Think Python, Chapter 8: Strings 笔记

目录

这是麻省理工大学（MIT）官方编程教程中Python Tutorial的内容，教材为《Think Python: How to Think Like a Computer Scientist》。这是我的学习笔记，因为水品有限，请大家多多包涵。如果有一起学习的同学可以一起交流。如笔记中错误，请一定要告诉我啊，我肯定及时改正。所有笔记的目录详见：MIT：Python Tutorial目录

这是MIT官方编程教程中Python Tutorial中Loops and List Comprehensions的内容。本篇博客为《 Think Python: How to Think Like a Computer Scientist》（Think Python, Chapter 8: Strings）

Python Tutorial：Loops and List Comprehensions

Before starting these problems, please read:

Think Python, Chapter 8: Strings

Think Python, Chapter 10: Lists

Think Python, Chapter 11: Dictionaries

Think Python, Chapter 12: Tuples

6.01 Python Notes, Section 4: Lists (PDF)

6.01 Python Notes, Section 5: Functional Style (PDF)

Chapter 8 Strings

8.1 A string is a sequence(字符串是一个序列)

object: Something a variable can refer to. For now, you can use “object” and “value” interchangeably.

sequence: An ordered set; that is, a set of values where each value is identified by an integer index.

A string is a sequence of characters. You can access the characters one at a time with the bracket operator:

#字符串是一个字母的序列,其中,第一个字符的位置是0,不是1.
>>> fruit='apple'
>>> letter=fruit[1]
>>> print(letter)
p
>>> letter=fruit[0]
>>> print(letter)
a

8.2 len(字符串的长度函数)

len is a built-in function that returns the number of characters in a string:

#len的长度不是从开始数的,故len(string)=string中最后一个字符的序列+1
>>> len(fruit)
5
>>> last=fruit[len(fruit)-1]
>>> print(last)
e
>>>

Alternatively, you can use negative indices, which count backward from the end of thestring. The expression fruit[-1] yields the last letter, fruit[-2] yields the second to last, and so on.

#但是当string
中n是负数的时候,则数列取得是倒数第几个(注意,这里不是从0开始的)
>>> fruit[-2]#倒数第二个
'l'
>>> fruit[-1]#倒数第一个
'e'

8.3 Traversal with a for loop

traversal(遍历): One way to write a traversal is with a while loop.

#8.3 traversal with while loop
def traversal_while(s):
index=0
while index<len(s):
letter=fruit[index]
print(letter)
index=index+1

#测试
>>> fruit='apple'
>>> traversal_while(fruit)
a
p
p
l
e
>>>

index: An integer value used to select an item in a sequence, such as a character in a string.

Another way to write a traversal is with a for loop:

#8.3 traversal with for loop
def traversal_for(s):
for char in fruit:#这里的char是取值的变量，应该是python自带的定义
print(char)

>>> fruit='apple'
>>> traversal_for(fruit)
a
p
p
l
e

8.4 String slices(字符串部分)

slice: A part of a string specified by a range of indices.

#string[M:N]代表取string中Mth到Nth的字符串片段.
#特别注意:开始从string[M]开始,一直取到string[N-1]
>>> s='THINK PYTHON'
>>> print(s[0:4])
THIN
>>> print(s[0:5])
THINK
>>> print(s[5:])
PYTHON
>>> print(s[:5])
THINK
>>> print(s[5:5])
''
>>> print(s[:])
THINK PYTHON

empty string: A string with no characters and length 0, represented by two quotation marks.

8.5 Strings are immutable(不可变的)

immutable: The property of a sequence whose items cannot be assigned.

>>> greeting="Hello World"
>>> greeting[0]='J'
TypeError: 'str' object does not support item assignment

The reason for the error is that strings are immutable, which means you can’t change anexisting string. The best you can do is create a new string that is a variation on the original(现有的字符串是不可变的,你最好是创建一个新的字符串):

>>> new_greeting='J'+greeting[1:]

>>> print(new_greeting)
Jello World

8.6 Searching(搜寻)

search: A pattern of traversal that stops when it finds what it is looking for.

#8.6 searching
def search_str(word,letter):
index=0
count=0
while index<len(word):
if word[index]==letter:
print(index)
count=count+1
index=index+1
print( '已搜寻完毕')
print('在',word,'中共有',count,'个',letter)

#测试
>>> search_str('THINK PYTHON','T')
0
8
已搜寻完毕
在 THINK PYTHON 中共有 2 个 T

8.7 Looping and counting(计数)

counter: A variable used to count something, usually initialized to zero and then incremented.

#8.7 Looping and counting
def count_str(word,letter):
index=0
count=0
while index<len(word):
if word[index]==letter:
count=count+1
index=index+1
return count

8.8 String methods(方法)

method: A function that is associated with an object and called using dot(点) notation.

>>> word='banana'
#upper()函数用于小写转大写
>>> new_word=word.upper()
>>> print(new_word)
BANANA
#find()函数的使用
>>> index=word.find('a')
>>> print(index)
1
#查找字符串
>>> word.find('na')
2
#从第N个开始查找
>>> word.find('na',3)
4
#在某个范围内查找,注意与s[M,N]一样,截止到从s[M]到s[N-1]
>>> word.find('n',3,4)
-1

8.9 The in operator

用’in’来判断某字符串或者字符是否在字符串内.

>>> 'a' in 'banana'
True
>>> 'seed'in 'banana'
False

例:寻找字符串word1和word2中共有的字母

#8.9 the in operator
def in_both(word1,word2):
for letter in word1:#遍历word1，赋值到letter
if letter in word2:#判断letter是否在word2里有
print(letter)

#测试
>>> in_both('think','python')
t
h
n

8.10 String comparison(字符串比较)

Python does not handle uppercase and lowercase letters the same way that people do. All the uppercase letters come before all the lowercase letters, so:

‘Banana’>’banana’

A common way to address this problem is to convert strings to a standard format, such as all lowercase, before performing the comparison. Keep that in mind in case you have to defend yourself against a man armed with a Pineapple.(一个统一的解决方法是在比较前将所有的string都转换成有一个形式,要么都是uppercase大写的,要么都是lowercase小写的,用upper()和lower())

8.13 Exercises

Exercise 8.10.

string[M,N,T]表示取在M-N的范围中,从string[M]开始,每T个字符(每隔T-1个字符)取第一个字符

>>> fruit='banana'
>>> fruit[0:5:2]
'bnn'

#神奇的倒着取.注意,-1代表在倒着的第一个中取第一个,-2代表倒着每两个中取第一个
>>> fruit[::-1]
'ananab'

>>> fruit[::-2]
'aaa'