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Python3 基础学习笔记第四章【if 语句】

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【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：

 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：

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

 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：

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

 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：

Kivleis not in user_name.

【4.2】if-else 语句

 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：

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

 请输入你的年龄查看是否可以去网吧：15You 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

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

输出结果如下：

 Your admission cost is $5.

【4.3.1】使用多个 elif 代码块

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：

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：

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：

 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：

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：

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：

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

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：

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：

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：

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：

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：

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