第4节.if语句

4.1 if语句的简单示例

cars=['audi','bmw','subaru','toyota']
for car in cars:
    if car=='bmw':
        print(car.upper())
    else:
        print(car.title())

4.2 if语句

if-else语句

age=17
if age>=18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
else:
    print("Sorry,you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")

if-elif-else结构

用于检查超过两个的情形

'''
我们以一个根据年龄收费的游乐场为例：
4岁以下免费
4~18岁收费25美元
18（含）岁以上收费40美元
'''
age=12
if age <4:
    print("Your admission cost is $0")
elif age < 18:                #只会执行通过测试的代码
    print("Your admission cost is $25")
else:
    print("Your admission cost is $40")
#现在我们将上面的代码进行简化
age =12
if age <4:
    price=0
elif age <18:
    price=25
else:
    price=40
print(f"Your admission cost is ${price}.")
#else代码块可以省略，只要不满足if或elif的测试条件，else中的代码就会执行
#如果知道最终要测试的条件，就可以考虑用一个elif代码块代替else代码块
#if-elif-else结构功能强大，但仅适合用于只有一个条件满足的情况，遇到了通过测试的情况，剩下的代码就会跳过执行

测试多个条件

requested_toppings=['mushrooms','extra cheese']
if 'mushrooms' in requested_toppings:
    print('Adding mushrooms.')
if 'pepperoni' in requested_toppings:
    print('Adding pepperoni.')
if 'extra cheese' in requested_toppings:
    print('Adding extra cheese.')
print("\nFinished making your pizza!")
#首先创建一个列表，然后依次检查某一元素是否在列表中最后输出结果
#如果只想执行一个代码块，就是用if-elif-else结构；如果要执行多个代码块，使用一系列独立的if语句

4.3使用if语句检查列表

检查特殊元素

requested_toppings=['nushrooms','green peppers','extra cheese']
for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Soory we are out of green peppers!")
    else:
        print(f"Adding {requested_topping}")
print(f"\nFinished making your pizza!")

确定列表不是空的

运行for循环前确定列表是否为空很重要

requested_toppings=[]
if requested_toppings:
    for requested_topping in requested_toppings:
        print(f"Adding {requested_topping}")
    print ("\nFinished making your piaaz!")
else:
    print("Are you sure want a plain pizza?")

使用多个列表：

available_toppings = ['mushrooms','olives','green peppers','pepperoni','pineapple','extra cheese']
requested_toppings = ['mushrooms','french fris','extra cheese']
for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print(f"Adding {requested_topping}")
    else:
        print(f"Sorry,we don't have {requested_topping}")
print("\nFinished making your pizza!")