5. if语句

if语句

条件测试

检查是否相等

car = 'Bmw'
print(car == 'Bmw')
print(car == 'bmw') #区分大小写
#返回
True
False
​
car = 'Bmw'
print(car.lower() == 'bmw') #不区分大小写
#返回
True

检查是否不相等

car = 'Bmw'
print(car != 'bmw')
#返回
True

比较数字：== , <, <= , >, >=

检查多个条件

#使用and检查多个条件
(age_0 >= 21) and (age_1 >= 21)
#使用or检查多个条件
(age_0 >= 21) or (age_1 >= 21)

检查特定值是否包含在列表中：关键字 in

检查特定值是否不包含在列表中：关键字 not in

bicyles = ['trek', 'cannondale', 'redline', 'specialized', 'lu']
print('lu' in bicyles)
print('lu' not in bicyles)

布尔表达式

True/False

if语句

简单的if语句

age = 19
if age >= 18:
    print("You are old enough to vote!")

if-else 语句

age = 19
if age >= 18:
    print("You are old enough to vote!")
else:
    print("Sorry, you are too young to vote!")

if-elif-else 语句: 只要有一个测试通过，就会跳过余下的测试

age = 12
if age < 4:
    print("You are too ypung!")
elif age < 15:
    print("You are young!")
else age >20:
    print("You are no young")

使用多个elif代码块：

if-elif-elif-else

省略else代码块：

if-elif-elif-elif

测试多个条件：不管前一个测试是否通过，都将进行这个测试

bicyles = ['trek', 'cannondale', 'redline', 'specialized', 'lu']
if 'trek' in bicyles:
    print("1")
if 'cannondale' in bicyles:
    print("2")
if 'redline' in bicyles:
    print("3")

if语句处理列表

检查特殊元素

bicyles = ['trek', 'cannondale', 'redline', 'specialized', 'lu']
for bicyle in bicyles:
    if bicyle == 'lu':
        print("It is mine")
    else:
        print("No")

确定列表不是空

bicyles = []
if bicyles:
    print("yes")
else:
    print("no")
#输出
no

使用多个列表

bicyles = ['trek', 'cannondale', 'redline', 'specialized', 'lu']
bicyles2 = ['trek', 'hong', 'lu']
for bicyle in bicyles2:
    if bicyle in bicyles:
        print(bicyle + " yes")
    else:
        print(bicyle + " no")

设置if语句的格式

在诸如==、>=、<=、>、<等比较运算符两边各添加一个空格，不会影响对代码的解读，而只是让代码阅读起来更容易