1. 循环退出

• break，跳出整个循环

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
for i in xrange(10):
    if i == 5:
        break
    print i
else :
    print "end"

对于上述程序，因为有break，后续语句不再执行，结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
0
1
2
3
4

Process finished with exit code 0

• continue，跳出当前的小循环
  把上面的代码加几行：

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
for i in xrange(10):
    if i == 3:
        continue
    if i == 5:
        break
    print i
else :
    print "end"

结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
0
1
2
4

Process finished with exit code 0

• pass占位，啥也不做

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
for i in xrange(10):
    if i == 3:
        continue
    if i == 5:
        continue
    if i == 6:
        pass
    print i,
else :
    print "end",
print "haha"

这样的话3和5都不会打印，可以看下结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
0 1 2 4 6 7 8 9 end haha

Process finished with exit code 0

• sys.exit()退出程序，使用时要先插入sys模块

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
import sys
for i in xrange(10):
    if i == 3:
        continue
    if i == 5:
        continue
    if i == 6:
        sys.exit()
    print i,
else :
    print "end",
print "haha"

结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
0 1 2 4

Process finished with exit code 0

• 猜数字游戏

系统生成一个20以内的随机整数，
玩家有6次机会进行猜猜看，每次猜测都有反馈（猜大了， 猜小了，猜对了-结束）
6次中，猜对了，玩家赢了。
否则系统赢了。

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
import random
import sys
random_num = random.randint(1, 20)
for i in xrange(1, 7):
    enter_num = int(raw_input('Please input a num between 1 and 20:  '))
    if random_num < enter_num:
        print 'the enter_num is bigger.'
    elif random_num > enter_num:
        print 'the enter_num is lower.'
    elif random_num == enter_num:
        print 'you are win'
        sys.exit()
print 'game over'

结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171228/猜数字.py
The random have been created
Please input a num between 1 and 20:  5
the enter_num is lower.
Please input a num between 1 and 20:  6
the enter_num is lower.
Please input a num between 1 and 20:  7
the enter_num is lower.
Please input a num between 1 and 20:  8
the enter_num is lower.
Please input a num between 1 and 20:  9
the enter_num is lower.
Please input a num between 1 and 20:  10
the enter_num is lower.
game over

Process finished with exit code 0

2. 流程控制--while

• while与for相比
  • for循环用在有次数的循环上。
  • while循环用在有条件的控制上。
• whle循环，直到表达式变为假，才退出while循环，表达式是一个逻辑表达式，必须返回一个True或False。
• 语法：
  while expression:
  statement(s)
• while很容易写成死循环，造成负载过大
  比如下面的脚本：

#!/usr/bin/python
while 1:
    print "hello"

如果执行，计算机会不停输出hello。

• 修改版本1：

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
n = 0
while 1:
    if n == 5:
         break
    print n,"hello"
    n += 1

输出结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
0 hello
1 hello
2 hello
3 hello
4 hello

Process finished with exit code 0

• 修改版本2：等待键盘输入

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
while 1:
    print "hello"
    if raw_input("please input sth,q for quit:  ") == "q":
        break

执行结果

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
hello
please input sth,q for quit:  a
hello
please input sth,q for quit:  q

Process finished with exit code 0

• 修改版本3：while条件是false，退出循环

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
n = "" #n最开始为空
while n != "q" :
    print "hello"
    n = raw_input("please input sth,q for quit:  ")

结果：

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
hello
please input sth,q for quit:  fgg
hello
please input sth,q for quit:  q

Process finished with exit code 0

• while里的else，类比其他循环

#!/usr/bin/python
# -*- coding:utf8 -*-
# author: chawn
# date:
n = ""
while n != "q" :  #第一次n为空，所以继续下面的语句
    print "hello"
    n = raw_input("please input sth,q for quit:  ")
    if n == "q":  #如果输入q，跳出while循环，执行else语句
        continue
else:
    print "world"

结果:

C:\Users\chawn\PycharmProjects\pyex\venv\Scripts\python.exe C:/Users/chawn/PycharmProjects/pyex/171226/ts.py
hello
please input sth,q for quit:  w
hello
please input sth,q for quit:  q
world

Process finished with exit code 0

3. 练习

习题

1. 将一个正整数分解质因数。例如：输入90,打印出90=233*5。
   程序分析：对n进行分解质因数,应先找到一个最小的质数i,然后按下述步骤完成：
   (1)如果分解后商为1,则说明分解质因数的过程已经结束,打印出即可。
   (2)如果商不为1,则应打印出i的值,并用n除以i的商,作为新的正整数进行分解,
   　重复执行第一步。
   (3)如果n不能被i整除,则i的值加1,重复执行第一步。

2. 猴子吃桃问题：猴子第一天摘下若干个桃子,当即吃了一半,还不瘾,又多吃了一个，第二天早上又将剩下的桃子吃掉一半,又多吃了一个。以后每天早上都吃了前一天剩下的一半零一个。到第10天早上想再吃时,见只剩下一个桃子了。求第一天共摘了多少。
   程序分析：采取逆向思维的方法,从后往前推断。

1. 解答：

def fun(n):

    for i in range(2, n / 2 + 1):

        if n % i == 0:

            print(i),

            print("*"),

            return fun(n / i)

    print(n),



if __name__ == "__main__":

    i = int(input("输入数的数为: "))

    fun(i)

2. 解答

num = 1

for i in range(9, 0, -1):

    num = (num + 1) * 2

print('第一天共摘了%s个桃子' %num)