterate through the key/value pairs in a dictionary.
python:
In Python 2.7 dict.items() returns a list of pairs and dict.iteritems() returns
an iterator on the list of pairs.
In Python 3 dict.items() returns an iterator and dict.iteritems() has
been removed.
How to convert the keys of a dictionary to an array; how to convert the values of a dictionary to an array.
python:
In Python 3 dict.keys() and dict.values() return
read-only views into the dict. The following code illustrates the change in behavior:
[/code]
How to sort the data in a dictionary by its values.
How to create a dictionary with a default value for missing keys; how to compute and store the value on lookup.
php:
Extend ArrayObject to compute values on lookup:
[/code]
perl:
How to use a tied hash. If the CPAN module Tie::ExtraHash is installed there is a
shorter way.
Python has both functions and methods. Ruby only has methods: functions defined at the top level are in fact methods on a special main object. Perl subroutines can be invoked with a function syntax or a method syntax.
How to define a function.
perl:
One can also use shift to put the arguments into local variables:
[/code]
How to invoke a function.
python:
Parens are mandatory, even for functions which take no arguments. Omitting the parens returns the function or method as an object. Whitespace can occur between the function name and the following left paren.
In Python 3 print is a function instead of a keyword; parens are mandatory around the argument.
ruby:
Ruby parens are optional. Leaving out the parens results in ambiguity when function invocations are nested. The interpreter resolves the ambiguity by assigning as many arguments as possible to the innermost function invocation, regardless
of its actual arity. It is mandatory that the left paren not be separated from the method name by whitespace.
How to apply a function to an array.
perl:
Perl passes the elements of arrays as individual arguments. In the following invocation, the function foo() does
not know which arguments came from which array. For that matter it does not know how many arrays were used in the invocation:
[/code]
If the elements must be kept in their respective arrays the arrays must be passed by reference:
[/code]
When hashes are used as arguments, each key and value becomes its own argument.
How incorrect number of arguments upon invocation are handled.
perl:
Perl collects all arguments into the @_ array, and subroutines normally don't declare the number of arguments they expect. However, this can be done with prototypes.
Prototypes also provide a method for taking an array from the caller and giving a reference to the array to the callee.
How to declare a default value for an argument.
How to write a function which accepts a variable number of argument.
python:
This function accepts one or more arguments. Invoking it without any arguments raises a TypeError:
[/code]
ruby:
This function accepts one or more arguments. Invoking it without any arguments raises an ArgumentError:
[/code]
How to write a function which uses named parameters and how to invoke it.
python:
The caller can use named parameter syntax at the point of invocation even if the function was defined using positional parameters.
The splat operator * collects the remaining arguments into a list. In a function invocation, the splat can be used to expand an array into separate arguments.
The double splat operator ** collects named parameters into a dictionary. In a function invocation, the double splat expands a dictionary into named parameters.
A double splat operator can be used to force the caller to use named parameter syntax. This method has the disadvantage that spelling errors in the parameter name are not caught:
[/code]
In Python 3 named parameters can be made mandatory:
[/code]
How to pass numbers or strings by reference.
The three common methods of parameter passing are pass by value, pass by reference, and pass by address. Pass by value is the default in most languages.
When a parameter is passed by reference, the callee can changed the value in the variable that was provided as a parameter, and the caller will see the new value when the callee returns. When the parameter is passed by value the callee
cannot do this.
When a language has mutable data types it can be unclear whether the language is using pass by value or pass by reference.
perl:
Here is a potential for confusion: if a reference is used in Perl to pass data, that is pass by address, not pass by reference. A Perl reference is comparable to a pointer in C, albeit one that knows the data type of
what it points to at runtime. C++ has both pointers and references and thus can pass data by address or by reference, though pass by value is the default.
How to pass an array or dictionary without making a copy of it.
perl:
Arrays and hashes are not passed by reference by default. If an array is provided as a argument, each element of the array will be assigned to a parameter. A change to the parameter will change the corresponding value in the original
array, but the number of elements in the array cannot be increased. To write a function which changes the size of the array the array must be passed by reference using the backslash notation.
When a hash is provided as a argument each key of the has will be assigned to a parameter and each value of the hash will be assigned to a parameter. In other words the number of parameters seen by the body of the function will be twice
the size of the hash. Each value parameter will immediately follow its key parameter.
How the return value of a function is determined.
How to return multiple values from a function.
How to define and invoke a lambda function.
python:
Python lambdas cannot contain newlines or semicolons, and thus are limited to a single statement or expression. Unlike named functions, the value of the last statement or expression is returned, and a return is not necessary
or permitted. Lambdas are closures and can refer to local variables in scope, even if they are returned from that scope.
If a closure function is needed that contains more than one statement, use a nested function:
[/code]
Python closures are read only.
A nested function can be returned and hence be invoked outside of its containing function, but it is not visible by its name outside of its containing function.
ruby:
The following lambda and Proc object behave identically:
[/code]
With respect to control words, Proc objects behave like blocks and lambdas like functions. In particular, when the body of a Proc object contains a return or break statement,
it acts like a return or break in
the code which invoked the Proc object. A return in a lambda merely causes the lambda to exit, and a break inside
a lambda must be inside an appropriate control structure contained with the lambda body.
Ruby are alternate syntax for defining lambdas and invoking them:
[/code]
How to store a function in a variable and pass it as an argument.
php:
If a variable containing a string is used like a function then PHP will look for a function with the name in the string and attempt to invoke it.
python:
Python function are stored in variables by default. As a result a function and a variable with the same name cannot share the same scope. This is also the reason parens are mandatory when invoking Python functions.
How to create a function with private state which persists between function invocations.
python:
Here is a technique for creating private state which exploits the fact that the expression for a default value is evaluated only once:
[/code]
How to create a first class function with access to the local variables of the local scope in which it was created.
python:
Python 2 has limited closures: access to local variables in the containing scope is read only and the bodies of anonymous functions must consist of a single expression.
Python 3 permits write access to local variables outside the immediate scope when declared with nonlocal.
How to create a function which can yield a value back to its caller and suspend execution.
perl:
CPAN provides a module called Coro which implements coroutines. Some notes
on the distinction between coroutines and generators.
python:
A Python generator is a function which returns an iterator.
An iterator is an object with two methods: iter(),
which returns the iterator itself, and next(), which returns the next item or raises a StopIteration exception.
Python sequences, of which lists are an example, define an iter() for
returned an iterator which traverses the sequence.
Python iterators can be used in for/in statements and list comprehensions.
In the table below, p and q are
variables for iterators.
ruby:
Ruby generators are called fibers.
A decorator replaces an invocation of one function with another in a way that that is imperceptible to the client.
Normally a decorator will add a small amount of functionality to the original function which it invokes. A decorator can modify the arguments before passing them to the original function or modify the return value before returning it
to the client. Or it can leave the arguments and return value unmodified but perform a side effect such as logging the call.
How to call an operator using the function invocation syntax.
This can be useful when dealing with an API which accepts a function as an argument.
python:
The operator module provides functions which perform the same operations as the various operators. Using
these functions is more efficient than wrapping the operators in lambdas.
ruby:
All operators can be invoked with method invocation syntax. The binary operator invocation syntax can be regarded as syntactic sugar.
The if statement.
php:
PHP has the following alternate syntax for if statements:
[/code]
perl:
When an if block is the last statement executed in a subroutine, the return value is the value of the
branch that executed.
ruby:
If an if statement is the last statement executed in a function, the return value is the value of the
branch that executed.
Ruby if statements are expressions. They can be used on the right hand side of assignments:
[/code]
The switch statement.
php:
PHP provides a do-while loop. The body of such a loop is guaranteed to execute at least once.
[/code]
perl:
Perl provides until, do-while,
and do-until loops.
An until or a do-until loop
can be replaced by a while or a do-while loop
by negating the condition.
ruby:
Ruby provides a loop with no exit condition:
[/code]
Ruby also provides the until loop.
Ruby loops can be used in expression contexts but they always evaluate to nil.
How to write a C-style for loop.
break exits a for or while loop immediately. continue goes to the next iteration of the loop. redo goes back to the beginning of the current iteration.
A list of control structure keywords. The loop control keywords from the previous line are excluded.
The list summarizes the available control structures. It excludes the keywords for exception handling, loading libraries, and returning from functions.
How the do keyword is used.
perl:
The do keyword can convert an if statement
to a conditional expression:
[/code]
Clauses added to the end of a statement to control execution.
Perl and Ruby have conditional statement modifiers. Ruby also has looping statement modifiers.
ruby:
Ruby has the looping statement modifiers while and until:
[/code]
How to raise exceptions.
ruby:
Ruby has a throw keyword in addition to raise. throw can have a symbol as an argument, and will not convert a string to a RuntimeError exception.
How to re-raise an exception preserving the original stack trace.
python:
If the exception is assigned to a variable in the except clause and the variable is used as the argument
to raise, then a new stack trace is created.
ruby:
If the exception is assigned to a variable in the rescue clause and the variable is used as the argument
to raise, then the original stack trace is preserved.
How to catch exceptions.
php:
PHP code must specify a variable name for the caught exception. Exception is the top of the exception hierarchy and will catch all exceptions.
Internal PHP functions usually do not throw exceptions. They can be converted to exceptions with this signal handler:
[/code]
ruby:
A rescue Exception clause will catch any exception. A rescue clause with no exception type specified will catch exceptions that are subclasses of StandardError. Exceptions outside StandardError are
usually unrecoverable and hence not handled in code.
In a rescue clause, the retry keyword will cause the begin clause to be re-executed.
In addition to begin and rescue, ruby has catch:
[/code]
The global variable name for the last exception raised.
How to define a new variable class.
How to catch exceptions of a specific type and assign the exception a name.
php:
PHP exceptions when caught must always be assigned a variable name.
Clauses that are guaranteed to be executed even if an exception is thrown or caught.
ruby:
Ruby MRI threads are operating system threads, but a global interpreter lock prevents more than one thread from executing Ruby code at a time.
How to make a thread wait for another thread to finish.
perldoc
core modules
The first character of a Perl variable $ @ % determines the type of value that can be stored in the variable:
scalar, array, hash. Using an array variable @foo in a scalar context yields the size of the array, and assigning a scalar to
an array will modify the array to contain a single element. $foo[0] accesses the first element of the array @foo,
and$bar{'hello'} accesses the value stored under 'hello' in
the hash %bar. $#foo is
the index of the last element in the array @foo.
Scalars can store a string, integer, or float. If an operator is invoked on a scalar which contains an incorrect data type, perl will perform an implicit conversion to the correct data type: non-numeric strings evaluate to zero.
Scalars can also contain a reference to a variable, which can be created with a backslash: $baz = \@foo; The
original value can be dereferenced with the correct prefix: @$baz. References are how perl creates complex data structures,
such as arrays of hashes and arrays of arrays. If $baz contains a reference to an array, then $baz->[0] is
the first element of the array. if $baz contains a reference to a hash, $baz->{'hello'} is
the value indexed by'hello'.
The literals for arrays and hashes are parens with comma separated elements. Hash literals must contain an even number of elements, and the => operator
can be used in placed of a comma between a key and its value. Square brackets, e.g. [1, 2, 3], create an array and return a
reference to it, and curly brackets, e.g. {'hello' => 5, 'bye' => 3} , create a hash and return a reference to it.
By default Perl variables are global. They can be made local to the containing block with the my keyword
or the localkeyword. my gives
lexical scope, and local gives dynamic scope. Also by default, the perl interpreter creates a variable whenever it encounters
a new variable name in the code. The use strict; pragma requires that all variables be declared with my, local,
or our. The last is used to declare global variables.
Perl functions do not declare their arguments. Any arguments passed to the function are available in the @_ array,
and the shift command will operate on this array if no argument is specified. An array passed as an argument is expanded: if the array contains 10 elements, the callee will have 10 arguments in its @_ array.
A reference (passing \@foo instead of@foo)
can be used to prevent this.
Some of Perl’s special variables:
$$: pid of the perl process
$0: name of the file containing the perl script (may be a full pathname)
$@: error message from last eval or require command
$& $` $’: what last regex matched, part of the string before and after the match
$1 … $9:
what subpatterns in last regex matched
PHP Manual
The PHP interpreter is packaged in 3 different ways: (1) as a standalone executable which can be executed as a CGI script, (2) as a dynamically linked library which adheres to the SAPI of a webserver such as Apache or IIS, and (3) as
a standalone executable which can be used to run PHP scripts from the command line. The latter executable is called PHP CLI.
From the perspective of a PHP programmer, there no important differences between PHP CGI and PHP SAPI. The programmer should be aware of the following differences between PHP CGI/SAPI and PHP CLI:
PHP CGI/SAPI writes HTTP headers to standard out before any output specified by the program. PHP CLI does not.
PHP CLI sets the constants STDIN, STDOUT, and STDERR. PHP CGI/SAPI do not.
PHP CLI has no timeout. PHP CGI/SAPI will typically timeout a script after 30 seconds.
PHP CGI/SAPI add HTML markup to error messages. PHP CLI does not.
PHP CLI does not buffer output, so calling flush is never necessary. PHP CGI/SAPI buffer output.
2.7: Language, Standard Library
3.2: Language, Standard Library
Python uses leading whitespace to indicate block structure. It is not recommended to mix tabs and spaces in leading whitespace, but when this is done, a tab is equal to 8 spaces. The command line options '-t' and '-tt' will warn and raise
an error respectively when tabs are used inconsistently for indentation.
Regular expressions and functions for interacting with the operating system are not available by default and must be imported to be used, i.e.
[/code]
Identifiers in imported modules must be fully qualified unless imported with from/import:
[/code]
There are two basic sequence types: the mutable list and the immutable tuple. The literal syntax for lists uses square brackets and commas [1,2,3] and the literal syntax for tuples uses parens and commas (1,2,3).
The dictionary data type literal syntax uses curly brackets, colons, and commas { “hello”:5, “goodbye”:7 }. Python 3 adds a literal syntax for sets which uses curly brackets and commas: {1,2,3}. This notation is also available in Python
2.7. Dictionaries and sets are implemented using hash tables and as a result dictionary keys and set elements must be hashable.
All values that can be stored in a variable and passed to functions as arguments are objects in the sense that they have methods which can be invoked using the method syntax.
Attributes are settable by default. This can be changed by defining a __setattr__ method for the class. The attributes of an object are stored in the __dict__ attribute.
Methods must declare the receiver as the first argument.
Classes, methods, functions, and modules are objects. If the body of a class, method, or function definition starts with is a string, it is available available at runtime via __doc__. Code examples
in the string which are preceded with '>>>' (the python repl prompt) can be executed by doctest and compared with the output that follows.
1.9.3 core, stdlib
Ruby has a type of value called a symbol. The literal syntax for a symbol is :identifier or :"arbitrary string". The methodsto_s and to_sym can
be used to convert symbols to strings and strings to symbols. Symbols can be used to pass functions or methods as arguments by name. They can be used as keys in Hash objects in place of strings, but the client must remember the type of the keys since :foo
!= "foo". Also note that converting a Hash object with symbols as keys to JSON and then back will yield a Hash object with strings as keys.
In Ruby all values that can be stored in a variable and passed to functions as arguments are objects in the sense that they have methods which can be invoked using the method syntax. Moreover classes are objects. The system provided classes
are open and as a result the user can add methods to classes such as String, Array,
or Fixnum. Ruby only permits single inheritance, but Ruby modules are mix-ins and can be used to add methods to a class via
theinclude statement.
Ruby methods can be declared private and this is enforced by the interpreter. Object attributes are private by default and attribute names have an ampersand @ prefix. The methods attr_reader, attr_writer,
and attr_accessor can be used in a class block to define a getter, setter, or both for an attribute.
When invoking a method the parens are optional. If there are two or more arguments they must still be separated by commas. If one of the arguments is an expression containing a method invocation with arguments, then the Ruby interpreter
will assign as many arguments as possible to the innermost method invocation.
Inside a Ruby method, the self keyword refers to the receiver. It is not declared when defining the method.
Ruby functions are implemented as methods on an object called main which has the special property that any methods defined on it become instance methods in the Object class
which is a base class of most Ruby objects. This makes the method available everywhere. Methods defined at the top level are also added to the main object and the Object class.
Functions which Ruby provides by default are instance methods defined the Object class or the Kernel module.
Ruby methods are not objects and cannot directly be stored in variables. It is worth emphasizing that the Python interpreter when encountering a method identifier with no parens returns the method as an object value, but the Ruby interpreter
invokes the method. As mentioned earlier, methods can be passed by name using symbols. If a method receives a symbol representing a method as an argument, it can invoke the method with the syntax :symbol.to_proc.call(args…).
Note that to_proc resolves the symbol to the method that is in scope where it is invoked.
Although passing a method or a function is a bit awkward, Ruby provides a convenient mechanism called blocks for simultaneously defining an anonymous function at the invocation of a method and providing it to the method as an
argument. The block appears immediately after the closing paren of the method invocation and uses either the { |args…|body } or do |args…| body end syntax.
The invoked method can in turn invoke the block with the yield keyword.
Ruby blocks are closures like lambda functions and can see local variables in the enclosing scope in which they were defined. The parameters of the block are local to the block. Semicolon syntax is available so that identifiers listed
after the arguments could be made local to the block even if already defined in the containing scope.
The lambda keyword or the Proc.new constructor
can be used to store an anonymous function in a variable. The function can be invoked with variable.call(). If such a function is passed to a method argument as the last argument and preceded with an ampersand, the
function will be used as the block for the method. Conversely, if the last argument in a method definition is preceded with an ampersand, any block provided to the function will be bound to the argument name as an anonymous function.
content of this page licensed under creative commons attribution-sharealike 3.0
a side-by-side reference sheet
sheet one: grammar and invocation | variables
and expressions | arithmetic and logic | strings | regexes | dates
and time| arrays | dictionaries | functions | execution
control | exceptions | concurrency
sheet two: file handles | files | file
formats | directories | processes and
environment | option parsing | libraries
and namespaces | objects | polymorphism | reflection | net
and web | unit tests | debugging and profiling | java
interop
| perl | php | python | ruby | |
|---|---|---|---|---|
| versions used | 5.14; 5.16 | 5.4; 5.5 | 2.7; 3.3 | 1.9; 2.0 |
| show version | $ perl --version | $ php --version | $ python -V $ python --version | $ ruby --version |
| implicit prologue | use strict; | none | import os, re, sys | none |
| grammar and invocation | ||||
| perl | php | python | ruby | |
| interpreter | $ perl foo.pl | $ php -f foo.php | $ python foo.py | $ ruby foo.rb |
| repl | $ perl -de 0 | $ php -a | $ python | $ irb |
| command line program | $ perl -e 'print("hi\n")' | $ php -r 'echo "hi\n";' | $ python -c 'print("hi")' | $ ruby -e 'puts "hi"' |
| block delimiters | {} | {} | : and offside rule | {} do end |
| statement separator | ; | ; statements must be semicolon terminated inside {} | newline or ; newlines not separators inside (), [], {}, triple quote literals, or after backslash: \ | newline or ; newlines not separators inside (), [], {}, ``, '', "", or after binary operator or backslash: \ |
| source code encoding | use utf8; | none | Python 3 source is UTF-8 by default # -*- coding: utf-8 -*- | Ruby 2.0 source is UTF-8 by default # -*- coding: utf-8 -*- |
| end-of-line comment | # comment | // comment # comment | # comment | # comment |
| multiple line comment | =for comment line another line =cut | /* comment line another line */ | use triple quote string literal: '''comment line another line''' | =begin comment line another line =end |
| variables and expressions | ||||
| perl | php | python | ruby | |
| local variable | my $v; my (@a, %d); my $x = 1; my ($y, $z) = (2, 3); | # in function body: $v = NULL; $a = []; $d = []; $x = 1; list($y, $z) = [2, 3]; | # in function body: v = None a, d = [], {} x = 1 y, z = 2, 3 | v = nil a, d = [], {} x = 1 y, z = 2, 3 |
| regions which define lexical scope | top level: file nestable: function body anonymous function body anonymous block | top level: function or method body nestable (with use clause): anonymous function body | nestable (read only): function or method body | top level: file class block module block method body nestable: anonymous function body anonymous block |
| global variable | our ($g1, $g2) = (7, 8); sub swap_globals { ($g1, $g2) = ($g2, $g1); } | list($g1, $g2) = [7, 8]; function swap_globals() { global $g1, $g2; list($g1, $g2) = [$g2, $g1]; } | g1, g2 = 7, 8 def swap_globals(): global g1, g2 g1, g2 = g2, g1 | $g1, $g2 = 7, 8 def swap_globals $g1, $g2 = $g2, $g1 end |
| constant | use constant PI => 3.14; | define("PI", 3.14); | # uppercase identifiers # constant by convention PI = 3.14 | # warning if capitalized # identifier is reassigned PI = 3.14 |
| assignment | $v = 1; | $v = 1; | assignments can be chained but otherwise don't return values: v = 1 | v = 1 |
| parallel assignment | ($x, $y, $z) = (1, 2, 3); # 3 is discarded: ($x, $y) = (1, 2, 3); # $z set to undef: ($x, $y, $z) = (1, 2); | list($x, $y, $z) = [1 ,2, 3]; # 3 is discarded: list($x, $y) = [1, 2, 3]; # $z set to NULL: list($x, $y, $z) = [1, 2]; | x, y, z = 1, 2, 3 # raises ValueError: x, y = 1, 2, 3 # raises ValueError: x, y, z = 1, 2 | x, y, z = 1, 2, 3 # 3 is discarded: x, y = 1, 2, 3 # z set to nil: x, y, z = 1, 2 |
| swap | ($x, $y) = ($y, $x); | list($x, $y) = [$y, $x]; | x, y = y, x | x, y = y, x |
| compound assignment arithmetic, string, logical, bit | += -= *= none /= %= **= .= x= &&= ||= ^= <<= >>= &= |= ^= | += -= *= none /= %= **= .= none &= |= none <<= >>= &= |= ^= | # do not return values: += -= *= /= //= %= **= += *= &= |= ^= <<= >>= &= |= ^= | += -= *= /= none %= **= += *= &&= ||= ^= <<= >>= &= |= ^= |
| increment and decrement | my $x = 1; my $y = ++$x; my $z = --$y; | $x = 1; $y = ++$x; $z = --$y; | none | x = 1 # x and y not mutated: y = x.succ z = y.pred |
| null | undef | NULL # case insensitive | None | nil |
| null test | ! defined $v | is_null($v) ! isset($v) | v == None v is None | v == nil v.nil? |
| undefined variable access | error under use strict; otherwise undef | NULL | raises NameError | raises NameError |
| conditional expression | $x > 0 ? $x : -$x | $x > 0 ? $x : -$x | x if x > 0 else -x | x > 0 ? x : -x |
| arithmetic and logic | ||||
| perl | php | python | ruby | |
| true and false | 1 "" | TRUE FALSE # case insensitive | True False | true false |
| falsehoods | undef 0 0.0 "" "0" () | FALSE NULL 0 0.0 "" "0" [] | False None 0 0.0 '' [] {} | false nil |
| logical operators | && || ! lower precedence: and or xor not | && || ! lower precedence: and or xor | and or not | && || ! lower precedence: and or not |
| relational operators | numbers only: == != > < >= <= | == != or <> > < >= <= no conversion: === !== | relational operators are chainable: == != > < >= <= | == != > < >= <= |
| compare strings | eq ne gt lt ge le | == != or <> > < >= <= no conversion: === !== | relational operators are chainable: == != > < >= <= | == != > < >= <= |
| min and max | use List::Util qw(min max); min(1, 2, 3); max(1, 2, 3); @a = (1, 2, 3); min(@a); max(@a); | min(1, 2, 3) max(1, 2, 3) $a = [1, 2, 3] min($a) max($a) | min(1, 2, 3) max(1, 2, 3) min([1, 2, 3]) max([1, 2, 3]) | [1, 2, 3].min [1, 2, 3].max |
| three value comparison | 0 <=> 1 "do" cmp "re" | none | removed from Python 3: cmp(0, 1) cmp('do', 're') | 0 <=> 1 "do" <=> "re" |
| arithmetic operators addition, subtraction, multiplication, float division, quotient, remainder | + - * / none % | + - * / none % | + - * see note // % Python 2 does not have an operator which performs float division on integers. In Python 3 / always performs float division. | + - * x.fdiv(y) / % |
| integer division | int(13 / 5) | (int)(13 / 5) | 13 // 5 | 13 / 5 |
| divmod | none | none | q, r = divmod(13, 5) | q, r = 13.divmod(5) |
| integer division by zero | error | returns FALSE with warning | raises ZeroDivisionError | raises ZeroDivisionError |
| float division | 13 / 5 | 13 / 5 | float(13) / 5 # Python 3: 13 / 5 | 13.to_f / 5 or 13.fdiv(5) |
| float division by zero | error | returns FALSE with warning | raises ZeroDivisionError | returns -Infinity, NaN, or Infinity |
| power | 2**32 | pow(2, 32) | 2**32 | 2**32 |
| sqrt | sqrt(2) | sqrt(2) | import math math.sqrt(2) | include Math sqrt(2) |
| sqrt -1 | error unless use Math::Complex in effect | NaN | # raises ValueError: import math math.sqrt(-1) # returns complex float: import cmath cmath.sqrt(-1) | raises Errno::EDOM |
| transcendental functions | use Math::Trig qw( tan asin acos atan); exp log sin cos tan asin acos atan atan2 | exp log sin cos tan asin acos atan atan2 | from math import exp, log, \ sin, cos, tan, asin, acos, atan, atan2 | include Math exp log sin cos tan asin acos atan atan2 |
| transcendental constants π and e | M_PI M_E | import math math.pi math.e | include Math PI E | |
| float truncation | # cpan -i Number::Format use Number::Format 'round'; use POSIX qw(ceil floor); int($x) round($x, 0) ceil($x) floor($x) | (int)$x round($x) ceil($x) floor($x) | import math int(x) int(round(x)) math.ceil(x) math.floor(x) | x.to_i x.round x.ceil x.floor |
| absolute value | abs($x) | abs($x) | abs(x) | x.abs |
| integer overflow | converted to float; use Math::BigInt to create arbitrary length integers | converted to float | becomes arbitrary length integer of type long | becomes arbitrary length integer of typeBignum |
| float overflow | inf | INF | raises OverflowError | Infinity |
| rational construction | use Math::BigRat; my $x = Math::BigRat->new("22/7"); | none | from fractions import Fraction x = Fraction(22, 7) | require 'rational' x = Rational(22, 7) |
| rational decomposition | $x->numerator(); $x->denominator(); | none | x.numerator x.denominator | x.numerator x.denominator |
| complex construction | use Math::Complex; my $z = 1 + 1.414 * i; | none | z = 1 + 1.414j | require 'complex' z = 1 + 1.414.im |
| complex decomposition real and imaginary component, argument, absolute value, conjugate | Re($z); Im($z); arg($z); abs($z); ~$z; | none | import cmath z.real z.imag cmath.phase(z) abs(z) z.conjugate() | z.real z.imag z.arg z.abs z.conj |
| random number uniform integer, uniform float, normal float | int(rand() * 100) rand() none | rand(0,99) lcg_value() none | import random random.randint(0, 99) random.random() random.gauss(0, 1) | rand(100) rand none |
| random seed set, get, restore | srand 17; my $seed = srand; srand($seed); | srand(17); none | import random random.seed(17) seed = random.getstate() random.setstate(seed) | srand(17) seed = srand srand(seed) |
| bit operators | << >> & | ^ ~ | << >> & | ^ ~ | << >> & | ^ ~ | << >> & | ^ ~ |
| binary, octal, and hex literals | 0b101010 052 0x2a | 0b101010 052 0x2a | 0b101010 052 0x2a | 0b101010 052 0x2a |
| radix convert integer to and from string with radix | # cpan -i Math::BaseCalc use Math::BaseCalc; $c = new Math::BaseCalc( digits => [0..6]); $c->to_base(42); $c->from_base("60"); | base_convert("42", 10, 7); base_convert("60", 7, 10); | none int("60", 7) | 42.to_s(7) "60".to_i(7) |
| strings | ||||
| perl | php | python | ruby | |
| string type | SCALAR which is also used for numeric types and references | string | str | String |
| string literal | "don't say \"no\"" 'don\'t say "no"' | "don't say \"no\"" 'don\'t say "no"' | 'don\'t say "no"' "don't say \"no\"" "don't " 'say "no"' '''don't say "no"''' """don't say "no\"""" | "don't say \"no\"" 'don\'t say "no"' "don't " 'say "no"' |
| newline in literal | 'first line second line' "first line second line" | 'first line second line' "first line second line" | triple quote literals only: '''first line second line''' """first line second line""" | 'first line second line' "first line second line" |
| literal escapes | double quoted: \a \b \cx \e \f \n \r \t \xhh \x{hhhh} \ooo\o{ooo} single quoted: \' \\ | double quoted: \f \n \r \t \v \xhh \$ \" \\ \ooo single quoted: \' \\ | single and double quoted: \newline \\ \' \" \a \b \f \n \r \t \v \ooo\xhh Python 3: \uhhhh \Uhhhhhhhh | double quoted: \a \b \cx \e \f \n \r \s \t \v \xhh \ooo\uhhhh \u{hhhhh} single quoted: \' \\ |
| custom delimiters | my $s1 = q(lorem ipsum); my $s2 = qq($s1 dolor sit amet); | none | none | s1 = %q(lorem ipsum) s2 = %Q(#{s1} dolor sit amet) |
| here document | $word = "amet"; $s = <<EOF; lorem ipsum dolor sit $word EOF | $word = "amet"; $s = <<<EOF lorem ipsum dolor sit $word EOF; | none | word = "amet" s = <<EOF lorem ipsum dolor sit #{word} EOF |
| variable interpolation | my $count = 3; my $item = "ball"; print "$count ${item}s\n"; | $count = 3; $item = "ball"; echo "$count ${item}s\n"; | count = 3 item = 'ball' print('{count} {item}s'.format( **locals())) | count = 3 item = "ball" puts "#{count} #{item}s" |
| expression interpolation | none | none | '1 + 1 = {}'.format(1 + 1) | "1 + 1 = #{1 + 1}" |
| format | my $fmt = "lorem %s %d %f"; sprintf($fmt, "ipsum", 13, 3.7) | $fmt = "lorem %s %d %f"; sprintf($fmt, "ipsum", 13, 3.7); | 'lorem %s %d %f' % ('ipsum', 13, 3.7) fmt = 'lorem {0} {1} {2}' fmt.format('ipsum', 13, 3.7) | "lorem %s %d %f" % ["ipsum", 13, 3.7] |
| are strings mutable? | $s = "bar"; $s2 = $s; # sets s to "baz"; s2 is unchanged: $s =~ s/r/z/; | $s = "bar"; $s2 = $s; # sets s to "baz"; s2 is unchanged: $s[2] = "z"; | no | s = "bar" s2 = s # sets s and s2 to "baz": s[2] = "z" |
| copy string | $s2 = $s; | $s2 = $s; | none | s = "bar" s2 = s.clone # s2 is not altered: s[2] = "z" |
| concatenate | my $s = "Hello, "; my $s2 = $s . "World!"; | $s = "Hello, "; $s2 = $s . "World!"; | s = 'Hello, ' s2 = s + 'World!' # juxtaposition can be used to # concatenate literals: s2 = 'Hello, ' "World!" | s = "Hello, " s2 = s + "World!" # juxtaposition can be used to # concatenate literals: s2 ="Hello, " 'World!' |
| replicate | my $hbar = "-" x 80; | $hbar = str_repeat("-", 80); | hbar = '-' * 80 | hbar = "-" * 80 |
| translate case to upper, to lower | uc("lorem") lc("LOREM") | strtoupper("lorem") strtolower("LOREM") | 'lorem'.upper() 'LOREM'.lower() | "lorem".upcase "LOREM".downcase |
| capitalize string, words | # cpan -i Text::Autoformat use Text::Autoformat; ucfirst("lorem") autoformat("lorem ipsum", {case => 'title'}) | ucfirst("lorem") ucwords("lorem ipsum") | import string 'lorem'.capitalize() string.capwords('lorem ipsum') | "lorem".capitalize none |
| trim both sides, left, right | # cpan -i Text::Trim use Text::Trim; trim " lorem " ltrim " lorem" rtrim "lorem " | trim(" lorem ") ltrim(" lorem") rtrim("lorem ") | ' lorem '.strip() ' lorem'.lstrip() 'lorem '.rstrip() | " lorem ".strip " lorem".lstrip "lorem ".rstrip |
| pad on right, on left, centered | # cpan -i Text::Format use Text::Format; sprintf("%-10s", "lorem") sprintf("%10s", "lorem") $text = Text::Format->new(columns => 10); $text->center("lorem"); | str_pad("lorem", 10) str_pad("lorem", 10, " ", STR_PAD_LEFT) str_pad("lorem", 10, " ", STR_PAD_BOTH) | 'lorem'.ljust(10) 'lorem'.rjust(10) 'lorem'.center(10) | "lorem".ljust(10) "lorem".rjust(10) "lorem".center(10) |
| number to string | "value: " . 8 | "value: " . 8 | 'value: ' + str(8) | "value: " + 8.to_s |
| string to number | 7 + "12" 73.9 + ".037" | 7 + "12" 73.9 + ".037" | 7 + int('12') 73.9 + float('.037') | 7 + "12".to_i 73.9 + ".037".to_f |
| join | join(" ", qw(do re mi fa)) | $a = ["do", "re", "mi", "fa"]; implode(" ", $a) | ' '.join(['do', 're', 'mi', 'fa']) # raises TypeError: ' '.join([1, 2, 3]) | %w(do re mi fa).join(' ') # implicitly converted to strings: [1, 2, 3].join(' ') |
| split | split(/\s+/, "do re mi fa") | explode(" ", "do re mi fa") | 'do re mi fa'.split() | "do re mi fa".split |
| split in two | split(/\s+/, "do re mi fa", 2) | preg_split('/\s+/', "do re mi fa", 2) | 'do re mi fa'.split(None, 1) | "do re mi fa".split(/\s+/, 2) |
| split and keep delimiters | split(/(\s+)/, "do re mi fa") | preg_split('/(\s+)/', "do re mi fa", NULL, PREG_SPLIT_DELIM_CAPTURE) | re.split('(\s+)', 'do re mi fa') | "do re mi fa".split(/(\s+)/) |
| length | length("lorem") | strlen("lorem") | len('lorem') | "lorem".length "lorem".size |
| index of substring first, last | # returns -1 if not found: index("lorem ipsum", "ipsum") rindex("do re re", "re") | # returns FALSE if not found: strpos("do re re", "re") strrpos("do re re", "re") | # raises ValueError if not found: 'do re re'.index('re') 'do re re'.rindex('re') # returns -1 if not found: 'do re re'.find('re') 'do re re'.rfind('re') | # returns nil if not found: "do re re".index("re") "do re re".rindex("re") |
| extract substring by start and length, by start and end, by successive starts | substr("lorem ipsum", 6, 5) none none | substr("lorem ipsum", 6, 5) none none | none none 'lorem ipsum'[6:11] | "lorem ipsum"[6, 5] "lorem ipsum"[6..10] "lorem ipsum"[6...11] |
| extract character | # can't use index notation with # strings: substr("lorem ipsum", 6, 1) | # syntax error to use index notation # directly on string literal: $s = "lorem ipsum"; $s[6]; | 'lorem ipsum'[6] | "lorem ipsum"[6] |
| chr and ord | chr(65) ord("A") | chr(65) ord("A") | chr(65) ord('A') | 65.chr "A".ord |
| to array of characters | split(//, "abcd") | str_split("abcd") | list('abcd') | "abcd".split("") |
| translate characters | $s = "hello"; $s =~ tr/a-z/n-za-m/; | $ins = implode(range("a", "z")); $outs = substr($ins, 13, 13) . substr($ins, 0, 13); strtr("hello", $ins, $outs) | from string import lowercase as ins from string import maketrans outs = ins[13:] + ins[:13] 'hello'.translate(maketrans(ins,outs)) | "hello".tr("a-z", "n-za-m") |
| delete characters | $s = "disemvowel me"; $s =~ tr/aeiou//d; | $vowels = str_split("aeiou"); $s = "disemvowel me"; $s = str_replace($vowels, "", $s); | "disemvowel me".translate(None, "aeiou") | "disemvowel me".delete("aeiou") |
| squeeze characters | $s = "too much space"; $s =~ tr/ //s; | $s = "too much space"; $s = = preg_replace('/(\s)+/', '\1', $s); | re.sub('(\s)+', r'\1', 'too much space') | "too much space".squeeze(" ") |
| regular expressions | ||||
| perl | php | python | ruby | |
| literal, custom delimited literal | /lorem|ipsum/ qr(/etc/hosts) | '/lorem|ipsum/' '(/etc/hosts)' | re.compile('lorem|ipsum') none | /lorem|ipsum/ %r(/etc/hosts) |
| character class abbreviations | . \d \D \h \H \s \S \v \V \w \W | . \d \D \h \H \s \S \v \V \w \W | . \d \D \s \S \w \W | . \d \D \h \H \s \S \w \W |
| anchors | ^ $ \A \b \B \z \Z | ^ $ \A \b \B \z \Z | ^ $ \A \b \B \Z | ^ $ \A \b \B \z \Z |
| match test | if ($s =~ /1999/) { print "party!\n"; } | if (preg_match('/1999/', $s)) { echo "party!\n"; } | if re.search('1999', s): print('party!') | if /1999/.match(s) puts "party!" end |
| case insensitive match test | "Lorem" =~ /lorem/i | preg_match('/lorem/i', "Lorem") | re.search('lorem', 'Lorem', re.I) | /lorem/i.match("Lorem") |
| modifiers | i m s p x | e i m s x | re.I re.M re.S re.X | i o m x |
| substitution | my $s = "do re mi mi mi"; $s =~ s/mi/ma/g; | $s = "do re mi mi mi"; $s = preg_replace('/mi/', "ma", $s); | s = 'do re mi mi mi' s = re.compile('mi').sub('ma', s) | s = "do re mi mi mi" s.gsub!(/mi/, "ma") |
| match, prematch, postmatch | if ($s =~ /\d{4}/p) { $match = ${^MATCH}; $prematch = ${^PREMATCH}; $postmatch = ${^POSTMATCH}; } | none | m = re.search('\d{4}', s) if m: match = m.group() prematch = s[0:m.start(0)] postmatch = s[m.end(0):len(s)] | m = /\d{4}/.match(s) if m match = m[0] prematch = m.pre_match postmatch = m.post_match end |
| group capture | $rx = qr/(\d{4})-(\d{2})-(\d{2})/; "2010-06-03" =~ $rx; ($yr, $mo, $dy) = ($1, $2, $3); | $s = "2010-06-03"; $rx = '/(\d{4})-(\d{2})-(\d{2})/'; preg_match($rx, $s, $m); list($_, $yr, $mo, $dy) = $m; | rx = '(\d{4})-(\d{2})-(\d{2})' m = re.search(rx, '2010-06-03') yr, mo, dy = m.groups() | rx = /(\d{4})-(\d{2})-(\d{2})/ m = rx.match("2010-06-03") yr, mo, dy = m[1..3] |
| named group capture | $s = "foo.txt"; $s =~ /^(?<file>.+)\.(?<suffix>.+)$/; $+{"file"} $+{"suffix"} | $s = "foo.txt"; $rx = '/^(?P<file>.+)\.(?P<suffix>.+)$/'; preg_match($rx, $s, $m); $m["file"] $m["suffix"] | rx = '^(?P<file>.+)\.(?P<suffix>.+)$' m = re.search(rx, ''foo.txt') m.groupdict()['file'] m.groupdict()['suffix'] | rx = /^(?<file>.+)\.(?<suffix>.+)$/ m = rx.match('foo.txt') m["file"] m["suffix"] |
| scan | my $s = "dolor sit amet"; @a = $s =~ m/\w+/g; | $s = "dolor sit amet"; preg_match_all('/\w+/', $s, $m); $a = $m[0]; | s = 'dolor sit amet' a = re.findall('\w+', s) | a = "dolor sit amet".scan(/\w+/) |
| backreference in match and substitution | "do do" =~ /(\w+) \1/ my $s = "do re"; $s =~ s/(\w+) (\w+)/$2 $1/; | preg_match('/(\w+) \1/', "do do") $s = "do re"; $rx = '/(\w+) (\w+)/'; $s = preg_replace($rx, '\2 \1', $s); | none rx = re.compile('(\w+) (\w+)') rx.sub(r'\2 \1', 'do re') | /(\w+) \1/.match("do do") "do re".sub(/(\w+) (\w+)/, '\2 \1') |
| recursive regex | /\(([^()]*|(?R))\)/ | '/\(([^()]*|($R))\)/' | none | /(?<foo>\(([^()]*|\g<foo>)*\))/ |
| dates and time | ||||
| perl | php | python | ruby | |
| date/time type | Time::Piece if use Time::Piece in effect, otherwise tm array | DateTime | datetime.datetime | Time |
| current date/time | use Time::Piece; my $t = localtime(time); my $utc = gmtime(time); | $t = new DateTime("now"); $utc_tmz = new DateTimeZone("UTC"); $utc = new DateTime("now", $utc_tmz); | import datetime t = datetime.datetime.now() utc = datetime.datetime.utcnow() | t = Time.now utc = Time.now.utc |
| to unix epoch, from unix epoch | use Time::Local; use Time::Piece; my $epoch = timelocal($t); my $t2 = localtime(1304442000); | $epoch = $t->getTimestamp(); $t2 = new DateTime(); $t2->setTimestamp(1304442000); | from datetime import datetime as dt epoch = int(t.strftime("%s")) t2 = dt.fromtimestamp(1304442000) | epoch = t.to_i t2 = Time.at(1304442000) |
| current unix epoch | $epoch = time; | $epoch = time(); | import datetime t = datetime.datetime.now() epoch = int(t.strftime("%s")) | epoch = Time.now.to_i |
| strftime | use Time::Piece; $t = localtime(time); $fmt = "%Y-%m-%d %H:%M:%S"; print $t->strftime($fmt); | strftime("%Y-%m-%d %H:%M:%S", $epoch); date("Y-m-d H:i:s", $epoch); $t->format("Y-m-d H:i:s"); | t.strftime('%Y-%m-%d %H:%M:%S') | t.strftime("%Y-%m-%d %H:%M:%S") |
| default format example | Tue Aug 23 19:35:19 2011 | no default string representation | 2011-08-23 19:35:59.411135 | 2011-08-23 17:44:53 -0700 |
| strptime | use Time::Local; use Time::Piece; $s = "2011-05-03 10:00:00"; $fmt = "%Y-%m-%d %H:%M:%S"; $t = Time::Piece->strptime($s,$fmt); | $fmt = "Y-m-d H:i:s"; $s = "2011-05-03 10:00:00"; $t = DateTime::createFromFormat($fmt, $s); | from datetime import datetime s = '2011-05-03 10:00:00' fmt = '%Y-%m-%d %H:%M:%S' t = datetime.strptime(s, fmt) | require 'date' s = "2011-05-03 10:00:00" fmt = "%Y-%m-%d %H:%M:%S" t = Date.strptime(s, fmt).to_time |
| parse date w/o format | # cpan -i Date::Parse use Date::Parse; $epoch = str2time("July 7, 1999"); | $epoch = strtotime("July 7, 1999"); | # pip install python-dateutil import dateutil.parser s = 'July 7, 1999' t = dateutil.parser.parse(s) | require 'date' s = "July 7, 1999" t = Date.parse(s).to_time |
| result of date subtraction | Time::Seconds object if use Time::Piece in effect; not meaningful to subtract tm arrays | DateInterval object if diff method used: $fmt = "Y-m-d H:i:s"; $s = "2011-05-03 10:00:00"; $then = DateTime::createFromFormat($fmt, $s); $now = new DateTime("now"); $interval = $now->diff($then); | datetime.timedelta object use total_seconds() method to convert to float representing difference in seconds | Float containing time difference in seconds |
| add time duration | use Time::Seconds; $now = localtime(time); $now += 10 * ONE_MINUTE() + 3; | $now = new DateTime("now"); $now->add(new DateInterval("PT10M3S"); | import datetime delta = datetime.timedelta( minutes=10, seconds=3) t = datetime.datetime.now() + delta | require 'date/delta' s = "10 min, 3 s" delta = Date::Delta.parse(s).in_secs t = Time.now + delta |
| local timezone | Time::Piece has local timezone if created with localtime and UTC timezone if created with gmtime; tm arrays have no timezone or offset info | DateTime objects can be instantiated without specifying the timezone if a default is set: $s = "America/Los_Angeles"; date_default_timezone_set($s); | a datetime object has no timezone information unless a tzinfo object is provided when it is created | if no timezone is specified the local timezone is used |
| arbitrary timezone | # pip install pytz import pytz import datetime tmz = pytz.timezone('Asia/Tokyo') utc = datetime.datetime.utcnow() utc_dt = datetime.datetime( *utc.timetuple()[0:5], tzinfo=pytz.utc) jp_dt = utc_dt.astimezone(tmz) | # gem install tzinfo require 'tzinfo' tmz = TZInfo::Timezone.get("Asia/Tokyo") jp_time = tmz.utc_to_local(Time.now.utc) | ||
| timezone name; offset from UTC; is daylight savings? | # cpan -i DateTime use DateTime; use DateTime::TimeZone; $dt = DateTime->now(); $tz = DateTime::TimeZone->new( name=>"local"); $tz->name; $tz->offset_for_datetime($dt) / 3600; $tz->is_dst_for_datetime($dt); | $tmz = date_timezone_get($t); timezone_name_get($tmz); date_offset_get($t) / 3600; $t->format("I"); | import time tm = time.localtime() time.tzname[tm.tm_isdst] (time.timezone / -3600) + tm.tm_isdst tm.tm_isdst | t.zone t.utc_offset / 3600 t.dst? |
| microseconds | use Time::HiRes qw(gettimeofday); ($sec, $usec) = gettimeofday; | list($frac, $sec) = explode(" ", microtime()); $usec = $frac * 1000 * 1000; | t.microsecond | t.usec |
| sleep | # a float argument will be truncated # to an integer: sleep 1; | # a float argument will be truncated # to an integer: sleep(1); | import time time.sleep(0.5) | sleep(0.5) |
| timeout | eval { $SIG{ALRM}= sub {die "timeout!";}; alarm 5; sleep 10; }; alarm 0; | use set_time_limit to limit execution time of the entire script; use stream_set_timeoutto limit time spent reading from a stream opened with fopen or fsockopen | import signal, time class Timeout(Exception): pass def timeout_handler(signo, fm): raise Timeout() signal.signal(signal.SIGALRM, timeout_handler) try: signal.alarm(5) time.sleep(10) except Timeout: pass signal.alarm(0) | require 'timeout' begin Timeout.timeout(5) do sleep(10) end rescue Timeout::Error end |
| arrays | ||||
| perl | php | python | ruby | |
| literal | @a = (1, 2, 3, 4); | $a = [1, 2, 3, 4]; # older syntax: $a = array(1, 2, 3, 4); | a = [1, 2, 3, 4] | a = [1, 2, 3, 4] |
| quote words | @a = qw(do re mi); | none | none | a = %w(do re mi) |
| size | $#a + 1 or scalar(@a) | count($a) | len(a) | a.size a.length # same as size |
| empty test | !@a | !$a | not a | NoMethodError if a is nil: a.empty? |
| lookup | $a[0] # returns last element: $a[-1] | $a[0] # PHP uses the same type for arrays and # dictionaries; indices can be negative # integers or strings | a[0] # returns last element: a[-1] | a[0] # returns last element: a[-1] |
| update | $a[0] = "lorem"; | $a[0] = "lorem"; | a[0] = 'lorem' | a[0] = "lorem" |
| out-of-bounds behavior | @a = (); # evaluates as undef: $a[10]; # increases array size to 11: $a[10] = "lorem"; | $a = []; # evaluates as NULL: $a[10]; # increases array size to one: $a[10] = "lorem"; | a = [] # raises IndexError: a[10] # raises IndexError: a[10] = 'lorem' | a = [] # evaluates as nil: a[10] # increases array size to 11: a[10] = "lorem" |
| index of element | use List::Util 'first'; @a = qw(x y z w); $i = first {$a[$_] eq "y"} (0..$#a); | $a = ["x", "y", "z", "w"]; $i = array_search("y", $a); | a = ['x', 'y', 'z', 'w'] i = a.index('y') | a = %w(x y z w) i = a.index("y") |
| slice by endpoints, by length | # select 3rd and 4th elements: @a[2..3] splice(@a, 2, 2) | # select 3rd and 4th elements: none array_slice($a, 2, 2) | # select 3rd and 4th elements: a[2:4] a[2:2 + 2] | # select 3rd and 4th elements: a[2..3] a[2, 2] |
| slice to end | @a[1..$#a] | array_slice($a, 1) | a[1:] | a[1..-1] |
| manipulate back | @a = (6, 7, 8); push @a, 9; pop @a; | $a = [6, 7, 8]; array_push($a, 9); $a[] = 9; # same as array_push array_pop($a); | a = [6, 7, 8] a.append(9) a.pop() | a = [6, 7, 8] a.push(9) a << 9 # same as push a.pop |
| manipulate front | @a = (6, 7, 8); unshift @a, 5; shift @a; | $a = [6, 7, 8]; array_unshift($a, 5); array_shift($a); | a = [6, 7, 8] a.insert(0, 5) a.pop(0) | a = [6, 7, 8] a.unshift(5) a.shift |
| concatenate | @a = (1, 2, 3); @a2 = (@a, (4, 5, 6)); push @a, (4,5,6); | $a = [1, 2, 3]; $a2 = array_merge($a, [4, 5, 6]); $a = array_merge($a, [4, 5, 6]); | a = [1, 2, 3] a2 = a + [4, 5, 6] a.extend([4, 5, 6]) | a = [1, 2, 3] a2 = a + [4, 5, 6] a.concat([4, 5, 6]) |
| replicate | @a = (undef) x 10; | a = [None] * 10 a = [None for i in range(0, 10)] | a = [nil] * 10 a = Array.new(10, nil) | |
| copy address copy, shallow copy, deep copy | use Storable 'dclone' my @a = (1,2,[3,4]); my $a2 = \@a; my @a3 = @a; my @a4 = @{dclone(\@a)}; | $a = [1, 2, [3, 4]]; $a2 =& $a; none $a4 = $a; | import copy a = [1,2,[3,4]] a2 = a a3 = list(a) a4 = copy.deepcopy(a) | a = [1,2,[3,4]] a2 = a a3 = a.dup a4 = Marshal.load(Marshal.dump(a)) |
| arrays as function arguments | each element passed as separate argument; use reference to pass array as single argument | parameter contains deep copy | parameter contains address copy | parameter contains address copy |
| iterate over elements | for $i (1, 2, 3) { print "$i\n" } | foreach ([1, 2, 3] as $i) { echo "$i\n"; } | for i in [1,2,3]: print(i) | [1,2,3].each { |i| puts i } |
| iterate over indices and elements | none; use range iteration from 0 to $#a and use index to look up value in the loop body | $a = ["do", "re", "mi" "fa"]; foreach ($a as $i => $s) { echo "$s at index $i\n"; } | a = ['do', 're', 'mi', 'fa'] for i, s in enumerate(a): print('%s at index %d' % (s, i)) | a = %w(do re mi fa) a.each_with_index do |s, i| puts "#{s} at index #{i}" end |
| iterate over range | for $i (1..1_000_000) { code } | not space efficient; use C-style for loop | # use range() in Python 3: for i in xrange(1, 1000001): code | (1..1_000_000).each do |i| code end |
| instantiate range as array | @a = 1..10; | $a = range(1, 10); | a = range(1, 11) Python 3: a = list(range(1, 11)) | a = (1..10).to_a |
| reverse non-destructive, in-place | @a = (1, 2, 3); reverse @a; @a = reverse @a; | $a = [1, 2, 3]; array_reverse($a); $a = array_reverse($a); | a = [1, 2, 3] a[::-1] a.reverse() | a = [1, 2, 3] a.reverse a.reverse! |
| sort non-destructive, in-place, custom comparision | @a = qw(b A a B); sort @a; @a = sort @a; sort { lc($a) cmp lc($b) } @a; | $a = ["b", "A", "a", "B"]; none sort($a); none, but usort sorts in place | a = ['b', 'A', 'a', 'B'] sorted(a) a.sort() # custom binary comparision # removed from Python 3: a.sort(key=str.lower) | a = %w(b A a B) a.sort a.sort! a.sort do |x, y| x.downcase <=> y.downcase end |
| dedupe non-destructive, in-place | use List::MoreUtils 'uniq'; my @a = (1, 2, 2, 3); my @a2 = uniq @a; @a = uniq @a; | $a = [1, 2, 2, 3]; $a2 = array_unique($a); $a = array_unique($a); | a = [1, 2, 2, 3] a2 = list(set(a)) a = list(set(a)) | a = [1, 2, 2, 3] a2 = a.uniq a.uniq! |
| membership | 7 ~~ @a | in_array(7, $a) | 7 in a | a.include?(7) |
| intersection | $a = [1, 2]; $b = [2, 3, 4] array_intersect($a, $b) | {1,2} & {2,3,4} | [1,2] & [2,3,4] | |
| union | $a1 = [1, 2]; $a2 = [2, 3, 4]; array_unique(array_merge($a1, $a2)) | {1,2} | {2,3,4} | [1,2] | [2,3,4] | |
| relative complement, symmetric difference | $a1 = [1, 2, 3]; $a2 = [2]; array_values(array_diff($a1, $a2)) none | {1,2,3} - {2} {1,2} ^ {2,3,4} | require 'set' [1,2,3] - [2] Set[1,2] ^ Set[2,3,4] | |
| map | map { $_ * $_ } (1,2,3) | array_map(function ($x) { return $x * $x; }, [1, 2, 3]) | map(lambda x: x * x, [1,2,3]) # or use list comprehension: [x * x for x in [1,2,3]] | [1,2,3].map { |o| o * o } |
| filter | grep { $_ > 1 } (1,2,3) | array_filter([1, 2, 3], function ($x) { return $x>1; }) | filter(lambda x: x > 1, [1,2,3]) # or use list comprehension: [x for x in [1,2,3] if x > 1] | [1,2,3].select { |o| o > 1 } |
| reduce | use List::Util 'reduce'; reduce { $x + $y } 0, (1,2,3) | array_reduce([1, 2, 3], function($x,$y) { return $x + $y; }, 0) | # import needed in Python 3 only from functools import reduce reduce(lambda x, y: x+y, [1,2,3], 0) | [1,2,3].inject(0) { |m,o| m+o } |
| universal and existential tests | # cpan -i List::MoreUtils use List::MoreUtils qw(all any); all { $_ % 2 == 0 } (1,2,3,4) any { $_ % 2 == 0 } (1,2,3,4) | use array_filter | all(i % 2 == 0 for i in [1,2,3,4]) any(i % 2 == 0 for i in [1,2,3,4]) | [1,2,3,4].all? {|i| i.even? } [1,2,3,4].any? {|i| i.even? } |
| shuffle and sample | use List::Util 'shuffle'; @a = (1, 2, 3, 4); shuffle(@a); none | $a = [1, 2, 3, 4]; shuffle($a); array_rand($a, 2) | from random import shuffle, sample a = [1, 2, 3, 4] shuffle(a) sample(a, 2) | [1, 2, 3, 4].shuffle [1, 2, 3, 4].sample(2) |
| zip | # cpan -i List::MoreUtils use List::MoreUtils 'zip'; @nums = (1, 2, 3); @lets = qw(a b c); # flat array of 6 elements: @a = zip @nums, @lets; | # array of 3 pairs: $a = array_map(NULL, [1, 2, 3], ["a", "b", "c"]); | # array of 3 pairs: a = zip([1,2,3], ['a', 'b', 'c']) | # array of 3 pairs: a = [1,2,3].zip(["a", "b", "c"]) |
| dictionaries | ||||
| perl | php | python | ruby | |
| literal | %d = ( "t" => 1, "f" => 0 ); # barewords permitted in front of => # under 'use strict' | $d = ["t" => 1, "f" => 0]; # older syntax: $d = array("t" => 1, "f" => 0); | d = { 't':1, 'f':0 } | d = { "t" => 1, "f" => 0 } |
| size | scalar(keys %d) | count($d) | len(d) | d.size d.length # same as size |
| lookup | $d{"t"} # barewords permitted inside { } # under 'use strict' | $d["t"] | d['t'] | d["t"] |
| out-of-bounds behavior | %d = (); evaluates as undef: $d{"lorem"}; adds key/value pair: $d{"lorem"} = "ipsum"; | $d = []; evaluates as NULL: $d["lorem"]; adds key/value pair: $d["lorem"] = "ipsum"; | d = {} raises KeyError: d['lorem'] adds key/value pair: d['lorem'] = 'ipsum' | d = {} evaluates as nil: d["lorem"] adds key/value pair: d["lorem"] = "ipsum" |
| is key present | exists $d{"y"} | array_key_exists("y", $d); | 'y' in d | d.has_key?("y") |
| delete entry | %d = ( 1 => "t", 0 => "f" ); delete $d{1}; | $d = [1 => "t", 0 => "f"]; unset($d[1]); | d = {1: True, 0: False} del d[1] | d = {1 => true, 0 => false} d.delete(1) |
| from array of pairs, from even length array | @a = (1,"a",2,"b",3,"c"); %d = @a; | a = [[1,'a'], [2,'b'], [3,'c']] d = dict(a) a = [1,'a',2,'b',3,'c'] d = dict(zip(a[::2], a[1::2])) | a = [[1,"a"], [2,"b"], [3,"c"]] d = Hash[a] a = [1,"a",2,"b",3,"c"] d = Hash[*a] | |
| merge | %d1 = (a=>1, b=>2); %d2 = (b=>3, c=>4); %d1 = (%d1, %d2); | $d1 = ["a"=>1, "b"=>2]; $d2 = ["b"=>3, "c"=>4]; $d1 = array_merge($d1, $d2); | d1 = {'a':1, 'b':2} d2 = {'b':3, 'c':4} d1.update(d2) | d1 = {"a"=>1, "b"=>2} d2 = {"b"=>3, "c"=>4} d1.merge!(d2) |
| invert | %to_num = (t=>1, f=>0); %to_let = reverse %to_num; | $to_num = ["t"=>1, "f"=>0]; $to_let = array_flip($to_num); | to_num = {'t':1, 'f':0} # dict comprehensions added in 2.7: to_let = {v:k for k, v in to_num.items()} | to_num = {"t"=>1, "f"=>0} to_let = to_num.invert |
| iteration | while (($k, $v) = each %d) { code } | foreach ($d as $k => $v) { code } | for k, v in d.iteritems(): code Python 3: for k, v in d.items(): code | d.each do |k,v| code end |
| keys and values as arrays | keys %d values %d | array_keys($d) array_values($d) | d.keys() d.values() Python 3: list(d.keys()) list(d.values()) | d.keys d.values |
| sort by values | foreach $k (sort { $d{$a} <=> $d{$b} } keys %d) { print "$k: $d{$k}\n"; } | asort($d); foreach ($d as $k => $v) { print "$k: $v\n"; } | from operator import itemgetter pairs = sorted(d.iteritems(), key=itemgetter(1)) for k, v in pairs: print('{}: {}'.format(k, v)) | d.sort_by {|k, v| v}.each do |k, v| puts "#{k}: #{v}" end |
| default value, computed value | my %counts; $counts{'foo'} += 1 define a tied hash for computed values and defaults other than zero or empty string | $counts = []; $counts['foo'] += 1; extend ArrayObject for computed values and defaults other than zero or empty string. | from collections import defaultdict counts = defaultdict(lambda: 0) counts['foo'] += 1 class Factorial(dict): def __missing__(self, k): if k > 1: return k * self[k-1] else: return 1 factorial = Factorial() | counts = Hash.new(0) counts['foo'] += 1 factorial = Hash.new do |h,k| k > 1 ? k * h[k-1] : 1 end |
| functions | ||||
| perl | php | python | ruby | |
| define function | sub add3 { $_[0] + $_[1] + $_[2] } sub add3 { my ($x1, $x2, $x3) = @_; $x1 + $x2 + $x3; } | function add3($x1, $x2, $x3) { return $x1 + $x2 + $x3; } | def add3(x1, x2, x3): return x1 + x2 + x3 | def add3(x1, x2, x3) x1 + x2 + x3 end # parens are optional and customarily # omitted when defining functions # with no parameters |
| invoke function | add3(1, 2, 3); # parens are optional: add3 1, 2, 3; | add3(1, 2, 3); # function names are case insensitive: ADD3(1, 2, 3); | add3(1, 2, 3) | add3(1, 2, 3) # parens are optional: add3 1, 2, 3 |
| apply function to array | @a = (2, 3); add3(1, @a); # arrays are always expanded when used # as arguments | $a = [1, 2, 3]; call_user_func_array("add3", $a); | a = [2, 3] add3(1, *a) # splat operator can only be used once # and must appear after other # unnamed arguments | a = [2, 3] add3(1, *a) # splat operator can be used multiple # times and can appear before regular # arguments |
| missing argument behavior | set to undef | set to NULL with warning | raises TypeError if number of arguments doesn't match function arity | raises ArgumentError if number of arguments doesn't match function arity |
| default argument | sub my_log { my $x = shift; my $base = shift // 10; log($x) / log($base); } my_log(42); my_log(42, exp(1)); | function my_log($x, $base=10) { return log($x) / log($base); } my_log(42); my_log(42, M_E); | import math def my_log(x, base=10): return math.log(x) / math.log(base) my_log(42) my_log(42, math.e) | def my_log(x, base=10) Math.log(x) / Math.log(base) end my_log(42) my_log(42, Math::E) |
| variable number of arguments | sub first_and_last { if ( @_ >= 1 ) { print "first: $_[0]\n"; } if ( @_ >= 2 ) { print "last: $_[-1]\n"; } } | function first_and_last() { $arg_cnt = func_num_args(); if ($arg_cnt >= 1) { $n = func_get_arg(0); echo "first: " . $n . "\n"; } if ($arg_cnt >= 2) { $a = func_get_args(); $n = $a[$arg_cnt-1]; echo "last: " . $n . "\n"; } } | def first_and_last(*a): if len(a) >= 1: print('first: ' + str(a[0])) if len(a) >= 2: print('last: ' + str(a[-1])) | def first_and_last(*a) if a.size >= 1 puts "first: #{a[0]}" end if a.size >= 2 puts "last: #{a[-1]}" end end |
| named parameters | none | none | def fequal(x, y, eps=0.01): return abs(x - y) < eps fequal(1.0, 1.001) fequal(1.0, 1.001, eps=0.1**10) | def fequal(x, y, opts={}) eps = opts[:eps] || 0.01 (x - y).abs < eps end fequal(1.0, 1.001) fequal(1.0, 1.001, :eps=>0.1**10) # Ruby 2.0: def fequals(x, y, eps: 0.01) (x - y).abs < eps end fequals(1.0, 1.001) fequals(1.0, 1.001, eps: 0.1**10) |
| pass number or string by reference | sub foo { $_[0] += 1; $_[1] .= "ly"; } my $n = 7; my $s = "hard"; foo($n, $s); | function foo(&$x, &$y) { $x += 1; $y .= "ly"; } $n = 7; $s = "hard"; foo($n, $s); | not possible | not possible |
| pass array or dictionary by reference | sub foo { $_[0][2] = 5; $_[1]{"f"} = -1; } my @a = (1,2,3); my %d = ("t"=> 1, "f" => 0); foo(\@a, \%d); | function foo(&$x, &$y) { $x[2] = 5; $y["f"] = -1; } $a = [1, 2, 3]; $d = ["t"=>1, "f"=>0]; foo($a, $d); | def foo(x, y): x[2] = 5 y['f'] = -1 a = [1,2,3] d = {'t':1, 'f':0} foo(a, d) | def foo(x, y) x[2] = 5 y["f"] = -1 end a = [1,2,3] d = {"t"=> 1, "f" => 0 } foo(a, d) |
| return value | return arg or last expression evaluated | return arg or NULL | return arg or None | return arg or last expression evaluated |
| multiple return values | sub first_and_second { return ($_[0], $_[1]); } @a = (1,2,3); ($x, $y) = first_and_second(@a); | function first_and_second(&$a) { return [$a[0], $a[1]]; } $a = [1, 2, 3]; list($x, $y) = first_and_second($a); | def first_and_second(a): return a[0], a[1] x, y = first_and_second([1,2,3]) | def first_and_second(a) return a[0], a[1] end x, y = first_and_second([1,2,3]) |
| lambda declaration | $sqr = sub { $_[0] * $_[0] } | $sqr = function ($x) { return $x * $x; }; | # body must be an expression: sqr = lambda x: x * x | sqr = lambda { |x| x * x } |
| lambda invocation | $sqr->(2) | $sqr(2) | sqr(2) | sqr.call(2) or sqr[2] |
| function as value | my $func = \&add; | $func = "add"; | func = add | func = lambda {|*args| add(*args)} |
| function with private state | use feature state; sub counter { state $i = 0; ++$i; } print counter() . "\n"; | function counter() { static $i = 0; return ++$i; } echo counter(); | # state not private: def counter(): counter.i += 1 return counter.i counter.i = 0 print(counter()) | none |
| closure | sub make_counter { my $i = 0; return sub { ++$i }; } my $nays = make_counter; print $nays->() . "\n"; | function make_counter() { $i = 0; return function () use (&$i) { return ++$i; }; } $nays = make_counter(); echo $nays(); | # Python 3: def make_counter(): i = 0 def counter(): nonlocal i i += 1 return i return counter nays = make_counter() | def make_counter i = 0 return lambda { i +=1; i } end nays = make_counter puts nays.call |
| generator | none | # PHP 5.5: function make_counter() { $i = 0; while (1) { yield ++$i; } } $nays = make_counter(); # does not return a value: $nays->next(); # runs generator if generator has not # yet yielded: echo $nays->current(); | # The itertools library contains # standard generators. # c.f. itertools.count() def make_counter(): i = 0 while True: i += 1 yield i nays = make_counter() print(nays.next()) | def make_counter return Fiber.new do i = 0 while true i += 1 Fiber.yield i end end end nays = make_counter puts nays.resume |
| decorator | def logcall(f): def wrapper(*a, **opts): print('calling ' + f.__name__) f(*a, **opts) print('called ' + f.__name__) return wrapper @logcall def square(x): return x * x | |||
| operator as function | import operator operator.mul(3, 7) a = ['foo', 'bar', 'baz'] operator.itemgetter(2)(a) | 3.*(7) a = ['foo', 'bar', 'baz'] a.[](2) | ||
| execution control | ||||
| perl | php | python | ruby | |
| if | if ( 0 == $n ) { print "no hits\n" } elsif ( 1 == $n ) { print "one hit\n" } else { print "$n hits\n" } | if ( 0 == $n ) { echo "no hits\n"; } elseif ( 1 == $n ) { echo "one hit\n"; } else { echo "$n hits\n"; } | if 0 == n: print('no hits') elif 1 == n: print('one hit') else: print(str(n) + ' hits') | if n == 0 puts "no hits" elsif 1 == n puts "one hit" else puts "#{n} hits" end |
| switch | use feature 'switch'; given ($n) { when (0) { print "no hits\n"; } when (1) { print "one hit\n"; } default { print "$n hits\n"; } } | switch ($n) { case 0: echo "no hits\n"; break; case 1: echo "one hit\n"; break; default: echo "$n hits\n"; } | none | case n when 0 puts "no hits" when 1 puts "one hit" else puts "#{n} hits" end |
| while | while ( $i < 100 ) { $i++ } | while ( $i < 100 ) { $i++; } | while i < 100: i += 1 | while i < 100 do i += 1 end |
| c-style for | for ( $i=0; $i <= 10; $i++ ) { print "$i\n"; } | for ($i = 1; $i <= 10; $i++) { echo "$i\n"; } | none | none |
| break, continue, redo | last next redo | break continue none | break continue none | break next redo |
| control structure keywords | do else elsif for foreach goto if unless until while | case default do else elseif for foreach goto if switch while | elif else for if while | case do else elsif end for loop when while unless until |
| what do does | executes following block and returns value of last statement executed | starts body of a do-while loop, a loop which checks the condition after the body is executed | raises NameError unless a value was assigned to it | starts an anonymous block. Also starts the body of a loop, while, or until loop |
| statement modifiers | print "positive\n" if $i > 0; print "nonzero\n" unless $i == 0; | none | none | puts "positive" if i > 0 puts "nonzero" unless i == 0 |
| exceptions | ||||
| perl | php | python | ruby | |
| raise exception | die "bad arg"; | throw new Exception("bad arg"); | raise Exception('bad arg') | # raises RuntimeError raise "bad arg" |
| re-raise exception | try: raise Exception('bam!') except: print('re-raising...') raise | begin raise "bam!" rescue puts "re-raising…" raise end | ||
| catch exception | eval { risky }; if ($@) { print "risky failed: $@\n"; } | try { risky(); } catch (Exception $e) { echo "risky failed: ", $e->getMessage(), "\n"; } | try: risky() except: print('risky failed') | # catches StandardError begin risky rescue print "risky failed: " puts $!.message end |
| global variable for last exception | $EVAL_ERROR: $@ $OS_ERROR: $! $CHILD_ERROR: $? | none | last exception: sys.exc_info()[1] | last exception: $! backtrace array of exc.: $@ exit status of child: $? |
| define exception | none | class Bam extends Exception { function __construct() { parent::__construct("bam!"); } } | class Bam(Exception): def __init__(self): super(Bam, self).__init__('bam!') | class Bam < Exception def initialize super("bam!") end end |
| catch exception by type | none | try { throw new Bam; } catch (Bam $e) { echo $e->getMessage(), "\n"; } | try: raise Bam() except Bam as e: print(e) | begin raise Bam.new rescue Bam => e puts e.message end |
| finally/ensure | none | none | acquire_resource() try: risky() finally: release_resource() | acquire_resource begin risky ensure release_resource end |
| concurrency | ||||
| perl | php | python | ruby | |
| start thread | use threads; $func = sub { sleep 10 }; $thr = threads->new($func); | none | class sleep10(threading.Thread): def run(self): time.sleep(10) thr = sleep10() thr.start() | thr = Thread.new { sleep 10 } |
| wait on thread | $thr->join; | thr.join() | thr.join | |
| ____________________________________________ | ____________________________________________ | _____________________________________________ | _____________________________________________ |
and environment | option parsing | libraries
and namespaces |objects | polymorphism | reflection | net
and web | unit tests | debugging and profiling | deployment
General
versions used
The versions used for testing code in the reference sheet.
show version
How to get the version.perl:
Also available in the predefined variable $], or in a different format in $^V and $PERL_VERSION.
php:
The function phpversion() will return the version number as a string.
python:
The following function will return the version number as a string:
import platform platform.python_version()
[/code]
ruby:
Also available in the global constant RUBY_VERSION.
implicit prologue
Code which examples in the sheet assume to have already been executed.perl:
We adopt the convention that if an example uses a variable without declaring it, it should be taken to have been previously declared with my.
python:
We assume that os, re,
and sys are always imported.
Grammar and Invocation
interpreter
The customary name of the interpreter and how to invoke it.php:
php -f will only execute portions of the source file within a <?php php code ?>
tag as php code. Portions of the source file outside of such tags is not treated as executable code and is echoed to standard out.
If short tags are enabled, then php code can also be placed inside <? php code ?> and <?= php code ?> tags.
<?= php code ?> is identical to <?php echo php code ?>.
repl
The customary name of the repl.perl:
The Perl REPL perl -de 0 does not save or display the result of an expression. perl
-d is the Perl debugger andperl -e runs code provided on the command line.
perl -de 0 does not by default have readline, but it can be added:
$ cpan -i Term::ReadLine::Perl <cpan output omitted> $ perl -de 0 DB<1> use Term::ReadLine::Perl; DB<2> print 1 + 1; 2
[/code]
php:
The php -a REPL does not save or display the result of an expression.
python:
The python repl saves the result of the last statement in _.
ruby:
irb saves the result of the last statement in _.
command line program
How to pass the code to be executed to the interpreter as a command line argument.
block delimiters
How blocks are delimited.perl:
Curly brackets {} delimit blocks. They are also used for:
hash literal syntax which returns a reference to the hash: $rh = { 'true' => 1, 'false' => 0 }
hash value lookup: $h{'true'}, $rh->{'true'}
variable name delimiter: $s = "hello"; print "${s}goodbye";
python:
Python blocks begin with a line that ends in a colon. The block ends with the first line that is not indented further than the initial line. Python raises an IndentationError if the statements in the block that are not in a nested block
are not all indented the same. Using tabs in Python source code is unrecommended and many editors replace them automatically with spaces. If the Python interpreter encounters a tab, it is treated as 8 spaces.
The python repl switches from a >>> prompt
to a … prompt inside a block. A blank line terminates the block.
Colons are also used to separate keys from values in dictionary literals and in sequence slice notation.
ruby:
Curly brackets {} delimit blocks. A matched curly bracket pair can be replaced by the do and end keywords.
By convention curly brackets are used for one line blocks.
The end keyword also terminates blocks started by def, class,
or module.
Curly brackets are also used for hash literals, and the #{ } notation is used to interpolate expressions into strings.
statement separator
How the parser determines the end of a statement.perl:
In a script statements are separated by semicolons and never by newlines. However, when using perl -de 0 a
newline terminates the statement.
php:
Inside braces statements must be terminated by a semicolon. The following causes a parse error:
<? if (true) { echo "true" } ?>[/code]
The last statement inside <?= ?> or <?
?> tags does not need to be semicolon terminated, however. The following code is legal:
<?= $a = 1 ?> <? echo $a ?>
[/code]
python:
Newline does not terminate a statement when:
inside parens
inside list [] or dictionary {} literals
Python single quote '' and double quote "" strings cannot contain newlines except as the two character escaped form \n. Putting a newline in these strings results in a syntax error. There is however a multi-line string literal which starts
and ends with three single quotes ''' or three double quotes: """.
A newline that would normally terminate a statement can be escaped with a backslash.
ruby:
Newline does not terminate a statement when:
inside single quotes '', double quotes "", backticks ``, or parens ()
after an operator such as + or , that expects another argument
Ruby permits newlines in array [] or hash literals, but only after a comma , or associator =>. Putting a newline before the comma or associator results in a syntax error.
A newline that would normally terminate a statement can be escaped with a backslash.
source code encoding
How to identify the character encoding for a source code file.Setting the source code encoding makes it possible to safely use non-ASCII characters in string literals and regular expression literals.
end-of-line comment
How to create a comment that ends at the next newline.
multiple line comment
How to comment out multiple lines.python:
The triple single quote ''' and triple double quote """ syntax is a syntax for string literals.
Variables and Expressions
local variable
How to declare variables which are local to the scope defining region which immediately contain them.perl:
Variables don't need to be declared unless use strict is in effect.
If not initialized, scalars are set to undef, arrays are set to an empty array, and hashes are set to an empty hash.
Perl can also declare variables with local. These replace the value of a global variable with the same name, if any, for the duration of the enclosing scope, after which the old value is restored.
php:
Variables do not need to be declared and there is no syntax for declaring a local variable. If a variable with no previous reference is accessed, its value is NULL.
python:
A variable is created by assignment if one does not already exist. If the variable is inside a function or method, then its scope is the body of the function or method. Otherwise it is a global.
ruby:
Variables are created by assignment. If the variable does not have a dollar sign ($) or ampersand (@) as its first character then its scope is scope defining region which most immediately contains it.
A lower case name can refer to a local variable or method. If both are defined, the local variable takes precedence. To invoke the method make the receiver explicit: e.g. self.name. However, outside of class and modules local
variables hide functions because functions are private methods in the class Object. Assignment to name will create a local variable if one with that name does not exist, even if there is a method name.
regions which define lexical scope
A list of regions which define a lexical scope for the local variables they contain.Local variables defined inside the region are only in scope while code within the region is executing. If the language does not have closures, then code outside the region has no access to local variables defined inside the region. If
the language does have closures, then code inside the region can make local variables accessible to code outside the region by returning a reference.
A region which is top level hides local variables in the scope which contains it from the code it contains. A region can also be top level if the syntax requirements of the language prohibit it from being placed inside another
scope defining region.
A region is nestable if it can be placed inside another scope defining region, and if code in the inner region can access local variables in the outer region.
perl:
A local variable can be defined outside of any function definition or anonymous block, in which case the scope of the variable is the file containing the source code. In this way Perl resembles Ruby and contrasts with PHP and Python.
In PHP and Python, any variable defined outside a function definition is global.
In Perl, when a region which defines a scope is nested inside another, then the inner region has read and write access to local variables defined in the outer region.
Note that the blocks associated with the keywords if, unless, while, until, for, and foreach are anonymous blocks, and thus any my declarations in them create variables local
to the block.
php:
Only function bodies and method bodies define scope. Function definitions can be nested, but when this is done lexical variables in the outer function are not visible to code in the body of the inner function.
Braces can be used to set off blocks of codes in a manner similar to the anonymous blocks of Perl. However, these braces do not define a scope. Local variables created inside the braces will be visible to subsequent code outside of the
braces.
Local variables cannot be created in class bodies.
python:
Only functions and methods define scope. Function definitions can be nested. When this is done, inner scopes have read access to variables defined in outer scopes. Attempting to write (i.e. assign) to a variable defined in an outer scope
will instead result in a variable getting created in the inner scope. Python trivia question: what would happen if the following code were executed?
def foo(): v = 1 def bar(): print(v) v = 2 print(v) bar() foo()
[/code]
ruby:
The keywords if, unless, case, while, and until each define a block which is terminated by an end keyword, but none of these blocks have their own scope.
Anonymous functions can be created with the lambda keyword. Ruby anonymous blocks can be provided after a function invocation and are bounded by curly brackets { } or the do and end keywords. Both anonymous
functions and anonymous blocks can have parameters which are specified at the start of the block within pipes. Here are some examples:
id = lambda { |x| x }
[3, 1, 2, 4].sort { |a,b| a <=> b }
10.times do |i|
print "#{i}..."
end[/code]
The scope of the parameters of an anonymous block or anonymous function is local to the block or function body.
It is possible to mark variables as local, even when they are already defined in the containing scope. Such variables are listed inside the parameter pipes, separated from the parameters by a semicolon:
x = 3
noop = lambda { |; x| x = 15 }
noop.call
# x is still 3[/code]
global variable
How to declare and access a variable with global scope.perl:
Undeclared variables, which are permitted unless use strict is in effect, are global. If use strict is in effect, a global can be declared at the top level of a package (i.e. outside any blocks or functions) with the our keyword.
A variable declared withmy inside a function will hide a global with the same name, if there is one.
php:
A variable is global if it is used at the top level (i.e. outside any function definition) or if it is declared inside a function with the global keyword. A function must use the global keyword to access the global variable.
python:
A variable is global if it is defined at the top level of a file (i.e. outside any function definition). Although the variable is global, it must be imported individually or be prefixed with the module name prefix to be accessed from
another file. To be accessed from inside a function or method it must be declared with the global keyword.
ruby:
A variable is global if it starts with a dollar sign: $.
constant
How to declare a constant.php:
A constant can be declared inside a class:
class Math {
const pi = 3.14;
}[/code]
Refer to a class constant like this:
Math::pi
[/code]
ruby:
Capitalized variables contain constants and class/module names. By convention, constants are all caps and class/module names are camel case. The ruby interpreter does not prevent modification of constants, it only gives a warning. Capitalized
variables are globally visible, but a full or relative namespace name must be used to reach them: e.g. Math::PI.
assignment
How to assign a value to a variable.perl:
Assignment operators have right precedence and evaluate to the right argument, so assignments can be chained:
$a = $b = 3;
[/code]
python:
If the variable on the left has not previously been defined in the current scope, then it is created. This may hide a variable in a containing scope.
Assignment does not return a value and cannot be used in an expression. Thus, assignment cannot be used in a conditional test, removing the possibility of using assignment (=) when an equality test (==) was intended. Assignments can nevertheless
be chained to assign a value to multiple variables:
a = b = 3
[/code]
ruby:
Assignment operators have right precedence and evaluate to the right argument, so they can be chained. If the variable on the left does not exist, then it is created.
parallel assignment
How to assign values to variables in parallel.python:
The r-value can be a list or tuple:
nums = [1,2,3] a,b,c = nums more_nums = (6,7,8) d,e,f = more_nums
[/code]
Nested sequences of expression can be assigned to a nested sequences of l-values, provided the nesting matches. This assignment will set a to 1, b to 2, and c to 3:
(a,[b,c]) = [1,(2,3)]
[/code]
This assignment will raise a TypeError:
(a,(b,c)) = ((1,2),3)
[/code]
In Python 3 the splat operator * can be used to collect the remaining right side elements in a list:
x, y, *z = 1, 2 # assigns [] to z x, y, *z = 1, 2, 3 # assigns [3] to z x, y, *z = 1, 2, 3, 4 # assigns [3, 4] to z
[/code]
ruby:
The r-value can be an array:
nums = [1,2,3] a,b,c = nums
[/code]
swap
How to swap the values held by two variables.
compound assignment
Compound assignment operators mutate a variable, setting it to the value of an operation which takes the previous value of the variable as an argument.If <OP> is a binary operator and the language has the compound assignment operator <OP>=,
then the following are equivalent:
x <OP>= y x = x <OP> y
[/code]
The compound assignment operators are displayed in this order:
First row: arithmetic operator assignment: addition, subtraction, multiplication, (float) division, integer division, modulus, and exponentiation.
Second row: string concatenation assignment and string replication assignment
Third row: logical operator assignment: and, or, xor
Fourth row: bit operator assignment: left shift, right shift, and, or, xor.
python:
Python compound assignment operators do not return a value and hence cannot be used in expressions.
increment and decrement
The C-style increment and decrement operators can be used to increment or decrement values. They return values and thus can be used in expressions. The prefix versions return the value in the variable after mutation, and the postfix versionreturn the value before mutation.
Incrementing a value two or more times in an expression makes the order of evaluation significant:
x = 1; foo(++x, ++x); // foo(2, 3) or foo(3, 2)? x = 1; y = ++x/++x; // y = 2/3 or y = 3/2?
[/code]
Python avoids the problem by not having an in-expression increment or decrement.
Ruby mostly avoids the problem by providing a non-mutating increment and decrement. However, here is a Ruby expression which is dependent on order of evaluation:
x = 1 y = (x += 1)/(x += 1)
[/code]
perl:
The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:
$x = 1; $x++; $x--;
[/code]
php:
The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:
$x = 1; $x++; $x--;
[/code]
ruby:
The Integer class defines succ, pred,
and next, which is a synonym for succ.
The String class defines succ, succ!, next,
and next!. succ! and next! mutate
the string.
null
The null literal.
null test
How to test if a variable contains null.perl:
$v == undef does not imply that $v is undef. Any comparison between undef and a falsehood will return true. The following comparisons are true:
$v = undef;
if ($v == undef) { print "true"; }
$v = 0;
if ($v == undef) { print "sadly true"; }
$v = '';
if ($v == undef) { print "sadly true"; }[/code]
php:
$v == NULL does not imply that $v is NULL, since any comparison between NULL and a falsehood will return true. In particular, the following comparisons are true:
$v = NULL;
if ($v == NULL) { echo "true"; }
$v = 0;
if ($v == NULL) { echo "sadly true"; }
$v = '';
if ($v == NULL) { echo "sadly true"; }[/code]
undefined variable access
The result of attempting to access an undefined variable.perl:
Perl does not distinguish between unset variables and variables that have been set to undef. In Perl, calling defined($a) does not result in a error if $a is undefined, even with the strict pragma.
php:
PHP does not provide the programmer with a mechanism to distinguish an undefined variable from a variable which has been set to NULL.
A test showing that isset is
the logical negation of is_null.
python:
How to test if a variable is defined:
not_defined = False try: v except NameError: not_defined = True
[/code]
ruby:
How to test if a variable is defined:
! defined?(v)
[/code]
conditional expression
How to write a conditional expression. A ternary operator is an operator which takes three arguments. Sincecondition ? true value : false value
is the only ternary operator in C, it is unambiguous to refer to it as the ternary operator.
python:
The Python conditional expression comes from Algol.
ruby:
The Ruby if statement is also an expression:
x = if x > 0 x else -x end
[/code]
Arithmetic and Logic
true and false
Literals for the booleans.These are the return values of the relational operators.
php:
Any identifier which matches TRUE case-insensitive can be used for the TRUE boolean. Similarly for FALSE.
In general, PHP variable names are case-sensitive, but function names are case-insensitive.
When converted to a string for display purposes, TRUE renders as "1" and FALSE as "". The equality tests TRUE
== 1and FALSE == "" evaluate as TRUE but the equality tests TRUE
=== 1 and FALSE === "" evaluate as FALSE.
falsehoods
Values which behave like the false boolean in a conditional context.Examples of conditional contexts are the conditional clause of an if statement and the test of a while loop.
python:
Whether a object evaluates to True or False in a boolean context can be customized by implementing a __nonzero__(Python 2) or __bool__ (Python 3) instance
method for the class.
logical operators
Logical and, or, and not.php, perl, ruby:
&& and || have higher precedence than assignment, compound assignment, and the ternary operator (?:), which have higher precedence than and and or.
relational operators
Equality, inequality, greater than, less than, greater than or equal, less than or equal.php:
Most of the relational operators will convert a string to a number if the other operand is a number. Thus 0 == "0" is true. The operators === and !== do not perform this conversion, so 0 === "0" is false.
perl:
The operators: == != > < >= <= convert strings to numbers before performing a comparison. Many string evaluate as zero in a numeric context and are equal according to the == operator. To perform a lexicographic string comparison, use: eq,ne, gt, lt, ge, le.
python:
Relational operators can be chained. The following expressions evaluate to true:
1 < 2 < 3 1 == 1 != 2
[/code]
In general if Ai are expressions and opi are relational operators, then
A1 op1 A2 op2 A3 …
An opn An+1
is true if and only if each of the following is true
A1 op1 A2
A2 op2 A3
…
An opn An+1
compare strings
How to compare strings.
min and max
How to get the min and max.
three value comparison
Binary comparison operators which return -1, 0, or 1 depending upon whether the left argument is less than, equal to, or greater than the right argument.The <=> symbol is called the spaceship operator.
arithmetic operators
The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation.
integer division
How to get the integer quotient of two integers.perl:
The integer pragma makes all arithmetic operations integer operations. Floating point numbers are truncated
before they are used. Hence integer division could be performed with:
use integer; my $a = 7 / 3; no integer;
[/code]
divmod
How to get the quotient and remainder with single function call.
integer division by zero
What happens when an integer is divided by zero.
float division
How to perform floating point division, even if the operands might be integers.
float division by zero
What happens when a float is divided by zero.
power
How to get the value of a number raised to a power.
sqrt
The square root function.
sqrt -1
The result of taking the square root of negative one.
transcendental functions
Some mathematical functions. Trigonometric functions are in radians unless otherwise noted. Logarithms are natural unless otherwise noted.python:
Python also has math.log10. To compute the log of x for base b, use:
math.log(x)/math.log(b)
[/code]
ruby:
Ruby also has Math.log2, Math.log10. To compute the log of x for base b, use
Math.log(x)/Math.log(b)
[/code]
transcendental constants
Constants for π and Euler's constant.
float truncation
How to truncate a float to the nearest integer towards zero; how to round a float to the nearest integer; how to find the nearest integer above a float; how to find the nearest integer below a float; how to take the absolute value.perl:
The CPAN module Number::Format provides a round function.
The 2nd argument specifies the number of digits to keep to the right of the radix. The default is 2.
use Number::Format 'round'; round(3.14, 0);
[/code]
absolute value
How to get the absolute value of a number.
integer overflow
What happens when the largest representable integer is exceeded.
float overflow
What happens when the largest representable float is exceeded.
rational numbers
How to create rational numbers and get the numerator and denominator.ruby:
Require the library mathn and integer division will yield rationals instead of truncated integers.
complex numbers
python:Most of the functions in math have analogues in cmath which will work correctly on complex numbers.
random integer, uniform float, normal float
How to generate a random integer between 0 and 99, include, float between zero and one in a uniform distribution, or a float in a normal distribution with mean zero and standard deviation one.
set random seed, get and restore seed
How to set the random seed; how to get the current random seed and later restore it.All the languages in the sheet set the seed automatically to a value that is difficult to predict. The Ruby MRI interpreter uses the current time and process ID, for example. As a result there is usually no need to set the seed.
Setting the seed to a hardcoded value yields a random but repeatable sequence of numbers. This can be used to ensure that unit tests which cover code using random numbers doesn't intermittently fail.
The seed is global state. If multiple functions are generating random numbers then saving and restoring the seed may be necessary to produce a repeatable sequence.
bit operators
The bit operators for left shift, right shift, and, inclusive or, exclusive or, and negation.
binary, octal, and hex literals
Binary, octal, and hex integer literals
radix
How to convert integers to strings of digits of a given base. How to convert such strings into integers.perl
Perl has the functions oct and hex which
convert strings encoded in octal and hex and return the corresponding integer. The oct function will handle binary or hex encoded
strings if they have "0b" or "0x" prefixes.
oct("60")
oct("060")
oct("0b101010")
oct("0x2a")
hex("2a")
hex("0x2a")[/code]
python
Python has the functions bin, oct,
and hex which take an integer and return a string encoding the integer in base 2, 8, and 16.
bin(42) oct(42) hex(42)
[/code]
Strings
string type
The type or types using for strings.perl:
Perl has supported multibyte characters since version 5.6.
php:
PHP assumes all strings have single byte characters.
python:
In Python 2.7 the str type assumes single byte characters. A separate unicode type
is available for working with Unicode strings.
In Python 3 the str type supports multibtye characters and the unicode type
has been removed.
There is a mutable bytearray type and an immutable bytes type
for working with sequences of bytes.
ruby:
The {String}} type supports multibtye characters. All strings have an explicit Encoding.
string literal
The syntax for string literals.perl:
When use strict is not in effect bareword strings are permitted.
Barewords are strings without quote delimiters. They are a feature of shells. Barewords cannot contain whitespace or any other character used by the tokenizer to distinguish words.
Before Perl 5 subroutines were invoked with an ampersand prefix & or the older do keyword. With Perl 5
neither is required, but this made it impossible to distinguish a bareword string from a subroutine without knowing all the subroutines which are in scope.
The following code illustrates the bareword ambiguity:
no strict;
print rich . "\n"; # prints "rich"; rich is a bareword string
sub rich { return "poor" }
print rich . "\n"; # prints "poor"; rich is now a subroutine[/code]
newline in literal
Whether newlines are permitted in string literals.python:
Newlines are not permitted in single quote and double quote string literals. A string can continue onto the following line if the last character on the line is a backslash. In this case, neither the backslash nor the newline are taken
to be part of the string.
Triple quote literals, which are string literals terminated by three single quotes or three double quotes, can contain newlines:
'''This is two lines''' """This is also two lines"""
[/code]
literal escapes
Backslash escape sequences for inserting special characters into string literals.| unrecognized backslash escape sequence | ||
|---|---|---|
| double quote | single quote | |
| PHP | preserve backslash | preserve backslash |
| Perl | drop backslash | preserve backslash |
| Python | preserve backslash | preserve backslash |
| Ruby | drop backslash | preserve backslash |
In addition to the character escapes, Perl has the following translation escapes:
| \u | make next character uppercase |
| \l | make next character lowercase |
| \U | make following characters uppercase |
| \L | make following characters lowercase |
| \Q | backslash escape following nonalphanumeric characters |
| \E | end \U, \L, or \Q section |
binmode(STDOUT, ':utf8');
use charnames ':full';
print "lambda: \N{GREEK SMALL LETTER LAMDA}\n";
use charnames ':short';
print "lambda: \N{greek:lamda}\n";
use charnames qw(greek);
print "lambda: \N{lamda}\n";[/code]
python:
When string literals have an r or R prefix
there are no backslash escape sequences and any backslashes thus appear in the created string. The delimiter can be inserted into a string if it is preceded by a backslash, but the backslash is also inserted. It is thus not possible to create a string with
an r or R prefix
that ends in a backslash. The r and R prefixes
can be used with single or double quotes:
r'C:\Documents and Settings\Admin' r"C:\Windows\System32"
[/code]
The \uhhhh escapes are also available inside Python 2 Unicode literals. Unicode literals have a u prefiix:
u'lambda: \u03bb'
[/code]
This syntax is also available in Python 3.3, but not Python 3.2. In Python 3.3 it creates a string of type str which
has the same features as the unicode type of Python 2.7.
custom delimiters
How to specify custom delimiters for single and double quoted strings. These can be used to avoid backslash escaping. If the left delimiter is (, [, or { the right delimiter must be ), ], or }, respectively.
here document
Here documents are strings terminated by a custom identifier. They perform variable substitution and honor the same backslash escapes as double quoted strings.perl:
Put the custom identifier in single quotes to prevent variable interpolation and backslash escape interpretation:
s = <<'EOF'; Perl code uses variables with dollar signs, e.g. $var EOF
[/code]
python:
Python lacks variable interpolation in strings. Triple quotes honor the same backslash escape sequences as regular quotes, so triple quotes can otherwise be used like here documents:
s = '''here document there computer '''
[/code]
ruby:
Put the customer identifier in single quotes to prevent variable interpolation and backslash escape interpretation:
s = <<'EOF'
Ruby code uses #{var} type syntax
to interpolate variables into strings.
EOF[/code]
variable interpolation
How to interpolate variables into strings.python:
str.format will take named or positional parameters. When used with named parameters str.format can
mimic the variable interpolation feature of the other languages.
A selection of variables in scope can be passed explicitly:
count = 3
item = 'ball'
print('{count} {item}s'.format(
count=count,
item=item))[/code]
Python 3 has format_map which accepts a dict as
an argument:
count = 3
item = 'ball'
print('{count} {item}s'.format_map(locals()))[/code]
expression interpolation
How to interpolate the result of evaluating an expression into a string.
format
How to create a string using a printf style format.python:
The % operator will interpolate arguments into printf-style format strings.
The str.format with positional parameters provides an alternative format using curly braces {0}, {1},
… for replacement fields.
The curly braces are escaped by doubling:
'to insert parameter {0} into a format, use {{{0}}}'.format(3)[/code]
If the replacement fields appear in sequential order and aren't repeated, the numbers can be omitted:
'lorem {} {} {}'.format('ipsum', 13, 3.7)[/code]
are strings mutable?
Are strings mutable?
copy string
How to copy a string such that changes to the original do not modify the copy.
concatenate
The string concatenation operator.
replicate
The string replication operator.
translate case
How to put a string into all caps or all lower case letters.
capitalize
How to capitalize a string and the words in a string.ruby:
Rails monkey patches the String class with the titleize method
for capitalizing the words in a string.
trim
How to remove whitespace from the ends of a string.perl:
An example of how to trim a string without installing a library:
$s = " lorem "; $s =~ s/^\s*(.*)\s*$/\1/;
[/code]
The return value of the =~ operator is boolean, indicating whether a match occurred. Also the left hand
side of the =~operator must be a scalar variable that can be modified. Using the =~ operator
is necessarily imperative, unlike theText::Trim functions which can be used in expressions.
pad
How to pad the edge of a string with spaces so that it is a prescribed length.
number to string
How to convert numeric data to string data.
string to number
How to convert string data to numeric data.php:
PHP converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.
perl:
Perl converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.
python:
float and int raise an error if called on a string and any part of the string is not numeric.
ruby:
to_i and to_f always succeed on a string, returning the numeric value of the digits at the start of the string, or zero if there are no initial digits.
join
How to concatenate the elements of an array into a string with a separator.
split
How to split a string containing a separator into an array of substrings.See also scan.
python:
str.split() takes simple strings as delimiters; use re.split() to
split on a regular expression:
re.split('\s+', 'do re mi fa')
re.split('\s+', 'do re mi fa', 1)[/code]
split in two
How to split a string in two.python:
Methods for splitting a string into three parts using the first or last occurrence of a substring:
'do re mi'.partition(' ') # returns ('do', ' ', 're mi')
'do re mi'.rpartition(' ') # returns ('do re', ' ', 'mi')[/code]
split and keep delimiters
How to split a string with the delimiters preserved as separate elements.
length
How to get the length in characters of a string.
index of substring
How to find the index of the leftmost occurrence of a substring in a string; how to find the index of the rightmost occurrence.
extract substring
How to extract a substring from a string by index.
extract character
How to extract a character from a string by its index.
chr and ord
Converting characters to ASCII codes and back.The languages in this reference sheet do not have character literals, so characters are represented by strings of length one.
to array of characters
How to split a string into an array of single character strings.
translate characters
How to apply a character mapping to a string.
delete characters
How to remove all specified characters from a string; how to remove all but the specified characters from a string.
squeeze characters
How to replace multiple adjacent occurrences of a character with a single occurrence.
Regular Expressions
PHP PCRE Regexesperlre and perlreref
Python re library: 2.7, 3.1
Ruby Regexp
Regular expressions or regexes are a way of specifying sets of strings. If a string belongs to the set, the string and regex "match". Regexes can also be used to parse strings.
The modern notation for regexes was introduced by Unix command line tools in the 1970s. POSIX standardized the notation into two types: extended regexes and the more archaic basic regexes. Perl regexes are extended regexes augmented by
new character class abbreviations and a few other features introduced by the Perl interpreter in the 1990s. All the languages in this sheet use Perl regexes.
Any string that doesn't contain regex metacharacters is a regex which matches itself. The regex metacharacters are: [
] . | ( ) * + ? { } ^ $ \
character classes: [ ] .
A character class is a set of characters in brackets: [ ]. When used in a regex it matches any character
it contains.
Character classes have their own set of metacharacters: ^ - \ ]
The ^ is only special when it is the first character in the character class. Such a character class matches
its complement; that is, any character not inside the brackets. When not the first character the ^ refers to itself.
The hyphen is used to specify character ranges: e.g. 0-9 or A-Z.
When the hyphen is first or last inside the brackets it matches itself.
The backslash can be used to escape the above characters or the terminal character class delimiter: ].
It can be used in character class abbreviations or string backslash escapes.
The period . is a character class abbreviation which matches any character except for newline. In all
languages the period can be made to match all characters. PHP and Perl use the m modifier. Python uses the re.M flag.
Ruby uses the s modifier.
character class abbreviations:
| abbrev | name | character class |
|---|---|---|
| \d | digit | [0-9] |
| \D | nondigit | [^0-9] |
| \h | PHP, Perl: horizontal whitespace character Ruby: hex digit | PHP, Perl: [ \t] Ruby: [0-9a-fA-F] |
| \H | PHP, Perl: not a horizontal whitespace character Ruby: not a hex digit | PHP, Perl: [^ \t] Ruby: [^0-9a-fA-F] |
| \s | whitespace character | [ \t\r\n\f] |
| \S | non whitespace character | [^ \t\r\n\f] |
| \v | vertical whitespace character | [\r\n\f] |
| \V | not a vertical whitespace character | [^\r\n\f] |
| \w | word character | [A-Za-z0-9_] |
| \W | non word character | [^A-Za-z0-9_] |
The vertical pipe | is used for alternation and parens () for grouping.
A vertical pipe takes as its arguments everything up to the next vertical pipe, enclosing paren, or end of string.
Parentheses control the scope of alternation and the quantifiers described below. They are also used for capturing groups, which are the substrings which matched parenthesized parts of the regular expression. Each language numbers the
groups and provides a mechanism for extracting them when a match is made. A parenthesized subexpression can be removed from the groups with this syntax: (?:expr)
quantifiers: * + ? { }
As an argument quantifiers take the preceding regular character, character class, or group. The argument can itself be quantified, so that ^a{4}*$ matches
strings with the letter a in multiples of 4.
| quantifier | # of occurrences of argument matched |
|---|---|
| * | zero or more, greedy |
| + | one or more, greedy |
| ? | zero or one, greedy |
| {m,n} | m to n, greedy |
| {n} | exactly n |
| {m,} | m or more, greedy |
| {,n} | zero to n, greedy |
| *? | zero or more, lazy |
| +? | one or more, lazy |
| {m,n}? | m to n, lazy |
| {m,}? | m or more, lazy |
| {,n}? | zero to n, lazy |
anchors: ^ $
| anchor | matches |
|---|---|
| ^ | beginning of a string. In Ruby or when m modifier is used also matches right side of a newline |
| $ | end of a string. In Ruby or when m modifier is used also matches left side of a newline |
| \A | beginning of the string |
| \b | word boundary. In between a \w and a \W character or in between a \w character and the edge of the string |
| \B | not a word boundary. In between two \w characters or two \W characters |
| \z | end of the string |
| \Z | end of the string unless it is a newline, in which case it matches the left side of the terminal newline |
To match a metacharacter, put a backslash in front of it. To match a backslash use two backslashes.
php:
PHP 5.3 still supports the EREG engine, though the functions which use it are deprecated. These include the splitfunction
and functions which start with ereg. The preferred functions are preg_split and
the other functions with apreg prefix.
literal, custom delimited literal
The literal for a regular expression; the literal for a regular expression with a custom delimiter.php:
PHP regex literals are strings. The first character is the delimiter and it must also be the last character. If the start delimiter is (, {, or [ the end delimiter must be ), }, or ], respectively.
Here are the signatures from the PHP manual for the preg functions used in this sheet:
array preg_split ( string $pattern , string $subject [, int $limit = -1 [, int $flags = 0 ]] ) int preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] ) mixed preg_replace ( mixed $pattern , mixed $replacement , mixed $subject [, int $limit = -1 [, int &$count ]] ) int preg_match_all ( string $pattern , string $subject [, array &$matches [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )
[/code]
python:
Python does not have a regex literal, but the re.compile function can be used to create regex objects.
Compiling regexes can always be avoided:
re.compile('\d{4}').search('1999')
re.search('\d{4}', '1999')
re.compile('foo').sub('bar', 'foo bar')
re.sub('foo', 'bar', 'foo bar')
re.compile('\w+').findall('do re me')
re.findall('\w+', 'do re me')[/code]
character class abbreviations
The supported character class abbreviations.Note that \h refers to horizontal whitespace (i.e. a space or tab) in PHP and Perl and a hex digit in
Ruby. Similarly \Hrefers to something that isn't horizontal whitespace in PHP and Perl and isn't a hex digit in Ruby.
anchors
The supported anchors.
match test
How to test whether a string matches a regular expression.python:
The re.match function returns true only if the regular expression matches the beginning of the string. re.searchreturns
true if the regular expression matches any substring of the of string.
ruby:
match is a method of both Regexp and String so
can match with both
/1999/.match("1999")[/code]
and
"1999".match(/1999/)
[/code]
When variables are involved it is safer to invoke the Regexp method because string variables are more
likely to containnil.
case insensitive match test
How to perform a case insensitive match test.
modifiers
Modifiers that can be used to adjust the behavior of a regular expression.The lists are not comprehensive. For all languages except Ruby there are additional modifiers.
| modifier | behavior |
|---|---|
| e | PHP: when used with preg_replace, the replacement string, after backreferences are substituted, is eval'ed as PHP code and the result is used as the replacement. |
| i, re.I | all: ignores case. Upper case letters match lower case letters and vice versa. |
| m, re.M | PHP, Perl, Python: makes the ^ and $ match the right and left edge of newlines in addition to the beginning and end of the string. Ruby: makes the period . match newline characters. |
| o | Ruby: performs variable interpolation #{ } only once per execution of the program. |
| p | Perl: sets ${^MATCH} ${^PREMATCH} and ${^POSTMATCH} |
| s, re.S | PHP, Perl, Python: makes the period . match newline characters. |
| x, re.X | all: ignores whitespace in the regex which permits it to be used for formatting. |
substitution
How to replace all occurrences of a matching pattern in a string with the provided substitution string.php:
The number of occurrences replaced can be controlled with a 4th argument to preg_replace:
$s = "foo bar bar";
preg_replace('/bar/', "baz", $s, 1);[/code]
If no 4th argument is provided, all occurrences are replaced.
perl:
The =~ operator performs the substitution in place on the string and returns the number of substitutions
performed.
The g modifiers specifies that all occurrences should be replaced. If omitted, only the first occurrence
is replaced.
python:
The 3rd argument to sub controls the number of occurrences which are replaced.
s = 'foo bar bar'
re.compile('bar').sub('baz', s, 1)[/code]
If there is no 3rd argument, all occurrences are replaced.
ruby:
The gsub operator returns a copy of the string with the substitution made, if any. The gsub! performs the substitution on the original string and returns the modified string.
The sub and sub! operators only replace the first occurrence of the match pattern.
match, prematch, postmatch
How to get the substring that matched the regular expression, as well as the part of the string before and after the matching substring.perl:
The special variables $&, $`,
and $' also contain the match, prematch, and postmatch.
ruby:
The special variables $&, $`,
and $' also contain the match, prematch, and postmatch.
group capture
How to get the substrings which matched the parenthesized parts of a regular expression.ruby:
Ruby has syntax for extracting a group from a match in a single expression. The following evaluates to "1999":
"1999-07-08"[/(\d{4})-(\d{2})-(\d{2})/, 1][/code]
named group capture
How to get the substrings which matched the parenthesized parts of a regular expression and put them into a dictionary.For reference, we call the (?P<foo>...) notation Python-style and
the (?<foo>...) notation Perl-style.
perl:
Perl 5.10 added support for both Python-style and Perl-style named groups.
php:
PHP originally supported Python-style named groups since that was the style that was added to the PCRE regex engine. Perl-style named groups were added to PHP 5.2.
python:
The Python interpreter was the first to support named groups.
scan
How to return all non-overlapping substrings which match a regular expression as an array.
backreference in match and substitution
How to use backreferences in a regex; how to use backreferences in the replacement string of substitution.
recursive regex
Examples of recursive regexes.The examples match substrings containing balanced parens.
Date and Time
In ISO 8601 terminology, a date specifies a day in the Gregorian calendar and a time does not contain date information; it merely specifies a time of day. A data type which combines both date and time information isprobably more useful than one which contains just date information or just time information; it is unfortunate that ISO 8601 doesn't provide a name for this entity. The word timestamp often gets used to denote a combined date and time. PHP and Python
use the compound noun datetime for combined date and time values.
An useful property of ISO 8601 dates, times, and date/time combinations is that they are correctly ordered by a lexical sort on their string
representations. This is because they are big-endian (the year is the leftmost element) and they used fixed-length fields for each term in the string representation.
The C standard library provides two methods for representing dates. The first is the UNIX epoch, which is the seconds since January 1, 1970 in UTC. If such a time were stored in a 32-bit signed integer, the rollover would happen on January
18, 2038.
The other method of representing dates is the tm struct, a definition of which can be found on Unix systems
in/usr/include/time.h:
struct tm {
int tm_sec; /* seconds after the minute [0-60] */
int tm_min; /* minutes after the hour [0-59] */
int tm_hour; /* hours since midnight [0-23] */
int tm_mday; /* day of the month [1-31] */
int tm_mon; /* months since January [0-11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday [0-6] */
int tm_yday; /* days since January 1 [0-365] */
int tm_isdst; /* Daylight Savings Time flag */
long tm_gmtoff; /* offset from CUT in seconds */
char *tm_zone; /* timezone abbreviation */
};[/code]
Perl and Python both use and expose the tm struct of the standard library. In the case of Perl, the first
nine values of the struct (up to the member tm_isdst) are put into an array. Python, meanwhile, has a module called time which
is a thin wrapper to the standard library functions which operate on this struct. Here is how get a tm struct in Python:
import time utc = time.gmtime(time.time()) t = time.localtime(time.time())
[/code]
The tm struct is a low level entity, and interacting with it directly should be avoided. In the case of
Python it is usually sufficient to use the datetime module instead. For Perl, one can use the Time::Piece module
to wrap the tm struct in an object.
date/time type
The data type used to hold a combined date and time.perl
Built in Perl functions work with either (1) scalars containing the Unix epoch as an integer or (2) arrays containing the first nine values of the standard C library tm struct. When use
Time::Piece is in effect functions which work with tm arrays are replaced with variant that work with the Time::Piece wrapper.
The modules Time::Local and Date::Parse can
create scalars containing the Unix epoch.
CPAN provides the DateTime module which provides objects with functionality comparable to the DateTime
objects of PHP and Python.
current date/time
How to get the combined date and time for the present moment in both local time and UTC.
to unix epoch, from unix epoch
How to convert the native date/time type to the Unix epoch which is the number of seconds since the start of January 1, 1970 UTC.python:
The Python datetime object created by now() and utcnow() has
no timezone information associated with it. Thestrftime() method assumes a receiver with no timezone information represents
a local time. Thus it is an error to callstrftime() on the return value of utcnow().
Here are two different ways to get the current Unix epoch. The second way is faster:
import calendar
import datetime
int(datetime.datetime.now().strftime('%s'))
calendar.timegm(datetime.datetime.utcnow().utctimetuple())[/code]
Replacing now() with utcnow() in
the first way, or utcnow() with now() in
the second way produces an incorrect value.
current unix epoch
How to get the current time as a Unix epoch timestamp.
strftime
How to format a date/time as a string using the format notation of the strftime function from the standard C library. This same format notation is used by the Unix date command.php:
PHP supports strftime but it also has its own time formatting system used by date, DateTime::format,
andDateTime::createFromFormat. The letters used in the PHP time formatting system are described
here.
default format example
Examples of how a date/time object appears when treated as a string such as when it is printed to standard out.The formats are in all likelihood locale dependent. The provided examples come from a machine running Mac OS X in the Pacific time zone of the USA.
php:
It is a fatal error to treat a DateTime object as a string.
strptime
How to parse a date/time using the format notation of the strptime function from the standard C library.
parse date w/o format
How to parse a date without providing a format string.
result date subtraction
The data type that results when subtraction is performed on two combined date and time values.
add time duration
How to add a time duration to a date/time.A time duration can easily be added to a date/time value when the value is a Unix epoch value.
ISO 8601 distinguishes between a time interval, which is defined by two date/time endpoints, and a duration, which is the length of a time interval and can be defined by a unit of time such as '10 minutes'. A time interval can also be
defined by date and time representing the start of the interval and a duration.
ISO 8601 defines notation for durations. This notation starts with a 'P' and uses a 'T' to separate the day and larger units from
the hour and smaller units. Observing the location relative to the 'T' is important for interpreting the letter 'M', which is used for both months and minutes.
local timezone
Do date/time values include timezone information. When a date/time value for the local time is created, how the local timezone is determined.A date/time value can represent a local time but not have any timezone information associated with it.
On Unix systems processes determine the local timezone by inspecting the file /etc/localtime.
php:
The default timezone can also be set in the php.ini file.
date.timezone = "America/Los_Angeles"
[/code]
Here is the list of timezones supported by PHP.
arbitrary timezone
How to convert a timestamp to the equivalent timestamp in an arbitrary timezone.
timezone name, offset from UTC, is daylight savings?
How to get time zone information: the name of the timezone, the offset in hours from UTC, and whether the timezone is currently in daylight savings.Timezones are often identified by three or four letter abbreviations. As can be seen from the list, many of the abbreviations
do not uniquely identify a timezone. Furthermore many of the timezones have been altered in the past. TheOlson database (aka Tz database) decomposes the world
into zones in which the local clocks have all been set to the same time since 1970 and it gives these zones unique names.
perl:
It is not necessary to create a DateTime object to get the local timezone offset:
use Time::Piece; $t = localtime(time); $offset_hrs = $t->tzoffset / 3600;
[/code]
ruby:
The Time class has a zone method
which returns the time zone abbreviation for the object. There is a tzinfo gem which can be used to create timezone objects
using the Olson database name. This can in turn be used to convert between UTC times and local times which are daylight saving aware.
microseconds
How to get the microseconds component of a combined date and time value. The SI abbreviations for milliseconds and microseconds are ms and μs,respectively. The C standard library uses the letter u as an abbreviation for micro.
Here is a struct defined in /usr/include/sys/time.h:
struct timeval {
time_t tv_sec; /* seconds since Jan. 1, 1970 */
suseconds_t tv_usec; /* and microseconds */
};[/code]
sleep
How to put the process to sleep for a specified number of seconds. In Python and Ruby the default version of sleepsupportsa fractional number of seconds.
php:
PHP provides usleep which takes an argument in microseconds:
usleep(500000);
[/code]
perl:
The Perl standard library includes a version of sleep which supports fractional seconds:
use Time::HiRes qw(sleep); sleep 0.5;
[/code]
timeout
How to cause a process to timeout if it takes too long.Techniques relying on SIGALRM only work on Unix systems.
Arrays
What the languages call their basic container types:| php | perl | python | ruby | |
|---|---|---|---|---|
| array | array | array, list | list, tuple, sequence | Array, Enumerable |
| dictionary | array | hash | dict, mapping | Hash |
PHP uses the same data structure for arrays and dictionaries.
perl:
array refers to a data type. list refers to a context.
python:
Python has the mutable list and the immutable tuple. Both are sequences. To be a sequence, a class must implement __getitem__, __setitem__, __delitem__, __len__, __contains__, __iter__, __add__, __mul__, __radd__,
and __rmul__.
ruby:
Ruby provides an Array datatype. If a class defines an each iterator and a comparison operator <=>, then it can mix in the Enumerable module.
literal
Array literal syntax.perl:
Square brackets create an array and return a reference to it:
$a = [1,2,3]
[/code]
quote words
The quote words operator, which is a literal for arrays of strings where each string contains a single word.
size
How to get the number of elements in an array.
empty test
How to test whether an array is empty.
lookup
How to access a value in an array by index.perl:
A negative index refers to the length - index element.
python:
A negative index refers to the length - index element.
>>> a = [1,2,3] >>> a[-1] 3
[/code]
ruby:
A negative index refers to to the length - index element.
update
How to update the value at an index.
out-of-bounds behavior
What happens when the value at an out-of-bounds index is referenced.
index of element
perl:Some techniques for getting the index of an array element.
slice
How to slice a subarray from an array by specifying a start index and an end index; how to slice a subarray from an array by specifying an offset index and a length index.perl:
Perl arrays can take an array of indices as the index value. The range of values selected can be discontinuous and the order of the values can be manipulated:
@nums = (1,2,3,4,5,6); @nums[(1,3,2,4)];
[/code]
python:
Slices can leave the first or last index unspecified, in which case the first or last index of the sequence is used:
>>> a=[1,2,3,4,5] >>> a[:3] [1, 2, 3]
[/code]
Python has notation for taking every nth element:
>>> a=[1,2,3,4,5] >>> a[::2] [1, 3, 5]
[/code]
The third argument in the colon-delimited slice argument can be negative, which reverses the order of the result:
>>> a = [1,2,3,4] >>> a[::-1] [4, 3, 2, 1]
[/code]
slice to end
How to slice to the end of an array.The examples take all but the first element of the array.
manipulate back
How to add and remove elements from the back or high index end of an array.These operations can be used to use the array as a stack.
manipulate front
How to add and remove elements from the front or low index end of an array.These operations can be used to use the array as a stack. They can be used with the operations that manipulate the back of the array to use the array as a queue.
concatenate
How to create an array by concatenating two arrays; how to modify an array by concatenating another array to the end of it.
replicate
How to create an array containing the same value replicated n times.
copy
How to make an address copy, a shallow copy, and a deep copy of an array.After an address copy is made, modifications to the copy also modify the original array.
After a shallow copy is made, the addition, removal, or replacement of elements in the copy does not modify of the original array. However, if elements in the copy are modified, those elements are also modified in the original array.
A deep copy is a recursive copy. The original array is copied and a deep copy is performed on all elements of the array. No change to the contents of the copy will modify the contents of the original array.
perl:
Taking a reference is customary way to make an address copy in Perl, but the Perl example is not equivalent to the other languages in that different syntax has to be used to access the original array and the address copy: @a and @$a1.
To make @a1 and @a refer to the same array, use typeglobs:
*a1 = *a;
[/code]
python:
The slice operator can be used to make a shallow copy:
a2 = a[:]
[/code]
list(v) always returns a list, but v[:] returns
a value of the same as v. The slice operator can be used in this manner on strings and tuples but there is little incentive
to do so since both are immutable.
copy.copy can be used to make a shallow copy on types that don't support the slice operator such as a
dictionary. Like the slice operator copy.copy returns a value with the same type as the argument.
arrays as function arguments
How arrays are passed as arguments.
iterate over elements
How to iterate through the elements of an array.perl:
for and foreach are
synonyms. Some use for exclusively for C-style for loops and foreach for
array iteration.
iterate over indices and elements
How to iterate through the elements of an array while keeping track of the index of each element.
iterate over range
Iterate over a range without instantiating it as a list.perl:
With Perl 5.005 the for and foreach operators
were optimized to not instantiate a range argument as a list.
instantiate range as array
How to convert a range to an array.Python 3 ranges and Ruby ranges implement some of the functionality of arrays without allocating space to hold all the elements.
python:
In Python 2 range() returns a list.
In Python 3 range() returns an object which implements the immutable sequence API.
ruby:
The Range class includes the Enumerable module.
reverse
How to create a reversed copy of an array, and how to reverse an array in place.python:
reversed returns an iterator which can be used in a for/in construct:
print("counting down:")
for i in reversed([1,2,3]):
print(i)[/code]
reversed can be used to create a reversed list:
a = list(reversed([1,2,3]))
[/code]
sort
How to create a sorted copy of an array, and how to sort an array in place. Also, how to set the comparison function when sorting.php:
usort sorts an array in place and accepts a comparison function as a 2nd argument:
function cmp($x, $y) {
$lx = strtolower($x);
$ly = strtolower($y);
if ( $lx < $ly ) { return -1; }
if ( $lx == $ly ) { return 0; }
return 1;
}
$a = ["b", "A", "a", "B"];
usort($a, "cmp");[/code]
python:
In Python 2 it is possible to specify a binary comparision function when calling sort:
a = [(1, 3), (2, 2), (3, 1)] a.sort(cmp=lambda a, b: -1 if a[1] < b[1] else 1) # a now contains: [(3, 1), (2, 2), (1, 3)]
[/code]
In Python 3 the cmp parameter was removed. One can achieve the same effect by defining cmp method
on the class of the list element.
dedupe
How to remove extra occurrences of elements from an array.python:
Python sets support the len, in,
and for operators. It may be more efficient to work with the result of the set constructor directly rather than convert it back
to a list.
membership
How to test for membership in an array.
intersection
How to compute an intersection.python:
Python has literal notation for sets:
{1,2,3}[/code]
Use set and list to
convert lists to sets and vice versa:
a = list({1,2,3})
ensemble = set([1,2,3])[/code]
ruby:
The intersect operator & always produces an array with no duplicates.
union
ruby:The union operator | always produces an array with no duplicates.
relative complement, symmetric difference
How to compute the relative complement of two arrays or sets; how to compute the symmetric difference.ruby:
If an element is in the right argument, then it will not be in the return value even if it is contained in the left argument multiple times.
map
Create an array by applying a function to each element of a source array.ruby:
The map! method applies the function to the elements of the array in place.
collect and collect! are
synonyms for map and map!.
filter
Create an array containing the elements of a source array which match a predicate.ruby:
The in place version is select!.
reject returns the complement of select. reject! is
the in place version.
The partition method returns two arrays:
a = [1, 2, 3]
lt2, ge2 = a.partition { |n| n < 2 }[/code]
reduce
Return the result of applying a binary operator to all the elements of the array.python:
reduce is not needed to sum a list of numbers:
sum([1,2,3])
[/code]
ruby:
The code for the reduction step can be provided by name. The name can be a symbol or a string:
[1,2,3].inject(:+)
[1,2,3].inject("+")
[1,2,3].inject(0, :+)
[1,2,3].inject(0, "+")[/code]
universal and existential tests
How to test whether a condition holds for all members of an array; how to test whether a condition holds for at least one member of any array.A universal test is always true for an empty array. An existential test is always false for an empty array.
A existential test can readily be implemented with a filter. A universal test can also be implemented with a filter, but it is more work: one must set the condition of the filter to the negation of the predicate and test whether the result
is empty.
shuffle and sample
How to shuffle an array. How to extract a random sample from an array.php:
The array_rand function returns a random sample of the indices of an array. The result can easily be converted
to a random sample of array values:
$a = [1, 2, 3, 4];
$sample = [];
foreach (array_rand($a, 2) as $i) { array_push($sample, $a[$i]); }[/code]
zip
How to interleave arrays. In the case of two arrays the result is an array of pairs or an associative list.perl:
zip expects arrays as arguments, which makes it difficult to define the arrays to be zipped on the same
line as the invocation. It can be done like this:
@a = zip @{[1,2,3]}, @{['a','b','c']};[/code]
Dictionaries
literal
perl:Curly brackets create a hash and return a reference to it:
$h = { 'hello' => 5, 'goodbye' => 7 }[/code]
size
How to get the number of dictionary keys in a dictionary.
lookup
How to lookup a dictionary value using a dictionary key.perl:
Use the ampersand prefix @ to slice a Perl hash. The index is a list of keys.
%nums = ('b'=>1, 't'=>2, 'a'=>3);
@nums{('b','t')}[/code]
out-of-bounds behavior
What happens when a lookup is performed on a key that is not in a dictionary.python:
Use dict.get() to avoid handling KeyError exceptions:
d = {}
d.get('lorem') # returns None
d.get('lorem', '') # returns ''[/code]
is key present
How to check for the presence of a key in a dictionary without raising an exception. Distinguishes from the case where the key is present but mapped to null or a value which evaluates to false.
delete entry
How to remove a key/value pair from a dictionary.
from array of pairs, from even length array
How to create a dictionary from an array of pairs; how to create a dictionary from an even length array.
merge
How to merge the values of two dictionaries.In the examples, if the dictionaries d1 and d2 share
keys then the values from d2 will be used in the merged dictionary.
invert
How to turn a dictionary into its inverse. If a key 'foo' is mapped to value 'bar' by a dictionary, then its inverse will map the key 'bar' to the value 'foo'. However, if multiple keys are mapped to the same value in the original dictionary,then some of the keys will be discarded in the inverse.
iteration
How to iterate through the key/value pairs in a dictionary.python:
In Python 2.7 dict.items() returns a list of pairs and dict.iteritems() returns
an iterator on the list of pairs.
In Python 3 dict.items() returns an iterator and dict.iteritems() has
been removed.
keys and values as arrays
How to convert the keys of a dictionary to an array; how to convert the values of a dictionary to an array.python:
In Python 3 dict.keys() and dict.values() return
read-only views into the dict. The following code illustrates the change in behavior:
d = {}
keys = d.keys()
d['foo'] = 'bar'
if 'foo' in keys:
print('running Python 3')
else:
print('running Python 2')[/code]
sort by values
How to sort the data in a dictionary by its values.
default value, computed value
How to create a dictionary with a default value for missing keys; how to compute and store the value on lookup.php:
Extend ArrayObject to compute values on lookup:
class Factorial extends ArrayObject {
public function offsetExists($i) {
return true;
}
public function offsetGet($i) {
if(!parent::offsetExists($i)) {
if ( $i < 2 ) {
parent::offsetSet($i, 1);
}
else {
$n = $this->offsetGet($i-1);
parent::offsetSet($i, $i*$n);
}
}
return parent::offsetGet($i);
}
}
$factorial = new Factorial();[/code]
perl:
How to use a tied hash. If the CPAN module Tie::ExtraHash is installed there is a
shorter way.
Functions
Python has both functions and methods. Ruby only has methods: functions defined at the top level are in fact methods on a special main object. Perl subroutines can be invoked with a function syntax or a method syntax.
define function
How to define a function.perl:
One can also use shift to put the arguments into local variables:
sub add {
my $a = shift;
my $b = shift;
$a + $b;
}[/code]
invoke function
How to invoke a function.python:
Parens are mandatory, even for functions which take no arguments. Omitting the parens returns the function or method as an object. Whitespace can occur between the function name and the following left paren.
In Python 3 print is a function instead of a keyword; parens are mandatory around the argument.
ruby:
Ruby parens are optional. Leaving out the parens results in ambiguity when function invocations are nested. The interpreter resolves the ambiguity by assigning as many arguments as possible to the innermost function invocation, regardless
of its actual arity. It is mandatory that the left paren not be separated from the method name by whitespace.
apply function to array
How to apply a function to an array.perl:
Perl passes the elements of arrays as individual arguments. In the following invocation, the function foo() does
not know which arguments came from which array. For that matter it does not know how many arrays were used in the invocation:
foo(@a, @b);
[/code]
If the elements must be kept in their respective arrays the arrays must be passed by reference:
sub foo {
my @a = @{$_[0]};
my @b = @{$_[1]};
}
foo(\@a, \@b);[/code]
When hashes are used as arguments, each key and value becomes its own argument.
missing argument behavior
How incorrect number of arguments upon invocation are handled.perl:
Perl collects all arguments into the @_ array, and subroutines normally don't declare the number of arguments they expect. However, this can be done with prototypes.
Prototypes also provide a method for taking an array from the caller and giving a reference to the array to the callee.
default argument
How to declare a default value for an argument.
variable number of arguments
How to write a function which accepts a variable number of argument.python:
This function accepts one or more arguments. Invoking it without any arguments raises a TypeError:
def poker(dealer, *players): ...
[/code]
ruby:
This function accepts one or more arguments. Invoking it without any arguments raises an ArgumentError:
def poker(dealer, *players) ... end
[/code]
named parameters
How to write a function which uses named parameters and how to invoke it.python:
The caller can use named parameter syntax at the point of invocation even if the function was defined using positional parameters.
The splat operator * collects the remaining arguments into a list. In a function invocation, the splat can be used to expand an array into separate arguments.
The double splat operator ** collects named parameters into a dictionary. In a function invocation, the double splat expands a dictionary into named parameters.
A double splat operator can be used to force the caller to use named parameter syntax. This method has the disadvantage that spelling errors in the parameter name are not caught:
def fequal(x, y, **kwargs):
eps = opts.get('eps') or 0.01
return abs(x - y) < eps[/code]
In Python 3 named parameters can be made mandatory:
def fequal(x, y, *, eps): return abs(x-y) < eps fequal(1.0, 1.001, eps=0.01) # True fequal(1.0, 1.001) # raises TypeError
[/code]
pass number or string by reference
How to pass numbers or strings by reference.The three common methods of parameter passing are pass by value, pass by reference, and pass by address. Pass by value is the default in most languages.
When a parameter is passed by reference, the callee can changed the value in the variable that was provided as a parameter, and the caller will see the new value when the callee returns. When the parameter is passed by value the callee
cannot do this.
When a language has mutable data types it can be unclear whether the language is using pass by value or pass by reference.
perl:
Here is a potential for confusion: if a reference is used in Perl to pass data, that is pass by address, not pass by reference. A Perl reference is comparable to a pointer in C, albeit one that knows the data type of
what it points to at runtime. C++ has both pointers and references and thus can pass data by address or by reference, though pass by value is the default.
pass array or dictionary by reference
How to pass an array or dictionary without making a copy of it.perl:
Arrays and hashes are not passed by reference by default. If an array is provided as a argument, each element of the array will be assigned to a parameter. A change to the parameter will change the corresponding value in the original
array, but the number of elements in the array cannot be increased. To write a function which changes the size of the array the array must be passed by reference using the backslash notation.
When a hash is provided as a argument each key of the has will be assigned to a parameter and each value of the hash will be assigned to a parameter. In other words the number of parameters seen by the body of the function will be twice
the size of the hash. Each value parameter will immediately follow its key parameter.
return value
How the return value of a function is determined.
multiple return values
How to return multiple values from a function.
lambda declaration and invocation
How to define and invoke a lambda function.python:
Python lambdas cannot contain newlines or semicolons, and thus are limited to a single statement or expression. Unlike named functions, the value of the last statement or expression is returned, and a return is not necessary
or permitted. Lambdas are closures and can refer to local variables in scope, even if they are returned from that scope.
If a closure function is needed that contains more than one statement, use a nested function:
def make_nest(x): b = 37 def nest(y): c = x*y c *= b return c return nest n = make_nest(12*2) print(n(23))
[/code]
Python closures are read only.
A nested function can be returned and hence be invoked outside of its containing function, but it is not visible by its name outside of its containing function.
ruby:
The following lambda and Proc object behave identically:
sqr = lambda { |x| x * x }
sqr = Proc.new {|x| x * x }[/code]
With respect to control words, Proc objects behave like blocks and lambdas like functions. In particular, when the body of a Proc object contains a return or break statement,
it acts like a return or break in
the code which invoked the Proc object. A return in a lambda merely causes the lambda to exit, and a break inside
a lambda must be inside an appropriate control structure contained with the lambda body.
Ruby are alternate syntax for defining lambdas and invoking them:
sqr = ->(x) {x*x}
sqr.(2)[/code]
function as value
How to store a function in a variable and pass it as an argument.php:
If a variable containing a string is used like a function then PHP will look for a function with the name in the string and attempt to invoke it.
python:
Python function are stored in variables by default. As a result a function and a variable with the same name cannot share the same scope. This is also the reason parens are mandatory when invoking Python functions.
function with private state
How to create a function with private state which persists between function invocations.python:
Here is a technique for creating private state which exploits the fact that the expression for a default value is evaluated only once:
def counter(_state=[0]): _state[0] += 1 return _state[0] print(counter())
[/code]
closure
How to create a first class function with access to the local variables of the local scope in which it was created.python:
Python 2 has limited closures: access to local variables in the containing scope is read only and the bodies of anonymous functions must consist of a single expression.
Python 3 permits write access to local variables outside the immediate scope when declared with nonlocal.
generator
How to create a function which can yield a value back to its caller and suspend execution.perl:
CPAN provides a module called Coro which implements coroutines. Some notes
on the distinction between coroutines and generators.
python:
A Python generator is a function which returns an iterator.
An iterator is an object with two methods: iter(),
which returns the iterator itself, and next(), which returns the next item or raises a StopIteration exception.
Python sequences, of which lists are an example, define an iter() for
returned an iterator which traverses the sequence.
Python iterators can be used in for/in statements and list comprehensions.
In the table below, p and q are
variables for iterators.
| itertools | |
|---|---|
| generator | description |
| count(start=0, step=1) | arithmetic sequence of integers |
| cyle(p) | cycle over p endlessly |
| repeat(v, ) | return v n times, or endlessly |
| chain(p, q) | p followed by q |
| compress(p, q) | p if q |
| groupby(p, func) | |
| ifilter(pred, p) | p if pred(p) |
| ifilterfalse(pred, p) | p if not pred(p) |
| islice(p, [start], stop, [step]) | |
| imap | |
| starmap() | |
| tee() | |
| takewhile() | |
| izip() | |
| izip_longest() | |
| product() | |
| permutations() | |
| combinations() | |
| combinations_with_replacement() |
Ruby generators are called fibers.
decorator
A decorator replaces an invocation of one function with another in a way that that is imperceptible to the client.Normally a decorator will add a small amount of functionality to the original function which it invokes. A decorator can modify the arguments before passing them to the original function or modify the return value before returning it
to the client. Or it can leave the arguments and return value unmodified but perform a side effect such as logging the call.
operator as function
How to call an operator using the function invocation syntax.This can be useful when dealing with an API which accepts a function as an argument.
python:
The operator module provides functions which perform the same operations as the various operators. Using
these functions is more efficient than wrapping the operators in lambdas.
ruby:
All operators can be invoked with method invocation syntax. The binary operator invocation syntax can be regarded as syntactic sugar.
Execution Control
if
The if statement.php:
PHP has the following alternate syntax for if statements:
if ($n == 0): echo "no hits\n"; elseif ($n == 1): echo "one hit\n"; else: echo "$n hits\n"; endif;
[/code]
perl:
When an if block is the last statement executed in a subroutine, the return value is the value of the
branch that executed.
ruby:
If an if statement is the last statement executed in a function, the return value is the value of the
branch that executed.
Ruby if statements are expressions. They can be used on the right hand side of assignments:
m = if n 1 else 0 end
[/code]
switch
The switch statement.
while
php:PHP provides a do-while loop. The body of such a loop is guaranteed to execute at least once.
$i = 0;
do {
echo $i;
} while ($i > 0);[/code]
perl:
Perl provides until, do-while,
and do-until loops.
An until or a do-until loop
can be replaced by a while or a do-while loop
by negating the condition.
ruby:
Ruby provides a loop with no exit condition:
def yes(expletive="y") loop do puts expletive end end
[/code]
Ruby also provides the until loop.
Ruby loops can be used in expression contexts but they always evaluate to nil.
c-style for
How to write a C-style for loop.
break, continue, redo
break exits a for or while loop immediately. continue goes to the next iteration of the loop. redo goes back to the beginning of the current iteration.
control structure keywords
A list of control structure keywords. The loop control keywords from the previous line are excluded.The list summarizes the available control structures. It excludes the keywords for exception handling, loading libraries, and returning from functions.
what do does
How the do keyword is used.perl:
The do keyword can convert an if statement
to a conditional expression:
my $m = do {
if ($n) { 1 }
else { 0 }
};[/code]
statement modifiers
Clauses added to the end of a statement to control execution.Perl and Ruby have conditional statement modifiers. Ruby also has looping statement modifiers.
ruby:
Ruby has the looping statement modifiers while and until:
i = 0 i += 1 while i < 10 j = 10 j -= 1 until j < 0
[/code]
Exceptions
raise exception
How to raise exceptions.ruby:
Ruby has a throw keyword in addition to raise. throw can have a symbol as an argument, and will not convert a string to a RuntimeError exception.
re-raise exception
How to re-raise an exception preserving the original stack trace.python:
If the exception is assigned to a variable in the except clause and the variable is used as the argument
to raise, then a new stack trace is created.
ruby:
If the exception is assigned to a variable in the rescue clause and the variable is used as the argument
to raise, then the original stack trace is preserved.
catch exception
How to catch exceptions.php:
PHP code must specify a variable name for the caught exception. Exception is the top of the exception hierarchy and will catch all exceptions.
Internal PHP functions usually do not throw exceptions. They can be converted to exceptions with this signal handler:
function exception_error_handler($errno, $errstr, $errfile, $errline ) {
throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
}
set_error_handler("exception_error_handler");[/code]
ruby:
A rescue Exception clause will catch any exception. A rescue clause with no exception type specified will catch exceptions that are subclasses of StandardError. Exceptions outside StandardError are
usually unrecoverable and hence not handled in code.
In a rescue clause, the retry keyword will cause the begin clause to be re-executed.
In addition to begin and rescue, ruby has catch:
catch (:done) do loop do retval = work throw :done if retval < 10 end end
[/code]
global variable for last exception
The global variable name for the last exception raised.
define exception
How to define a new variable class.
catch exception by type
How to catch exceptions of a specific type and assign the exception a name.php:
PHP exceptions when caught must always be assigned a variable name.
finally/ensure
Clauses that are guaranteed to be executed even if an exception is thrown or caught.
Concurrency
start thread
ruby:Ruby MRI threads are operating system threads, but a global interpreter lock prevents more than one thread from executing Ruby code at a time.
wait on thread
How to make a thread wait for another thread to finish.
Perl
perldoccore modules
The first character of a Perl variable $ @ % determines the type of value that can be stored in the variable:
scalar, array, hash. Using an array variable @foo in a scalar context yields the size of the array, and assigning a scalar to
an array will modify the array to contain a single element. $foo[0] accesses the first element of the array @foo,
and$bar{'hello'} accesses the value stored under 'hello' in
the hash %bar. $#foo is
the index of the last element in the array @foo.
Scalars can store a string, integer, or float. If an operator is invoked on a scalar which contains an incorrect data type, perl will perform an implicit conversion to the correct data type: non-numeric strings evaluate to zero.
Scalars can also contain a reference to a variable, which can be created with a backslash: $baz = \@foo; The
original value can be dereferenced with the correct prefix: @$baz. References are how perl creates complex data structures,
such as arrays of hashes and arrays of arrays. If $baz contains a reference to an array, then $baz->[0] is
the first element of the array. if $baz contains a reference to a hash, $baz->{'hello'} is
the value indexed by'hello'.
The literals for arrays and hashes are parens with comma separated elements. Hash literals must contain an even number of elements, and the => operator
can be used in placed of a comma between a key and its value. Square brackets, e.g. [1, 2, 3], create an array and return a
reference to it, and curly brackets, e.g. {'hello' => 5, 'bye' => 3} , create a hash and return a reference to it.
By default Perl variables are global. They can be made local to the containing block with the my keyword
or the localkeyword. my gives
lexical scope, and local gives dynamic scope. Also by default, the perl interpreter creates a variable whenever it encounters
a new variable name in the code. The use strict; pragma requires that all variables be declared with my, local,
or our. The last is used to declare global variables.
Perl functions do not declare their arguments. Any arguments passed to the function are available in the @_ array,
and the shift command will operate on this array if no argument is specified. An array passed as an argument is expanded: if the array contains 10 elements, the callee will have 10 arguments in its @_ array.
A reference (passing \@foo instead of@foo)
can be used to prevent this.
Some of Perl’s special variables:
$$: pid of the perl process
$0: name of the file containing the perl script (may be a full pathname)
$@: error message from last eval or require command
$& $` $’: what last regex matched, part of the string before and after the match
$1 … $9:
what subpatterns in last regex matched
PHP
PHP ManualThe PHP interpreter is packaged in 3 different ways: (1) as a standalone executable which can be executed as a CGI script, (2) as a dynamically linked library which adheres to the SAPI of a webserver such as Apache or IIS, and (3) as
a standalone executable which can be used to run PHP scripts from the command line. The latter executable is called PHP CLI.
From the perspective of a PHP programmer, there no important differences between PHP CGI and PHP SAPI. The programmer should be aware of the following differences between PHP CGI/SAPI and PHP CLI:
PHP CGI/SAPI writes HTTP headers to standard out before any output specified by the program. PHP CLI does not.
PHP CLI sets the constants STDIN, STDOUT, and STDERR. PHP CGI/SAPI do not.
PHP CLI has no timeout. PHP CGI/SAPI will typically timeout a script after 30 seconds.
PHP CGI/SAPI add HTML markup to error messages. PHP CLI does not.
PHP CLI does not buffer output, so calling flush is never necessary. PHP CGI/SAPI buffer output.
Python
2.7: Language, Standard Library3.2: Language, Standard Library
Python uses leading whitespace to indicate block structure. It is not recommended to mix tabs and spaces in leading whitespace, but when this is done, a tab is equal to 8 spaces. The command line options '-t' and '-tt' will warn and raise
an error respectively when tabs are used inconsistently for indentation.
Regular expressions and functions for interacting with the operating system are not available by default and must be imported to be used, i.e.
import re, sys, os
[/code]
Identifiers in imported modules must be fully qualified unless imported with from/import:
from sys import path from re import *
[/code]
There are two basic sequence types: the mutable list and the immutable tuple. The literal syntax for lists uses square brackets and commas [1,2,3] and the literal syntax for tuples uses parens and commas (1,2,3).
The dictionary data type literal syntax uses curly brackets, colons, and commas { “hello”:5, “goodbye”:7 }. Python 3 adds a literal syntax for sets which uses curly brackets and commas: {1,2,3}. This notation is also available in Python
2.7. Dictionaries and sets are implemented using hash tables and as a result dictionary keys and set elements must be hashable.
All values that can be stored in a variable and passed to functions as arguments are objects in the sense that they have methods which can be invoked using the method syntax.
Attributes are settable by default. This can be changed by defining a __setattr__ method for the class. The attributes of an object are stored in the __dict__ attribute.
Methods must declare the receiver as the first argument.
Classes, methods, functions, and modules are objects. If the body of a class, method, or function definition starts with is a string, it is available available at runtime via __doc__. Code examples
in the string which are preceded with '>>>' (the python repl prompt) can be executed by doctest and compared with the output that follows.
Ruby
1.9.3 core, stdlibRuby has a type of value called a symbol. The literal syntax for a symbol is :identifier or :"arbitrary string". The methodsto_s and to_sym can
be used to convert symbols to strings and strings to symbols. Symbols can be used to pass functions or methods as arguments by name. They can be used as keys in Hash objects in place of strings, but the client must remember the type of the keys since :foo
!= "foo". Also note that converting a Hash object with symbols as keys to JSON and then back will yield a Hash object with strings as keys.
In Ruby all values that can be stored in a variable and passed to functions as arguments are objects in the sense that they have methods which can be invoked using the method syntax. Moreover classes are objects. The system provided classes
are open and as a result the user can add methods to classes such as String, Array,
or Fixnum. Ruby only permits single inheritance, but Ruby modules are mix-ins and can be used to add methods to a class via
theinclude statement.
Ruby methods can be declared private and this is enforced by the interpreter. Object attributes are private by default and attribute names have an ampersand @ prefix. The methods attr_reader, attr_writer,
and attr_accessor can be used in a class block to define a getter, setter, or both for an attribute.
When invoking a method the parens are optional. If there are two or more arguments they must still be separated by commas. If one of the arguments is an expression containing a method invocation with arguments, then the Ruby interpreter
will assign as many arguments as possible to the innermost method invocation.
Inside a Ruby method, the self keyword refers to the receiver. It is not declared when defining the method.
Ruby functions are implemented as methods on an object called main which has the special property that any methods defined on it become instance methods in the Object class
which is a base class of most Ruby objects. This makes the method available everywhere. Methods defined at the top level are also added to the main object and the Object class.
Functions which Ruby provides by default are instance methods defined the Object class or the Kernel module.
Ruby methods are not objects and cannot directly be stored in variables. It is worth emphasizing that the Python interpreter when encountering a method identifier with no parens returns the method as an object value, but the Ruby interpreter
invokes the method. As mentioned earlier, methods can be passed by name using symbols. If a method receives a symbol representing a method as an argument, it can invoke the method with the syntax :symbol.to_proc.call(args…).
Note that to_proc resolves the symbol to the method that is in scope where it is invoked.
Although passing a method or a function is a bit awkward, Ruby provides a convenient mechanism called blocks for simultaneously defining an anonymous function at the invocation of a method and providing it to the method as an
argument. The block appears immediately after the closing paren of the method invocation and uses either the { |args…|body } or do |args…| body end syntax.
The invoked method can in turn invoke the block with the yield keyword.
Ruby blocks are closures like lambda functions and can see local variables in the enclosing scope in which they were defined. The parameters of the block are local to the block. Semicolon syntax is available so that identifiers listed
after the arguments could be made local to the block even if already defined in the containing scope.
The lambda keyword or the Proc.new constructor
can be used to store an anonymous function in a variable. The function can be invoked with variable.call(). If such a function is passed to a method argument as the last argument and preceded with an ampersand, the
function will be used as the block for the method. Conversely, if the last argument in a method definition is preceded with an ampersand, any block provided to the function will be bound to the argument name as an anonymous function.
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