a. while else
b. continue break
continue ,终止当前循环,开始下一次循环
break ,终止所有循环
c.用户登陆(三次机会重试)
count = 0
while count < 3:
user = input('>>>')
pwd = input('>>>')
if user == 'alex' and pwd == '123':
print('欢迎登陆')
print('..........')
break
else:
print('用户名或者密码错误')
count = count + 1
d.