Python基础1：循环

while 条件:
    循环体
else:
    代码快

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

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)

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

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)

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)

 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‘)

 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