简介

if语句可以根据条件采取一定的措施，每条if语句的核心都是一个值为True或False的表达式，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，即为True时执行if语句后面的代码，为Flase则忽略。

if

languages = ['java', 'c', 'python']
for language in languages:
      if language == 'java':
            print(language.upper())
      else:
            print(language.title())

条件测试

# 检查是否相等
## python中检查是否相等区分大小写
language = 'java'
if language == 'java':
      print("language == 'java'")

# 检查是否相等
language = 'java'
print(language == 'java')

# 检查是否不相等
language = 'java'
if language != 'python':
      print('It is false')

# 检查数字
age = 18
print(age == 18)

# 检查多个条件（and）
age_0 = 22
age_1 = 18
print(age_0 >= 22 and age_1 >= 18)
print(age_0 >= 23 and age_1 >= 18)
	
# 检查多个条件（or）
age_0 = 22
age_1 = 18
print(age_0 >= 22 or age_1 >= 19)
print(age_0 >= 23 or age_1 >= 19)

# 检查特定值是否包含在列表中
languages = ['java', 'c', 'python']
print('java' in languages)
print('python' in languages)

if-elif-if结构

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

省略else代码块

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

判断列表是否为空

languages = []
if languages:
      print("languages is not empty!")
else:
      print("languages is empty!")

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