input和while循环——Python编程从入门到实践

input( )

input()函数：让程序运行暂停，等待用户输入。

message = input('Tell me something, and I will repeat it back to you: ')
print(message)

运行结果：

Tell me something, and I will repeat it back to you: Hello Python!
Hello Python!

1. 编写清晰的程序

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

Please enter your name: hery
Hello, hery!

提示信息超过一行时：

prompt = "If you tell us who you are, we can personalize the messages you see."
prompt += '\nWhat is your name? '
name = input(prompt)
print("\nHello, " + name + "!")

2. 获取数值的输入

age = input("How old are you? ")
print(type(age))

How old are you? 12
<class 'str'>

通过input()函数输入的信息以字符串的形式存储，若需要将输入作为数值使用怎么办呢？

可以使用int()函数将其转换为数值表示：

height = input("How tall are you, in inches? ")
height = int(height)
if height >= 36:
    print("\nYou're tall enough to ride!")
else:
    print("\nYou'll be able to ride when you're a little older.")

3. 求模运算符

求模运算符（%）：求得两数相除返回的余数。

可用于判断一个数是奇数还是偶数：

number = input("Enter a number, and I'll tell you if it is 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.')

运算符两端的元素类型要一致，故print语句中又需要将数值型通过str()函数转换为字符型。

while循环

for循环是针对集合中的每个元素的一个代码块，而while循环是不断的运行，直到指定条件不满足。

1. 使用while循环

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

运行结果：

1
2
3
4
5

2. 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)
    print(message)

运行结果：

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

Tell me something , and I will repeat it back to you:
Enter 'quit' to end the program. Hello 0629
Hello 0629

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

输入为 quit 时循环结束。

若不想将 quit 也作为一条消息打印出来，则：

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)

3. 使用标志

在要求很多条件都满足的情况下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
    else:
        print(message)

敲代码的时候把 active = False 敲成了 active = 'False'，然后输入quit还一直执行循环，哈哈哈

4. 使用break退出循环

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("I'd love to go to " + city.title() + "!")

Note: Python循环（while循环、for循环）中都可使用break语句来推出循环。

5. 在循环中使用continue

循环中使用continue，会返回大循环开头，并根据条件测试结果决定是否继续执行循环：

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

运行结果：

1
3
5
7
9

6. 避免无限循环

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

上述的代码块中，若漏写了代码行 x += 1，这个程序将没完没了地运行。可按Ctrl + C，也可关闭显示程序输出的终端窗口，或关闭编辑器，结束无限循环。