《Python编程：从入门到实践（第二版）》自学笔记（六）

if语句结构

对于if语句结构，有以下几种情况，不过不管是哪种结构，在使用的过程中都应当注意条件测试和执行程序的格式，尤其是缩进

首先最简单的就是，一个条件测试，一个操作，不过个人喜欢把操作称为执行程序

##仅为示范
if conditional_test:
	do something

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

此处缩进的作用与在for循环中相同，如果测试通过了，将执行if语句后面所有缩进的代码行，否则将忽略它们

其次if语句结构，在英语中经常接触到，就是if……else……，此结构一般运用在条件测试通过时执行一个操作，在没有通过时执行另一个操作

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!")
输出：
Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!

另外，在常见的if语句结构中还会看到：if……elif……else，这种结构。它的意思是：依次检查每个条件测试，直到遇到通过了的条件测试。测试通过后，Python将执行紧跟在它后面的代码，并跳过余下的测试

age = 12
if age < 4:
	print("Your admission cost is $0.")
elif age < 18:
	print("Your admission cost is $25.")
else:
	print("Your admission cost is $40.")
输出：Your admission cost is $25.

为使代码更加简单轻量化，对原代码进行优化

age = 12
if age < 4: 
	price = 0 
elif age < 18:
	price = 25 
else:
	price = 40
print(f"Your admission cost is ${price}.")
输出：Your admission cost is $25.

这里需要插一句嘴的是：elif代码块并不是只能有一个，它是可以有多个穿插在if和else中间的，但带来的缺点就是：臃肿

age = 12
if age < 4:
	price = 0
elif age < 18:
	price = 25
elif age < 65:
	price = 40
else:
	price = 20
print(f"Your admission cost is ${price}.")
输出：Your admission cost is $25.

与此同时，既然elif代码可以有多个，那么else代码块也可以省略。在Python中并不要求if-elif结构后面必须有else代码块，在有些情况下，else代码块很有用，而在其他一些情况下，使用一条elif语句来处理特定的情形更清晰

age = 12
if age < 4:
	price = 0
elif age < 18:
	price = 25
elif age < 65:
	price = 40
elif age >= 65:
	price = 20
print(f"Your admission cost is ${price}.")
输出：Your admission cost is $25.

总之，如果只想执行一个代码块，就使用if……elif……else结构；如果要 执行多个代码块，就使用一系列独立的if语句

当遇到需要if语句去处理列表时，也可以结合之前的遍历这一块的内容，也就是使用for去进行处理

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(f"Adding {requested_topping}.")
print("\nFinished making your pizza!")
输出：
Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.

Finished making your pizza!

但是也有可能会遇到一种个特殊的情况，就是：列表为空这种情况

requested_toppings = []
if requested_toppings:
	for requested_topping in requested_toppings:
		print(f"Adding {requested_topping}.")
	print("\nFinished making your pizza!")
else:
	print("Are you sure you want a plain pizza?")
输出：Are you sure you want a plain pizza?

同样的，还有一种情况就是，对多个列表进行处理时，又将如何去进行处理

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(f"Adding {requested_topping}.")
	else:
		print(f"Sorry, we don't have {requested_topping}.")

print("\nFinished making your pizza!")
输出：
Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.

Finished making your pizza!

以上所有使用到for进行遍历和if语句结构的，都要仔细检查最后面的冒号，冒号，冒号！！！

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