2018.09.14python学习第四天part2

流程控制之while循环

1.什么是循环？（what）

循环是指重复做某一件事

2.为何要有循环？（why）

为了让计算机能像人一样重复去做某一件事

3.如何使用循环？（how）

#语法一：while条件

如：用户登录时信息输入错误是需要重新输入

name_bd="tony"
pwd_bd=123
while True:
    name=input("please your name: ")
    pwd=input("plesse your password: ")
    if name == name_bd and pwd == pwd_bd:
        print("login successful")
    else:
        print("please input again")

输入这段代码后，会发现无论你登录是否成功，都是无限循环登录，所以要让登录成功后就结束循环应该：

name_bd="tony"
pwd_bd="123"
a=True
while a:
    name=input("please your name: ")
    pwd=input("please your password: ")
    if name == name_bd and pwd == pwd_bd:
        print("login successful")
        a=False
    else:
        print("please input again")

#语法二：while+break（break表示强制结束本层循环）

name_bd="tony"
pwd_bd="123"
while True:
    name=input("please your name: ")
    pwd=input("plesse your password: ")
    if name == name_bd and pwd == pwd_bd:
        print("login successful")
        break
    else:
        print("please input again")

这样也实现了登陆成功是终止循环

#语法三：while+continue（continue表示停止本次循环，直接进入下一次）

count=0
while count<10:
    if count ==5:
        count+=1
        continue
    print(count)
    count+=1

#语法四：while+else（else会在while循环没有被break终止的情况下执行）

name_bd="tony"
pwd_bd="123"
count=0
while count<=2:        
        name=input("please your name: ")
        pwd=input("plesse your password: ")
        if name == name_bd and pwd == pwd_bd:
            print("login successful")
            break
        else:
            print("please input again")
            count+=1
else:
    print("输错次数太多，请稍后"）

#语法五：while循环的嵌套

name_bd="tony"
pwd_bd="123"
count=0

while count<=2:
    name=input("please your name: ")
    pwd=input("plesse your password: ")
    if name == name_bd and pwd == pwd_bd:
        print("login successful")
        while True:
                print("1.浏览商品，2.添加购物车，3.支付，4.退出")
                choice=input("请输入你的选择： ")
                if choice=="1":
                        print("正在浏览商品")
                if choice=="2":
                        print("正在添加购物车")
                if choice=="3":
                        print("正在支付")
                if choice=="4":
                        break
        break
    else:
        print("please input again")
        count+=1
else:
    print("输错次数太多，请稍后")