学习python笔记——控制流

简介

在到目前为止我们所见到的程序中，总是有一系列的语句，Python忠实地按照它们的顺序执行它们。如果你想要改变语句流的执行顺序，该怎么办呢？例如，你想要让程序做一些决定，根据不同的情况做不同的事情，例如根据时间打印“早上好”或者“晚上好”。

你可能已经猜到了，这是通过控制流语句实现的。在Python中有三种控制流语句——if、for和while。

1. 选择结构

      用if-elif-else实现：

if i < 1:
print("less than 1")
elif i < 3:
print("less than 3")
else:
print("else")
print("Done")

 

例子：if.py

#!/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

运行结果：

$ 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

需要注意的地方：

      （1）正确的缩进格式，否则运行报错

      （2）选择分支后的冒号

2. 循环结构

      循环结构可以用while或者for来实现。

      while循环：

当while循环条件变为False的时候，else块才被执行——这甚至也可能是在条件第一次被检验的时候。如果while循环有一个else从句，它将始终被执行，除非你的while循环将永远循环下去不会结束！

 

running = True
i = 1
while running:
if i == 5:
running = False
i = i + 1
else:
print("while loop done")

例子：while.py

#!/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'

运行结果：

$ 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

需要注意的地方：

      （1）正确的缩进格式，否则运行报错

      （2）while 条件后有：

for循环：

#!/usr/bin/python
# Filename: for.py
for i in range(1, 5):

print i
else:

print 'The for loop is over'

输出：

输出

$ python for.py
1
2
3
4
The for loop is over

我们所做的只是提供两个数，range返回一个序列的数。这个序列从第一个数开始到第二个数为止。例如，range(1,5)给出序列[1, 2, 3, 4]。默认地，range的步长为1。如果我们为range提供第三个数，那么它将成为步长。例如，range(1,5,2)给出[1,3]。记住，range 向上 延伸到第二个数，即它不包含第二个数。

 

break语句

break语句是用来 终止 循环语句的，即哪怕循环条件没有称为False或序列还没有被完全递归，也停止执行循环语句。

一个重要的注释是，如果你从for或while循环中 终止 ，任何对应的循环else块将不执行。

例子：break.py

#!/usr/bin/python
# Filename: break.py
while True:

s = raw_input('Enter something : ')

if s == 'quit':

break

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

输出：

$ 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

 

continue语句

continue语句被用来告诉Python跳过当前循环块中的剩余语句，然后 继续 进行下一轮循环。

使用continue语句

例子：continue.py

#!/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...

结果：

 

$ python continue.py
Enter something : a
Enter something : 12
Enter something : abc
Input is of sufficient length
Enter something : quit