Python基础1：循环

while循环

基本结构

while 条件:
    循环体
else:
    代码快

简单循环

while True:
    print('1')
    print('2')
    print('3')
    print('4')

循环的停止

1.改变循坏中的条件
    flag = True
    while flag:
        print('1')
        print('2')
        print('3')
        flag = False
        print('4')
2.break：跳出循环
    while flag:
        print('1')
        print('2')
        print('3')
     break
        print('4')
3.continue，退出本次循环，继续下一次循环
    while flag:
        print('1')
        print('2')
        print('3')
     continue
        print('4')    
4.while else循环：如果循环被break打断，则不执行else
    count = 1
    while count < 5:
        print(count)
        if count == 2:
            break
        count = count + 1
    else:
        print(666)

for循环

基本结构

有限循环：
    for 变量 iterable;
        语句
也可以使用break/contiune
if else 和 while else的用法一样

示例：

一，输出1到100的奇数

1 for i in range(1,101):
2     if i % 2 == 0: 3 continue 4 else: 5 print('loop:',i)

1 for i in range(1,101):
2     if i % 2 == 1: 3 print('loop:',i)

1 for i in range(1,101,2):
2     print('loop:',i)

二，输出1到100的奇书，并且不输出50~70

1 for i in range(1,101):
2     if i % 2 == 0: 3 continue 4 elif i >=50 and i <= 70: 5 continue 6 else: 7 print('loop:',i)

1 for i in range(1,101):
2     if i > 70 or i < 50: 3 print('loop:',i)

三，模仿一个账号登陆程序，三次错误跳出

 1 user = 'CatdeXin'
 2 passwd = 'abc123'  3  4 passwd_authentication = False  5  6 for i in range(3):  7 username = input('username: ')  8 password = input('password: ')  9 10 if username == user and password == passwd: 11 print("welcome %s login..."% user) 12 passwd_authentication = True 13 break 14 else: 15 print("Invalid username or password !") 16 17 if not passwd_authentication: 18 print('Youve tried too many times')

 1 user = 'CatdeXin'
 2 passwd = 'abc123'  3  4 for i in range(3):  5 username = input('username: ')  6 password = input('password: ')  7  8 if username == user and password == passwd:  9 print("welcome %s login..."% user) 10 break #break for过后，就不会执行后面的else语句 11 else: 12 print("Invalid username or password !") 13 else: #只要上面的for循环执行完毕，中间没有被打断，就会执行else语句 14 print('Youve tried too many times')

四，双层跳出

 1 exit_flag = False
 2 
 3 for i in range(10):  4 if i < 5:  5 continue #跳出当次循环  6 print(i)  7 for j in range(10):  8 print('Tow level:',j)  9 if j == 6: 10 exit_flag = True　　#you jump 11 break 12 if exit_flag == True:　　#i jump 13 break #双层跳出

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