《Python从入门到实践》第五章

使用if来进行判断：

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

if代表条件测试，根据条件测试的值是True还是False来决定是否运行if语句中的代码

①检查是否相等

==表示判断是否相等

=表示赋值

在Python中检查是否相等是区分大小写的，两个大小写不同的值被视为不相等

但如果大小写无关紧要，你只想检查变量的值，可以使用lower.()把变量的值全部转为小写的，值得注意的是，lower()方法不会修改存储在变量中的值，不会影响原来的变量

car='Audi'
if car.lower()=='audi':
    print('great')
print(car)

②检查是否不相等

！=

③数值比较

>  <  ==  <=  >=

检查多个条件：

①使用and检查多个条件

只有and左右两边的条件都成立才为True，为了改善可读性，可将每个条件测试都分别放在一对括号内，但并非必须这样做

②使用or检查多个条件

只要任意一个条件能够满足，即为True

检查特定的值是否在列表中：

要判断特定的值是否在列表中，可使用关键字in

requested_toppings=['mushrooms','onions','pineapple']
if 'mushrooms' in requested_toppings:
    print('ture')

检查特定的值是否不在列表中：

可以使用关键词not in

banned_users=['andrew','carolina','david']
user='marie'
if user not in banned_users:
    print(f"{user.title()},you can post a response if you want!")

布尔表达式：

布尔表达式u条件表达式一样，布尔表达式的结果要么为True，要么为False

练习5.1

car='subaru'
print("Is car=='subaru'? I predict True.")
print(car=='subaru')

if-else语句：面临两种可能的情况

age=19
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 your turn 18!")

if-elif-else 语句：面临两种以上可能的情况，可以使用多个elif

age=12
if age<4:
    price=0
elif age<18:
    price=25
else:
    price=40

print(f"Your admission cost is {price} yuan.")

Python并不要求if-elif结构后面必须有else代码块，在一些情况下，else代码块很有用，而在其它情况下，使用一条elif语句来处理特定的情形更清晰

测试多个条件：

requested_toppings=['mushrooms','extra cheese']

if 'mushrooms' in requested_toppings:
    print("Adding mushrooms")
if 'extra cheese' in requested_toppings:
    print("Adding extra cheese")
if 'pepperoni' in requested_toppings:
    print("Adding pepperoni")

print("\nFinished making your pizza")

这些if语句会一个一个按顺序执行下去进行判断（多个代码块），如果采用if-elif语句，则只会执行其中一个语句（一个代码块）。

练习5.3

alien_color='green'
if alien_color=='red':
    print("you get five points")

练习5.4

alien_color='red'
if alien_color=='green':
    print("you get five points")
else:
    print("you get ten points")

练习5.5

alien_color='yellow'
if alien_color=='green':
    print("you get five points")
elif alien_color=='red':
    print("you get ten points")
else:
    print("you get fifteen points")

练习5.6

age=19
if age < 2:
    print("that's a baby")
elif 2 <= age < 4:
    print("that's a little boy")
elif 4 <= age <13:
    print("that's a child")
elif 13 <= age<18:
    print("that's a teenager")
elif 18 <= age<65:
    print("that's a adult")
else:
    print("that's an old")

使用if语句处理列表：

requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Sorry, we are out of green peppers right now!")
    else:
        print(f"Adding {requested_topping}")

print("\nFinished making your pizza")

确定列表非空：

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

使用多个列表：

available_toppings=['mushrooms','olives','green peppers','pepperoni','pineapple','extra cheese']
requested_toppings=['mushrooms','french fries','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!")

练习5.8

users=['Join','Mike','admin','Alan','Kobe']
for user in users:
    if user=='admin':
        print("Hello admin, would you like to see a status report.")
    else:
        print(f"Hello {user}, thank you for logging in again.")

练习5.9：

current_users = ['Join', 'Mike', 'admin', 'Alan', 'Kobe']
new_users = ['Admin', 'Nike', 'adidas', 'Alan', 'duke']
current_users1=[item.lower() for item in current_users]
for new_user in new_users:
    if new_user.lower() in current_users1:
        print("该昵称已经被占用")
    else:
        print(f"你的昵称为：{new_user}")

练习5.10

numbers=[1,2,3,4,5,6,7,8,9]
for number in numbers:
    if number==1:
        print("1st")
    elif number==2:
        print("2nd")
    elif number==3:
        print("3rd")
    else:
        print(f"{number}th")