Python 学习笔记9 循环语句 while

While循环是哟中利用条件语句，不断的执行某一段代码块，达到批量操作输出等一系列的操作，直到条件不满足或者被强制退出为止。

其工作流程如下： （图片来源菜鸟教程：http://www.runoob.com/python/python-while-loop.html  ）

我们来看一个例子：

current_number = 10
while current_number <= 20:
    print("Current number is : " + str(current_number))
    current_number += 1

print("Final number: " + str(current_number))

'''
输出：
Current number is : 10
Current number is : 11
Current number is : 12
Current number is : 13
Current number is : 14
Current number is : 15
Current number is : 16
Current number is : 17
Current number is : 18
Current number is : 19
Current number is : 20 
Final number: 21
'''

我们可以看到 变量current_number 的初始值为10， 在小于等于20的情况下，不断的被打印，被增长，直至增长到21的时候，跳出了循环。

我们也可以使用标志，比如True,False 等布尔值来进行循环：

flag = True
while flag:
    number = int(input("Input a number less than 10:
"))
    if number < 10:
        flag = True
    else:
        flag = False

print("The game is end...")

'''
输出：
Input a number less than 10:
2
Input a number less than 10:
11
The game is end...
Final number: 21
'''

使用 break 退出循环。在while循环中，你可以通过特定的条件语句，选择终止循环，不再继续运行余下的代码。

flag = True
while flag:
    city = input("Input a city which you like: 
" + "Input 'Q' to quit...
")
    if city == 'Q':
        break
    else:
        print("You like " + city)

print("Thanks for your information.")

'''
输出：
Input a city which you like: 
Input 'Q' to quit...
ShangHai
You like ShangHai
Input a city which you like: 
Input 'Q' to quit...
HangZhou
You like HangZhou
Input a city which you like: 
Input 'Q' to quit...
Q
Thanks for your information.
'''

使用 continue 不再继续执行余下的代码块，返回到循环开头，继续下一轮循环。

number = 0
while number <= 10:
    if number % 2 == 0:
        print("The number is even: " + str(number))
        number += 1
        continue
        print("Next...")
    else:
        number += 1



'''
输出：
The number is even: 0
The number is even: 2
The number is even: 4
The number is even: 6
The number is even: 8
The number is even: 10
'''

 循环也可以和else 一起使用：

number = 0
while number <= 10:
    if number % 2 == 0:
        print("The number is even: " + str(number))
        number += 1
        continue
        print("Next...")
    else:
        number += 1
else:
    print("The number is equal or more than 10, stop loop.")


'''
输出：
The number is even: 0
The number is even: 2
The number is even: 4
The number is even: 6
The number is even: 8
The number is even: 10
The number is equal or more than 10, stop loop.
'''

使用while 操作列表 List

peoples = ['Ralf', 'Clark', 'Leon']
while peoples:
    people = peoples.pop()
    print(people)

'''
输出：
Leon
Clark
Ralf
'''

peoples = ['Ralf', 'Clark', 'Leon', 'Terry']
while 'Terry' in peoples:
    peoples.remove('Terry')
    print(peoples)

'''
输出：
['Ralf', 'Clark', 'Leon']
'''

while循环语句可以解决程序中需要重复执行的操作。其循环执行的次数由循环条件确定，当循环条件满足时，重复执行某程序段，直到循环条件不成立为止。反复执行的程序段称为循环体，循环条件必须要在循环体中改变，否则可能会出现无限循环的结果