7 第七章 用户输入和while循环
函数input()的工作原理
函数input() 让程序暂停运行,等待用户输入文本,在获取用户输入后,函数input() 将输入内容返回,可用变量接收。
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!
每当使用函数input() 时,都应指定"清晰而易于明白的提示",准确地指出你希望用户提供什么样的信息——指出用户该输入任何信息的提示都行,如下所示:
name = input("Please enter your name: ")
print("Hello, " + name + "!")
Please enter your name: Eric
Hello, Eric!
有时候,提示可能超过一行,例如,你可能需要指出获取特定输入的原因。在这种情况下,可将提示存储在一个变量中,再将该变量传递给函数input() 。这样,即便提示超过
一行,input() 语句也非常清晰。
prompt = "If you tell us who you are, we can personalize the messages you see."
prompt += "\nWhat is your first name? "
name = input(prompt)
print("\nHello, " + name + "!")
If you tell us who you are, we can personalize the messages you see.
What is your first name? Eric
Hello, Eric!
使用函数input() 时,Python将用户输入解读为字符串;
函数int() 将 数字的字符串表示 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.")
How tall are you, in inches? 71
You're tall enough to ride!
求模运算符(%)是一个很有用的工具,它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.")
Enter a number, and I'll tell you if it's even or odd: 42
The number 42 is even.
while简介
for 循环用于针对集合中的每个元素都一个代码块;
while 循环不断地运行,直到指定的条件不满足为止。
current_number = 1
while current_number <= 5:
print(current_number)
current_number += 1
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)
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
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 ,充当了程序的交通信号灯。你可让程序在标志
为True 时继续运行,并在任何事件导致标志的值为False 时让程序停止运行。这样,在while 语句中就只需检查一个条件——标志的当前值是否为True ,并将所有测试(是
否发生了应将标志设置为False 的事件)都放在其他地方,从而让程序变得更为整洁。
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)
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 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() + "!")
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。
continue语句: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
current_number = 0
while current_number < 10:
current_number += 1
if current_number % 2 == 0:
continue
print(current_number)
使用while循环来处理列表和字典
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。
# 首先,创建一个待验证用户列表
# 和一个用于存储已验证用户的空列表
unconfirmed_users = ['alice', 'brian', 'candace']
confirmed_users = []
# 验证每个用户,直到没有未验证用户为止
# 将每个经过验证的列表都移到已验证用户列表中
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("\nThe following users have been confirmed:")
for confirmed_user in confirmed_users:
print(confirmed_user.title())
删除包含特定值的所有列表元素
pets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
pets.remove('cat')
print(pets)
使用用户输入来填充字典
responses = {}
# 设置一个标志,指出调查是否继续
polling_active = True
while polling_active:
# 提示输入被调查者的名字和回答
name = input("\nWhat is your name? ")
response = input("Which mountain would you like to climb someday? ")
# 将答卷存储在字典中
responses[name] = response
# 看看是否还有人要参与调查
repeat = input("Would you like to let another person respond? (yes/ no) ")
if repeat == 'no':
polling_active = False
# 调查结束,显示结果
print("\n--- Poll Results ---")
for name, response in responses.items():
print(name + " would like to climb " + response + ".")
小结
如何在程序中使用 input() 来让用户提供信息;
如何处理文本和数字输入,以及如何使用while 循环让程序按用户的要求不断地运行;
多种控制while循环流程的方式:设置活动标志、使用 break 语句以及使用 continue 语句;
如何使用 while 循环在列表之间移动元素,以及如何从列表中删除所有包含特定值的元素;
如何结合使用 while 循环和字典