第三章 if语句

一、条件测试

1.检查是否相等（“=”是赋值，“==”是判断值是否相等）

2.检查相等时不考虑大小写

在Python中检查是否相等时区分大小写，但如果大小写无关紧要，而只想检查变量的值，可

将变量的值转换为小写。

car = 'Audi'
car.lower() == ''audi
#输出结果：True
#########
 car = 'Audi'
car.lower() == 'audi'
True 
 car
'Audi'
#不会影响原来的变量

3.检查是否不相等（！=）

4.比较数字（>、<、>=、<=）

<u>为符合编程规范，在以上符号中的两端各空一格。</u>

5.检查多个条件

使用and和or(相当于java 中的&&、||)

5.1 使用and检查多个条件

age_0 = 22 
 age_1 = 18 
(age_0 >= 21) and (age_1 >= 21 )
False 
age_1 = 22 
 (age_0 >= 21) and (age_1 >= 21) 
True

5.2 使用or检查多个条件

 age_0 = 22 
 age_1 = 18 
 age_0 >= 21 or age_1 >= 21 
True 
 age_0 = 18 
 age_0 >= 21 or age_1 >= 21 
False

6. 检查特定值是否包含/不包含在列表中(关键字：in、not in)

 ###################包含（in）
 requested_toppings = ['mushrooms', 'onions', 'pineapple']
'mushrooms' in requested_toppings 
True 
 'pepperoni' in requested_toppings 
False
###############不包含（not in）
banned_users = ['andrew', 'carolina', 'david'] 
user = 'marie' 
if user not in banned_users: 
 print(user.title() + ", you can post a response if you wish.")
 #结果：Marie, you can post a response if you wish.

7. 布尔表达式

   game_active = True 
   can_edit = False
   

   二、if 语句

   1. 简单if语句

   if conditional_test:
    do something1
       do something2
   

   缩进：缩进的作用与for循环中相同。如果测试通过了，将执行if语句后面所有缩进

   ​ 的代码行，否则将忽略它们。

   2. 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!")
   ##输出：
       Sorry, you are too young to vote. 
       Please register to vote as soon as you turn 18!
   

   3. if-elif-else 结构

      检查超过一个或者多个的情形

      age = 12 
      if age < 4: 
       price = 0 
      elif age < 18: 
       price = 5 
      elif age < 65: 
       price = 10 
       else: 
       price = 5 
      print("Your admission cost is $" + str(price) + ".")
      

      注意：<u>elif 的作用类似于java中的 else if</u>，其中最后的else（第8行）也可以胜率

age = 12 
if age < 4: 
 price = 0 
elif age < 18: 
 price = 5 
elif age < 65: 
 price = 10 
elif age >= 65: 
 price = 5 
print("Your admission cost is $" + str(price) + ".")

4. 测试多个条件

#不同于 if-elif-else结构，这里的每个if条件都会得到判断（类似于java中的switch语句）
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!")

如果你只想执行一个代码块，就使用if-elif-else结构；如果要运行多个代码块，就

使用一系列独立的if语句。

三、使用if语句处理列表

1. 检查特殊元素

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("Adding " + requested_topping + ".") 
print("\nFinished making your pizza!")
#输出结果
Adding mushrooms. 
Sorry, we are out of green peppers right now. 
Adding extra cheese. 
Finished making your pizza!

2. 确定列表是否为空

#在运行for循环前确定列表是否为空
 requested_toppings = [] 
if requested_toppings: 
     for requested_topping in requested_toppings: 
        print("Adding " + requested_topping + ".") 
     print("\nFinished making your pizza!") 
else: 
    print("Are you sure you want a plain pizza?")
#结果：Are you sure you want a plain pizza?

3. 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("Adding " + requested_topping + ".") 
    else: 
        print("Sorry, we don't have " + requested_topping + ".")
 print("\nFinished making your pizza!")
 #输出：
 Adding mushrooms. 
Sorry, we don't have french fries. 
Adding extra cheese. 
Finished making your pizza!