python基础 (3)if 和 while的使用学习内容：一.if语句二.用户输入和while循环

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概述

学习内容：

1.if语句
2.用户输入和while循环

一.if语句

（1）1.简单的if语句：
格式：

if conditional_test:
 do something

在第1行中，可包含任何条件测试，而在紧跟在测试后面的缩进代码块中，可执行任何操作。
如果条件测试的结果为True，Python就会执行紧跟在if语句后面的代码；否则Python将忽略这些代码。
例如：

age = 19
if age >= 18:
   print("You are old enough to vote!")

运行结果为:

You are old enough to vote!

2.if-else语句：
经常需要在条件测试通过了时执行一个操作，并在没有通过时执行另一个操作；在这种情况下，可使用Python提供的if-else语句。
例如：

age = 17
if age >= 18:
  print("You are old enough to vote!")
  print("Have you registered to vote yet?")
else:
  print("Sorry, you are too young to vote.")
  print("Please register to vote as soon as you turn 18!")

运行结果为：

Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!

3.if-elif-else语句：
Python只执行if-elif-else结构中的一个代码块（即if或elif或else中的为真语句），它依次检查每个条件测试，直到遇到通过了的条件测试。
例如：

age = 12
if age < 4:
 print("Your admission cost is $0.")
elif age < 18:
 print("Your admission cost is $5.")
else:
 print("Your admission cost is $10.")

运行结果为：

Your admission cost is $5.

也可以使用多个elif的代码块
例如：

age = 12
if age < 4:
 price = 0
elif age < 18:
 price = 5
elif age < 65:
 price = 10
else:
 price = 5
print("Your admission cost is $" + str(price) + ".")

运行结果为：

Your admission cost is $5.

同时模块中也可以省略else改用elif。
if-elif-else结构功能强大，但仅适合用于只有一个条件满足的情况：遇到通过了的测试后，Python就跳过余下的测试。这种行为很好，效率很高，让你能够测试一个特定的条件。
4.测试多个条件：
使用一系列不包含elif和else代码块的简单if语句。在可能有多个条件为True，且你需要在每个条件为True时都采取相应措施
时，适合使用这种方法。
例如：

 requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
 print("Adding mushrooms.")
if 'pepperoni' in requested_toppings:
 print("Adding pepperoni.")
if 'extra cheese' in requested_toppings:
 print("Adding extra cheese.")

print("nFinished making your pizza!")

运行结果为：

Adding mushrooms.
Adding extra cheese.
Finished making your pizza!

if-elif-else情况下：

requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
 print("Adding mushrooms.")
elif 'pepperoni' in requested_toppings:
 print("Adding pepperoni.")
elif 'extra cheese' in requested_toppings:
 print("Adding extra cheese.")

print("nFinished making your pizza!")

运行结果为：

Adding mushrooms.
Finished making your pizza!

在俩者的对比之下，更易体会何时使用何种语句。
（2）使用if语句处理列表
1.检查列表中特殊元素：
例如：

requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping == 'green peppers':
 print("Sorry, we are out of green peppers right now.")
else:
 print("Adding " + requested_topping + ".")
print("nFinished making your pizza!")

运行结果为：

Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.
Finished making your pizza!

这样就可以找出列表中的特殊元素。
2.确定列表不为空：
在if语句中将列表名用在条件表达式中时，Python将在列表
至少包含一个元素时返回True，并在列表为空时返回False。
例如：

requested_toppings = []
if requested_toppings:
 print("nFinished making your pizza!")
else:
 print("Are you sure you want a plain pizza?")

运行结果为：

Are you sure you want a plain pizza?

二.用户输入和while循环

（1）input（）函数
1.input（）函数原理：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，以方便你使用。
函数input()接受一个参数：即要向用户显示的提示或说明，让用户知道该如何做。
例如：

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

运行结果为：

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

2.使用int（）来获取数值输入：
使用函数input()时，Python将用户输入解读为字符串。然后要得到我们需要的值（整型，浮点型等）就需要强制转换。
用法：int（input（））
（2）while循环
1.首先循环是什么结构组成，其次如何使用循环，循环可以干什么。
循环结构：测试条件+循环体
例如：

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

运行结果：

以上为一个打印1到5的简单循环。
2.while循环break语句：
当程序执行遇到break时将会退出循环结构。

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() + "!")

运行结果为：

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

3.使用while循环处理列表和字典：
在列表之间移动元素
例如：

unconfirmed_users = ['alice', 'brian', 'candace']
confirmed_users = []
while unconfirmed_users: 
   current_user = unconfirmed_users.pop()
   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())

输出结果为：

Verifying user: Candace
Verifying user: Brian
Verifying user: Alice
The following users have been confirmed:
Candace
Brian
Alice

删除包含特定值的所有列表元素
例如：

pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
      pets.remove('cat')

print(pets)

运行结果为：

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

使用用户输入来填充字典
例如：

responses = {}
polling_active = True
while polling_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) ")
       if repeat == 'no':
          polling_active = False

print("n--- Poll Results ---")
for name, response in responses.items():
     print(name + " would like to climb " + response + ".")

运行结果为：

What is your name? Eric
Which mountain would you like to climb someday? Denali
Would you like to let another person respond? (yes/ no) yes

What is your name? Lynn
Which mountain would you like to climb someday? Devil's Thumb
Would you like to let another person respond? (yes/ no) no
--- Poll Results ---
Lynn would like to climb Devil's Thumb.
Eric would like to climb Denali.