python自学笔记1

求1-100的和

>首先定义一个索引变量
start = 1
>定义一个变量记录每次的的和
sum = 0
while True:
    print(start)
    sum = sum + start
    print(sum)
    if start == 100:
        break
    start += 1

输出1-100的内所有奇数

start = 1
while True:
    if start % 2 != 0:
        print(start)
    if start == 100:
        break
    start += 1

输出1-100的内所有偶数

start = 1
while True:
    if start % 2 == 0:
        print(start)
    if start == 100:
        break
    start += 1

求1-2+3-4+5-6.....99 所有数的和

start = 1
sum = 0
while True:
    if start % 2 == 0:
        sum = sum - start
    else:
        sum = sum + start
    print(sum，start)
    print(start)
    if start == 99:
        break
    start += 1

用户登录（三次机会重试）
i = 0
while i < 3:
    user = input("userName:")
    pwd = input("password:")
    if user == "alex" and pwd == "123":
        print("yes")
        break
    else:
        print("try again")
    i += 1