简明Python教程学习笔记_第6章 控制流

 

在Python中有三种控制流语句——if、for和while。

--------------------------------------------------------------------------------

if语句
if语句用来检验一个条件， 如果 条件为真，我们运行一块语句（称为 if-块 ）， 否则 我们处理另外一块语句（称为 else-块 ）。 else 从句是可选的。

 

 

#!/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

 

while语句
只要在一个条件为真的情况下，while语句允许你重复执行一块语句。while语句是所谓 循环 语句的一个例子。while语句有一个可选的else从句。

#!/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'

 

for循环
for..in是另外一个循环语句，它在一序列的对象上 递归 即逐一使用队列中的每个项目。我们会在后面的章节中更加详细地学习序列。

 

#!/usr/bin/python
# Filename: for.py

for i in range(1, 5):
    print i
else:
    print 'The for loop is over'

 

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。

#!/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'

 

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。

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...