《Python编程：从入门到实践（第二版）》自学笔记（十一）

While循环

编程界的三大循环：if、for、while。if之前是在遍历的时候开始接触，for之前是在元素是否在列表中开始接触，而while从此刻开始认识。while循环是不断运行，直到指定的条件不满足为止。

current_number = 1 
while current_number <= 5: 
	print(current_number) 
	current_number += 1
输出：
1
2
3
4
5

当然，无限的循环在程序中执行时，也会受到人为的干扰或者终止。其中，我们也可以在被执行程序中添加由用户决定什么时候停止循环

prompt = "\nTell me something, and I will repeat it back to you:" 
prompt += "\nEnter 'quit' to end the program. "
message = ""
while message != 'quit': 
	message = input(prompt) 
	print(message)
输出：
Tell me something, and I will repeat it back to you: 
Enter 'quit' to end the program. Hello everyone! 
Hello everyone!

为使被执行程序，更加简洁直接，因为上述代码中，美中不足的是：将quit也作为输出展示了出来

prompt = "\nTell me something, and I will repeat it back to you:" 
prompt += "\nEnter 'quit' to end the program. " 
message = "" 
while message != 'quit': 
	message = input(prompt) 
	if message != 'quit': 
		print(message)
输出：
Tell me something, and I will repeat it back to you: 
Enter 'quit' to end the program. Hello everyone! 
Hello everyone! 

Tell me something, and I will repeat it back to you: 
Enter 'quit' to end the program. Hello again. 
Hello again. 

Tell me something, and I will repeat it back to you: 
Enter 'quit' to end the program. quit

那么对于有些被执行程序，并不是被编码完成后就必须要进行执行，一般是达到某个条件或者说是触发了某个开关后，再去执行。我们把这一过程称作为：标志（相当于总开关）

prompt = "\nTell me something, and I will repeat it back to you:" 
prompt += "\nEnter 'quit' to end the program. " 
active = True
while active: 
	message = input(prompt) 
	if message == 'quit': 
		active = False
	else:
		print(message)

假设一下：如果循环正在进行，人为的立即退出循环，可否实现？通常利用break去实现。

break语句用于控制程序流程，可用来控制哪些代码行将执行、哪些代码行不执行，从而让程序按你的要求执行你要执行的代码

prompt = "\nPlease enter the name of a city you have visited:" 
prompt += "\n(Enter 'quit' when you are finished.) "
while True: 
city = input(prompt) 
if city == 'quit': 
	break 
else:
	print(f"I'd love to go to {city.title()}!")
输出：
Please enter the name of a city you have visited: 
(Enter 'quit' when you are finished.) New York 
I'd love to go to New York!

Please enter the name of a city you have visited: 
(Enter 'quit' when you are finished.) quit

当然，break语句不仅仅局限于用于while循环中，在Python的任意循环中都可以使用。

既然有退出，相对应的就有继续，continue。要返回循环开头，并根据条件测试结果决定是否继续执行循环，就可使用continue语句，它不像break语句那样不再执行余下的代码并退出整个循环。

current_number = 0 
while current_number < 10: 
	current_number += 1 
	if current_number % 2 == 0: 
		continue 
	print(current_number)
输出：
1
3
5
7
9

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