用户输入和while循环
一、函数input的工作原理
函数input()让程序暂停运行，等待用户输入一些文本。获取用户输入后，Python将其存储在一个变量中，以方便使用。
message = input("Tell me something, I will repeat it back to you: ")
print(message)
输出：
Tell me something, I will repeat it back to you: Hello python!
Hello python!
编写清晰的程序
当提示超过一行时：
prompt = "If you tell us who you are, we can personalize the messages you see."
prompt += "\nWhat is your first name? "
message = input(prompt)
print(message)
输出：
If you tell us who you are, we can personalize the messages you see.
What is your first name? Liu
Liu
使用int（）来获取数值输入
age = input("How old are you? ")
print(age >= 18)
此时会提示：
TypeError: '>=' not supported between instances of 'str' and 'int'
因为请求Python提供变量age的值时，它返回的是‘21’——用户输入的数值的字符串表示，Python将输入解读成了字符串。
为了解决这个问题，可使用函数int()，它让Python将输入视为数值。
函数int()将数字的字符串转换为数值表示。
age = input("How old are you? ")
age = int(age)
print(age >= 18)
输出：
How old are you? 20
True
求模运算符
求模运算符(%)，将两个数相除并返回余数。
number = input("Enter a number, and I'll tell you if it's even or odd: ")
number = int(number)
if number % 2 ==0:
print("\nThe number " + str(number) + " is even.")
else:
print("\nThe number " + str(number) + " is odd.")
输出：
Enter a number, and I'll tell you if it's even or odd: 30
The number 30 is even.
二、while循环简介
for 循环用于针对集合中的每个元素都一个代码块。
而 while 循环不断地运行，直到指定的条件不满足为止。
使用while循环
current_number = 1
while current_number <= 5:
print(current_number)
current_number += 1
输出：
1
2
3
4
5
让用户选择何时退出
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)
else:
print("Program Quit!")
输出：
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. Hello
Hello
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. Bye
Bye
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. quit
Program Quit!
使用标志
在要求很多条件都满足才继续运行的程序中，可定义一个变量，用于判断整个程序是否处于活动状态。这个变量被称为标志。
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
print("Program Quit!")
else:
print(message)
输出：
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. Hello
Hello
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. Bye
Bye
Tell me something, and I will repeat it back to you:
Enter 'quit' to end the program. quit
Program Quit!
使用break退出循环
要立即退出while循环，不再运行循环中余下的代码，也不管条件测试的结果如何，可使用 break语句。
prompt = "\nPlease enter the name of city you have visited: "
prompt += "\n(Enter 'quit' when you finished.)"
while True:
message = input(prompt)
if message != 'quit':
print("I'd love to go to " + message)
else:
print("Program Quit!")
break
输出：
Please enter the name of city you have visited:
(Enter 'quit' when you finished.)Changsha
I'd love to go to Changsha
Please enter the name of city you have visited:
(Enter 'quit' when you finished.)Beijing
I'd love to go to Beijing
Please enter the name of city you have visited:
(Enter 'quit' when you finished.)quit
Program Quit!
在循环中使用continue
要返回到循环开头，并根据条件测试结果决定是否继续执行循环，可使用continue语句。
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
避免无限循环
如果程序陷入无限循环，可按Ctrl + C，也可关闭显示程序输出的终端窗口。
要避免编写无限循环，务必对每个while循环进行测试，确保它按预期那样结束。
三、使用while循环来处理列表和字典
要记录大量的用户和信息， 需要在while循环中使用列表和字典。
for循环是一种遍历列表的有效方式，但在for循环中不应修改列表，否则将导致Python难以跟踪其中的元素。
要在遍历列表的同时对其进行修改，可使用while循环。
通过将while循环同列表和字典结合起来使用，可收集、存储并组织大量输入，供以后查看和显示。
在列表之间移动元素
假设有一个列表，其中包含新注册但还未验证的网站用户；验证这些用户后，如何将他们移到另一个已验证用户列表中呢？一种办法是使用一个while循环，在验证用户的同时将其从未验证用户列表中提取出来，再将其加入到另一个已验证用户列表中。
unconfirmed_users = ['alien', 'brian', 'candace']
confirmed_users = []
while unconfirmed_users:
current_user = unconfirmed_users.pop() #pop()以每次一个的方式从列表末尾删除未验证的用户
print("Verifying user: " + current_user.title())
confirmed_users.append(current_user)
print("\nThe following users have been confirmed:")
for confirmed_user in confirmed_users:
print(confirmed_user.title())
输出：
Verifying user: Candace
Verifying user: Brian
Verifying user: Alien
The following users have been confirmed:
Candace
Brian
Alien
删除包含特定值的所有列表元素
函数remove()来删除列表中的特定值，是因为要删除的值在列表中只出现了一次。
假设有一个宠物列表，其中包含多个值为’cat’的元素。要删除所有这些元素，可不断运行一个while循环，直到列表中不再包含值’cat’。
pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
pets.remove('cat')
print(pets)
输出：
['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
['dog', 'dog', 'goldfish', 'rabbit']
使用用户输入来填充字典
可使用while循环提示用户输入任意数量的信息。
创建一个调查程序，其中的循环每次执行时都提示输入被调查者的名字和回答。我们将收集的数据存储在一个字典中，以便将回答同被调查者关联起来：
responses = {}
active = True
while active:
name = input("\nWhat is your name? ")
response = input("Which mountain would you like to climb someday? ")
responses[name] = response
repeat = input("Would you like to let another person respond? (yes/ no) ")
while repeat != 'no' and repeat != 'yes':
repeat = input("Would you like to let another person respond? (yes/ no) ")
if repeat == 'no':
active = False
print("\n--- Poll Results ---")
for name, response in responses.items():
print(name + "would like to climb " + response + ".")
输出：
What is your name? 1
Which mountain would you like to climb someday? 1
Would you like to let another person respond? (yes/ no) yes
What is your name? 2
Which mountain would you like to climb someday? 2
Would you like to let another person respond? (yes/ no) haode
Would you like to let another person respond? (yes/ no) buhao
Would you like to let another person respond? (yes/ no) yes
What is your name? 3
Which mountain would you like to climb someday? 3
Would you like to let another person respond? (yes/ no) no
--- Poll Results ---
1would like to climb 1.
2would like to climb 2.
3would like to climb 3.
