Python3 基础学习笔记第四章 —— 【if语句】

- 4.1 一个简单的数列

给定一个汽车列表，将其中每一辆汽车的名称打印出来，要求打印 ‘bmw’ 时所有字母都要大写，其余名称只需要首字母大写：

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

输出结果如下：

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Audi
BMW
Subaru
Toyota

- 4.1.1 检查特定值是否包含在列表当中

要判断特定的值是否已包含在列表当中，可使用关键字 in

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user_names = ['andia' , 'david' , 'liwa']
user = 'andia'
if user in user_names:
	print(user.title() + "is in user_name.")

输出结果如下：

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Andiais in user_name.

要判断特定的值是否不包含在列表当中，可使用关键字 not in

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user_names = ['andia' , 'david' , 'liwa']
user = 'kivle'
if user not in user_names:
	print(user.title() + "is not in user_name.")

输出结果如下：

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Kivleis not in user_name.

- 4.2 if-else 语句 

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age = input("请输入你的年龄查看是否可以去网吧：")
if int(age) >= 18:
    print("You are old enough to go to the net bar!")
    print("You should go to net bar less,study more!")
else:
    print("You are too young to go to the net bar!")
    print("Wait until you are 18 to go to the net bar!")

分别输入19和15，输出结果如下：

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请输入你的年龄查看是否可以去网吧：19
You are old enough to go to the net bar!
You should go to net bar less,study more!
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请输入你的年龄查看是否可以去网吧：15
You are too young to go to the net bar!
Wait until you are 18 to go to the net bar!

- 4.3 if-elif-else 结构 

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

输出结果如下：

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Your admission cost is $5.

- 4.3.1 使用多个 elif 代码块 

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age = 20
if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 15
else:
	price = 10
print("Your admission cost is $" + str(price) + ".")

输出结果如下：

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Your admission cost is $15.

- 4.3.2 省略 else 代码块

Python并不要求 if-elif 结构后面必须有 else 代码块：

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age = 20
if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 15
elif age >= 65:
	price = 10
print("Your admission cost is $" + str(price) + ".")

输出结果仍与3.3.1一样

- 4.4 测试多个条件

if-elif-else结构功能强大，但仅适用于只有一个条件满足的情况：遇到通过了的测试后，Python就会跳过余下的测试：

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 names = ['Zhangshan' , 'Wanger']
if 'Zhangshan' in names:
    print("Zhangshan is here!")
if 'Wanger' in names:
    print("Wanger is here!")
if 'Xiaoming' in names:
    print("Xiaoming is here!")
print("All the students are here!")

输出结果如下：

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Zhangshan is here!
Wanger is here!
All the students are here!

相同的程序，如果使用 if-elif-else 结构，代码将不能正确运行：

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names = ['Zhangshan' , 'Wanger']
if 'Zhangshan' in names:
    print("Zhangshan is here!")
elif 'Wanger' in names:
    print("Wanger is here!")
elif 'Xiaoming' in names:
    print("Xiaoming is here!")
print("All the students are here!")

输出结果如下：

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Zhangshan is here!
All the students are here!

总之，如果我们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，就必须使用一系列独立的 if 语句！

- 4.5 使用 if 语句处理列表

- 4.5.1 检查特殊元素

对3.4例子改版，加入姓名 ‘Xiaoming’，当检索到Xiaoming时告诉他，他妈妈叫他回家吃饭

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names = ['Zhangshan' , 'Wanger' , 'Xiaoming']
for name in names:
    if name == 'Xiaoming':
        print("Xiaoming,Your mother told you to go home for dinner!")
    else:
        print(name +"is here!")
print("All the students are here!")

输出结果如下：

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Zhangshanis here!
Wangeris here!
Xiaoming,Your mother told you to go home for dinner!
All the students are here!

- 4.5.2 确定列表不是空的

在检索姓名前检查姓名是否为空，不为空则打印出所有姓名，为空则提示没有姓名：

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names = []
if names:
    for name in names:
        print(name +" is here!")
    print("All the students are here!")
else:
    print("There is no students!")

输出结果如下：

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There is no students!
在if语句中将列表名用在条件表达式中时，Python将在列表至少包含一个元素时返回Ture，并在列表为空时返回False

- 4.5.3 使用多个列表

两个列表names_1和names_2，要求输出既在names_2中又在names_1中的元素：

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names_1 = ['Zhangshan' , 'Liyang'  , 'Wanger' , 'Tangyang' , 'Xiaoming']
names_2 = ['Liyang' , 'Zhangwei' , 'Tangyang']
for names in names_2:
    if names in names_1:
        print(names +" is here!")
print("All the students are here!")

输出结果如下：

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Liyang is here!
Tangyang is here!
All the students are here!

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