8.流程控制之while循环

1.循环的语法与基本使用

while 条件：
	代码1
	代码2
	代码3

例如：输出0~4

count = 0
while count < 5:
	print(count)
	count += 1

2.死循环与效率问题

#纯计算无IO的死循环会导致致命的效率问题
while True:
	1 + 1

3.循环的应用

• 输出1~100的自然数

count = 1
flag = True
while flag:
    print(count)
    count = count +1
    if count == 101:
        flag = False

注意点：python中是没有++和--的，自加自减使用count +=1（count = count+1）

• 输出1~100的和

s = 0
count = 1
while count < 101
	s = s + count
	count = count +1
print(s)

• 输出1~100的偶数

count = 1
while count < 101:
	if count % 2 ==0:
		print(count)
	count = count + 1
	

4.循环终止

• 将条件改为False，等到下次循环判断条件时才会生效

flag = True
while flag:
	flag = False
	print('狼的诱惑')
	print('月亮之上')
	print('庐州月')

• break:循环中遇到break直接退出循环

while True:
	print('狼的诱惑')
	print('我们不一样')
	print('月亮之上')
	break
	print('庐州月')
	print('人间')

5.continue

continue：结束本次循环，进入下一次

强调：在continue之后添加同级代码毫无意义，因为永远无法运行

flag = True
while flag:
	print('111')
	print('222')
	flag = False
	continue
	print('333')		# 111  222

number = 0
while number < 10:
	number += 1
	if number % 2 == 0:
		continue
	print(number)			#13579

number = 0
while number < 10:
   number += 1
   if number % 2 == 0:
      print(number)			#246810

6.while else

while 循环如果被break打断，则不执行else语句

while True:
	...
else:
	print('else包含的代码会在while循环结束后'，
	并且while循环在没有被break打断的情况下才会运行)

count = 1
while (count < 5):
	print (count)
	if count == 2:
		break
    count = count + 1
else:
	print('666')		#1 2

7.例题：三次账号密码输入

count = 1
while count <= 3:
    username = input('用户名：')
    password = input('密码：')
    code = 'qwer'
    your_code = input('验证码：')
    if your_code == code:
        if username == ('alex') and password == ('123'):
            print('登录成功')
            break
        else:
            print('用户名或密码错误')
    else:
        print('验证码错误')
    count = count + 1

成功则跳出本次循环