Python编程：从入门到实践（学习笔记-Charpter 5)

Python编程：从入门到实践（学习笔记-Charpter 5)

十月 17, 2019

Chapter 5 : if 语句

5.1初识 if语句

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cars = ['audi', 'bmw', 'subaru', 'toyota']

for car in cars:
    if car == 'bmw':
        print(car.upper())
	else:
        print(car.title())
        
---------
Audi
BMW
Subaru
Toyota
---------

每条 if 语句的核心都是一个值为 True 或 False 的表达式,这种表达式被称为条件测试 。 Python 根据条件测试的值为 True 还是 False 来决定是否执行 if 语句中的代码。如果条件测试的值为 True , Python 就执行紧跟在 if 语句后面的代码;如果为 False , Python 就忽略这些代码。

5.2条件检测

  • 检测是否相等（一个等号是陈述（赋值），两个等号是发问（判断））
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car ='bmw'
car == 'bmw'

#使用两个等号( == )检查 car 的值是否为 'bmw'
#这个相等运算符 在它两边的值相等时返回 True ,否则返回 False 。
  • 检测是否相等时不考虑大小写(转换大小写在比较)
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car = 'Adui'
car == 'adui'
---
False
---

car = 'Adui'
car.lower() == 'adui'
---
True
---

#如果大小写很重要,这种行为有其优点。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,再进行比较:
  • 检测是否不相等（!=）
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requested_topping = 'mushrooms'
if requested_topping != 'anchovies':
    print("Hold the anchovies!")
    
#要判断两个值是否不等,可结合使用惊叹号和等号( != ),其中的惊叹号表示不 ,在很多编程语言中都如此。
  • 比较数字(大于、小于、大于等于、小于等于、等于)
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>>> age = 18
>>> age == 18
True

>>>age = 19
>>>age < 21
Ture
>>>age <= 21
False
>>>age >= 21
False

  • 检测多个条件：

    • 使用and检测多个条件（必须两个同时成立）
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    >>> age_0 = 22
    >>> age_1 = 18
    >>> age_0 >= 21 and age_1 >= 21  # (age_0 >= 21) and (age_1 >= 21)增强可读性
    False
    >>> age_1 = 22
    >>> age_0 >= 21 and age_1 >= 21
    True
    • 使用or检测多个条件 （一个成立即为True）
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    >>> 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

  • in检测特定值是否包含在列表中

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>>>requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>>'mushrooms' in requested_toppings
True
>>>'pepperoni' in requested_tooppings
False
  • not in检查特定值是否不包含在列表中
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banned_users = ['andrew', 'carolina', 'david']
user = 'marie'

if user not in banned_users:
    print(user.title() + ", you can post a response if you wish.")
    
---
Marie， you can post a response if you wish.
---   
#如果user的值未包含在列表banned_users中，Python将返回True，进而执行缩进的代码行
  • 布尔表达式

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

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game_active = True
can_edit = False

注：在跟踪程序状态或程序中重要条件方面，布尔值提供了一种高效的方式。

5.3 if 语句

在第 1 行中,可包含任何条件测试,而在紧跟在测试后面的缩进代码块中,可执行任何操作。如果条件测试的结果为 True , Python 就会执行紧跟在 if 语句后面的代码;否则Python 将忽略这些代码。

  • 简单的if 语句
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age = 19
if age >= 18:
    print("You are old enough to vote!")
    
----   
You are old enough to vote!
----

age = 19
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
----    
You are old enough to vote!
Have you registered to vote yet?
----
  • if-else语句
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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!")
    
#Sorry, you are too young to vote.
#Please register to vote as soon as you turn 18
  • if-elif-else 结构
    • 4 岁以下免费;
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    • 18 岁(含)以上收费 10 美元。
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age = input("please input your age: ")

if age < 4 :
    print("Your admission cost is $0.")       #  price = 0
elif age < 18:
    print("Your admission cost is $5.")       #  price = 5
else:
    print("Your admission cost is $10.")      #  price = 10
    
#print("Your admission cost is $" + str(price) + ".")
  • 使用多个elif代码块 
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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) + ".")
  • 省略else代码块
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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) + ".")
  • 测试多个条件
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requested_toppings = ['mushroom', 'extra cheese']

if 'mushroom' in requested_tooppings:
    print("Adding mushroom.")
if 'pepperoni' in requested_toppings:
    print("Adding prpperoni.")
if 'extra cheese' in requested_toppings:
    print("Adding extra cheese.")
    
print ("\n Finished making your pizza!")

5.4使用if语句处理列表

  • 检查特殊元素
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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!")
  • 确定列表不是空的
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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?")
  • 使用多个列表
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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!")