【Python基础知识】（六）用户输入与while循环

用户输入

　　使用input()函数可获取用户输入的文本，并以字符串的形式存入变量中。

　　注意：input括号内的语句只是输入提示，与input接收并存储进变量中的内容无关。

name = input("Please enter your name: ")
print("Hello, " + name + "!")

当输入Carl时的输出：

Please enter your name: Carl
Hello, Carl!

　　有时候，输入提示可能超过一行，此时可在外部编辑要输入的提示语句，并将其存储在一个变量中，再将该变量传递给函数input()。

　　下例就是使用+=运算符创建多行字符串的一种方式（可理解为字符串的拼接）。

prompt = "Please tell us who you are."
prompt += "
What's your first name? "

name = input(prompt)
print("
Hello, " + name + "!")

　　由于Python将用户输入解读为字符串，所以当要让Python将输入视为数值时，需要调用int()函数做类型转换。

age = input("How old are you?")    '''输入19'''
age = int(age)
age >= 18    '''True'''

while循环

1、简单的while循环

current_number = 1
while current_number <= 5:
    print(current_number)
    current_number += 1

输出为：

1
2
3
4
5

　　在要求很多条件都满足才能继续运行的程序中，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。当标志的值为True时，程序可继续进行；当标志的值为False时，程序停止运行。

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

active = True
while active:
    message = input(prompt)
    
    if message == 'quit':
         active = False
    else:
        print(message)

 多次输入，得到输出为：

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

2、break语句

　　在循环中使用break语句，可直接退出循环。

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

while True:
    message = input(prompt)
    
    if message == 'quit':
        break
    else:
        print(message)

3、continue语句

　　在循环中使用continue语句，可让程序跳过在循环体中位于continue语句后面的所有语句，直接回到循环开始并开始下一轮的循环。

current_number = 0
while current_number < 10:
    current_number += 1
    if current_number %2 == 0:
        continue
    print(current_number)

输出为：

1
3
5
7
9

使用while循环处理列表和字典

1、在列表之间移动元素

'''验证用户'''
unconfirmed_users = ['alice', 'brian', 'candance']
confirmed_users = []

'''逐一验证用户'''
while unconfirmed_users:
current_user = unconfirmed_users.pop()

print("Verifying user: " + current_user.title())
confirmed_users.append(current_user)

'''显示所有已验证的用户'''
print（"
The following users have been confirmed: ")
for confirmed_user in confirmed_users:
    print(confirmed_users.title())

2、删除列表中的特定值

　　使用循环和.remove()方法，可剔除列表中所有的特定元素。

pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)

while 'cat' in pets:
    pets.remove('cat')
    
print(pets)

输出为：

['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
['dog', 'dog', 'goldfish', 'rabbit']

3、将输入存储进字典

responses = {}

'''设置标志'''
polling_active = True

while polling_active:
    '''获取需要加入字典的键-值对信息'''
    name = input("
What's your name? ")
    response = input("which mountain would you like to climb someday?")
    
    '''将键-值对录入字典'''
    responses[name] = response
    
    '''是否要执行下一轮循环'''
    repeat = input("Asking another person?(yes/no)")
    if repeat == 'no':
        polling_active = False

'''调查结束，显示结果'''        
print("
--- Poll Results ---")
for name, response in responses.items():
    print(name + " would like to climb " + response + ".")

输入一些信息，得到输出为：

What's your name? Eric
which mountain would you like to climb someday?Denali
Asking another person?(yes/no)yes

What's your name? Lynn
which mountain would you like to climb someday?Devil's Thumb
Asking another person?(yes/no)no

--- Poll Results ---
Eric would like to climb Denali.
Lynn would like to climb Devil's Thumb.

参考书籍：《Python编程：从入门到实践》

2020-07-12