if语句

判断用户名和密码是否正确：

_username="liulu"_password="123456"

username=input("username:")password=input("password:")

if username==_username and password==_password:    print("welcome {name} login...".format(name=username))else:    print("invalid username and password!")

猜年龄：

my_age=23

guess_age=int(input("please guess my age:")) #因为input默认的是字符串，所以需要int

if guess_age==my_age:    print("you got it!")elif guess_age>my_age:    print("think smaller...")else:    print("think bigger...")

while循环猜年龄，最多猜三次，猜不中不能再猜

my_age=23

count=0

while True:        #True要大写                #可以简化为while count <3:    if count==3:        break#break 可结束运行本次循环。continue是跳出本次循环。

    guess_age=int(input("guess age:"))

    if guess_age==my_age:        print("you got it")        break    elif guess_age>my_age:        print("think smaller...")    else:        print("think bigger...")

    count+=1

else:    print("you have no more chance to guess...")

for循环猜年龄，最多猜三次，猜不中不能再猜

my_age=23

for i in range(3)

    guess_age=int(input("guess age:"))

    if guess_age==my_age:        print("you got it")        break    elif guess_age>my_age:        print("think smaller...")    else:        print("think bigger...")

else:    print("you have no more chance to guess...")

步长

# 从0到10for i in range(10):    print("loop",i)

# 只打02468for i in range(0,10,2):    print("loop",i)

if 与 while 混合使用案例：

猜年龄，最多猜三次，猜不中询问是否继续猜，输入n后不再猜。

my_age=23

count=0

while count<3:

    guess_age=int(input("guess age:"))

    if guess_age==my_age:        print("you got it")        break    elif guess_age>my_age:        print("think smaller...")    else:        print("think bigger...")

    count+=1

    if count==3:        continue_confirm=input("would you like to keep guessing?")        if continue_confirm!="n":            count=0

for循环嵌套

for i in range(10):    print("------",i)    for j in range(10):        print(j)        if j>5:            break