python学习旅程笔记3-控制流

Python的基本运算表达式

（1）判断语句

关于if语句的规则我不再介绍，只在这里提出Python下if语句的用法，以及特点。

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#!/usr/bin/python

# Filename: if.py

number = 23

guess = int(raw_input('Enter an integer : '))

if guess == number:

print 'Congratulations, you guessed it.' # New block starts here

print "(but you do not win any prizes!)" # New block ends here

elif guess < number:

print 'No, it is a little higher than that' # Another block

# You can do whatever you want in a block ...

else:

print 'No, it is a little lower than that'

# you must have guess > number to reach here

print 'Done'

# This last statement is always executed, after the if statement is executed

以下为输出

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$ python if.py

Enter an integer : 50

No, it is a little lower than that

Done

$ python if.py

Enter an integer : 22

No, it is a little higher than that

Done

$ python if.py

Enter an integer : 23

Congratulations, you guessed it.

(but you do not win any prizes!)

Done

在这里，我们使用了一个函数raw_input（），用以获取用户的输入。

我们为内建的

raw_input

函数提供一个字符串，这个字符串被打印在屏幕上，然后等待用户的输入。一旦我们输入一些东西，然后按回车键之后，函数返回输入。对于

raw_input

函数来说是一个字符串。我们通过

int

把这个字符串转换为整数，并把它存储在变量

guess

中。事实上，

int

是一个类，不过你对它所需了解的只是它把一个字符串转换为一个整数。

注意：我们使用了缩进层次来告诉Python每个语句分别属于哪一个块。这就是为什么缩进在Python如此重要的原因。

在Python中没有

switch

语句。不过你可以使用

if..elif..else

语句来完成同样的工作。

（2）循环语句

1.while语句

Python的while语句，与其他语言的while语句区别不大。

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#!/usr/bin/python

# Filename: while.py

number = 23

running = True

while running:

guess = int(raw_input('Enter an integer : '))

if guess == number:

print 'Congratulations, you guessed it.'

running = False # this causes the while loop to stop

elif guess < number:

print 'No, it is a little higher than that'

else:

print 'No, it is a little lower than that'

else:

print 'The while loop is over.'

# Do anything else you want to do here

print 'Done'

输出为：

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$ python while.py

Enter an integer : 50

No, it is a little lower than that.

Enter an integer : 22

No, it is a little higher than that.

Enter an integer : 23

Congratulations, you guessed it.

The while loop is over.

Done

注意：在Python中，你可以在

while

循环中使用一个

else

从句。

2.for语句

for语句，在这里，我通过一个例子在作出说明

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#!/usr/bin/python

# Filename: for.py

for i in range(1, 5):

print i

else:

print 'The for loop is over'

输出为：

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$ python for.py

1

2

3

4

The for loop is over

注意：

else

部分是可选的。如果包含else，它总是在

for

循环结束后执行一次，除非遇到break语句。

与C/C++语言相比，Python的if语句，无疑简单了许多。

（3）其他语句

1.break语句

break语句在for循环和while循环中使用。

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while True:

s = raw_input('Enter something : ')

if s == 'quit':

break

print 'Length of the string is', len(s)

print 'Done'

输出为

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$ python break.py

Enter something : Programming is fun

Length of the string is 18

Enter something : When the work is done

Length of the string is 21

Enter something : if you wanna make your work also fun:

Length of the string is 37

Enter something : use Python!

Length of the string is 12

Enter something : quit

Done

2.continue语句

在这里，我只通过一个例子来说明。

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#!/usr/bin/python

# Filename: continue.py

while True:

s = raw_input('Enter something : ')

if s == 'quit':

break

if len(s) < 3:

continue

print 'Input is of sufficient length'

# Do other kinds of processing here...

输出为：

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$ python continue.py

Enter something : a

Enter something : 12

Enter something : abc

Input is of sufficient length

Enter something : quit