学习《简明Python教程》第三天

第6章 控制流

Python 中有三种控制语句--if、for、while。

if语句来检验一个条件，如果为真，执行if块，否则执行else块（elif、else从句是可选的）。

一个最简单有效的if语句是：

if True:
    print 'Yes,it is true'

 

#!/usr/bin/python
#Filename if.py

number = 23
guess = int(raw_input('Enter an integer:'))  # 用raw_input()函数取用户输入

if guess == number:  #if语句在结尾处包含一个冒号--告知Python下面跟一个语句块
    print 'Congratulations,you guessed it.'
    print "(but you do noet win any prizes!)"
elif guess < number:
    print 'No,it is a little higher than that'
else:
    print 'No,it is a little lower than that'

print 'Done'

$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 littler higher than that

Done

$python if.py

Enter an intger: 23

Congratulations,you guessed it.

(but you do not win any prizes!)

Done

 

while语句在一个条件为真的情况下，允许重复反执行一块语句，其有一个可选的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
    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.'
print 'Done'

for..in递归使用序列对象中的每个项目。

 

#!/usb/bin/python
#Filename for.py

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

break语句是强制终止循环的。

 

#!/usr/bin/python
#Filename break.py

whil True:
    s = raw_input('Enter something:')
    if s == 'quit':
        break
    print 'Length of the string is',len(s)
print 'Done'

continue语句告诉Python跳过当前循环中的剩余语句，进行下一轮循环。

 

#!/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'