用户输入和while循环

• Q：函数input()工作
• A：函数让程序暂停运行，等待用户输入一些文本

# 下面的例子让用户输入一些文本，再将这些文本呈现给用户
message = input("Tell me something, and I will repeat it back to you: ")
print(message)
--------------------------------------------------------------------
Tell me something, and I will repeat it back to you: Hello everyone!
Hello everyone!

• Q：函数input()的应用时的注意
• A：应该指定清晰而易于明白的提示

# 下面是例子
prompt = "If you tell us who you are, we can personalize the messages you see."
prompt += "
What is your first name? "
name = input(prompt)
print("
Hello, " + name + "!")
------------------------------------------------------------------
If you tell us who you are, we can personalize the messages you see.
What is your first name? Eric

Hello, Eric!

• Q：使用int()来获取数值输入
• A：input()本身是获取字符串的

# 下面是int()获取数值输入的例子
height = input("How tall are you, in inches? ")
height = int(height)

if height >= 36:
    print("
You're tall enough to ride!")
else:
    print("
you'll be abke to ride when you're a little older.")
------------------------------------------------------------------
How tall are you, in inches? 71

You're tall enough to ride!

• Q：求模运算符(%)
• A：将两个数相除返回余数

# 可以利用这个判断一个数是奇数还是偶数,下面是例子
number = input("Enter a number, and I'll tell you if it's even or odd: ")
number = int(number)

if number % 2 == 0:
    print("
The number " + str(number) + " is even.")
else:
    print("
The number " + str(number) + " is odd.")
------------------------------------------------------------------
Enter a number, and I'll tell you if it's even or odd: 42

The number 42 is even.

• Q：简单地使用while循环
• A：不断的运行,直到指定的条件不满足为止

# 下面例子是从一数到五
current_number = 1

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

• Q：让用户选择什么时候退出
• A：定义一个退出值，只要用户输入的不是这个退出值，程序就接着运行

# 下面是选择什么时候退出的例子(粗体部分为用户输入)
prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter 'quit' to end the program. "
message = ""

while message != 'quit':
    message = input(prompt)
    print(message)
------------------------------------------------------------------
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
quit

# 这里将quit也打印出来了,我们可以加一条条件测试
prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter 'quit' to end the program. "
message = ""
while message != 'quit':
    message = input(prompt)

    if message != 'quit':
        print(message)
------------------------------------------------------------------------------------
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

• Q：要求很多条件都满足才能运行的程序怎么办？
• A：使用标志  把相关逻辑由程序的其他部分处理

# 下面是标志的例子
prompt = "
Tell me something, and I will repeat it back to you:"
prompt += "
Enter 'quit' to end the program. "
active = True
#我们将变量active设置成了True，让程序最初处于活动状态。

while active:
    message = input(prompt)

    if message == 'quit':
        active = False
    else:
        print(message)
# 我们使用了一个标志来指出程序是否处于活动状态，这样如果要添加测试
（如elif语句）以检查是否发生了其他导致active变为False的事件，将很容易。

• Q：退出循环
• A：break语句用于控制程序流程，可使用它来控制哪些代码行将执行，哪些代码行不执行

# 退出循环的例子
prompt = "
Please enter the name of a city you have visited:"
prompt += "
(Enter 'quit' when you are finished.) "

while True:   #以while True打头的循环将不断运行，直到遇到break语句。
    city = input(prompt)

    if city == 'quit':
        break
else：
    print("i'd love to go to " + city.title() + "!")
------------------------------------------------------------------
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.) San Francisco
I'd love to go to San Francisco!

Please enter the name of a city you have visited:
(Enter 'quit' when you are finished.) quit
#在任何Python循环中都可使用break语句。例如，可使用break语句来退出遍历列表或字典的for循环。



Q：是否继续执行循环
A：使用continue语句

#下面是使用continue从1数到10.但只打印其中的奇数的例子
current_number = 0
while current_number < 10:
    current_number += 1
    if current_number % 2 == 0:
        continue
#if语句检查current_number与2的求模运算结果。如果结果为0（意味着current_number可被2整除），就执行continue语句，让Python忽略余下的代码，并返回到循环的开头。

    print(current_number)
------------------------------------------------------------------
1
3
5
7
9

• Q：避免无限循环
• A：对每个while条件测试

# 这个循环将没完没了地运行！
x = 1
while x <= 5:
    print(x)
------------------------------------------------------------------
1
1
1
1
--snip--
#Ctrl + C可以退出循环，也可以关闭显示程序的终端窗口