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