《Python编程：从入门到实践（第3版）》读书笔记6：用户输入和while循环

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《Python编程：从入门到实践（第3版）》读书笔记6：用户输入和while循环 [复制链接]

第7章     用户输入和while循环

7.1    input()函数的工作原理

在 Python 中，‘input()’ 函数用于从标准输入（通常是键盘）读取用户输入，并将输入内容以字符串形式返回。

1. 基本工作原理
程序暂停：当执行到 ’input()`‘时，程序会暂停，等待用户输入。
输入确认：用户输入后需按 回车键（Enter），输入内容会被读取。
返回值：输入的文本会作为字符串返回（不包含末尾的换行符）。

示例

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name = input("请输入你的名字：")  # 显示提示信息并等待输入
print(f"你好，{name}！")

2. 输入内容处理
始终返回字符串：无论输入数字、符号还是文本，’input()‘ 均返回字符串。
类型转换：需手动转换数据类型（如 ‘int()’、‘float()’、‘list()’ 等）。

示例

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age = input("请输入年龄：")
age_int = int(age)  # 转为整数（若输入非数字会报错）

3. 安全输入与错误处理
验证输入格式：防止类型转换错误（如用户输入非数字时转’int‘会崩溃）。
使用’try-except‘捕获异常

示例

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while True:
    try:
        num = int(input("请输入整数："))
        break
    except ValueError:
        print("输入无效，请重新输入！")

练习 7.1 汽车租赁

编写一个程序，询问用户要租什么样的汽车，并打印一条消息，如下所示。

Let me see if I can find you a Subaru

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car = input("What kind of car would you like? ")
print(f"Let me see if I can find you a {car.title()}.")

运行结果：

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What kind of car would you like? audi
Let me see if I can find you a Audi.

练习 7.2 餐馆订位

编写一个程序，询问用户有多少人用餐。如果超过8个人，就打印一条消息，指出没有空桌；否则指出有空桌。

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party_size = input("How many people are in your dinner party tonight? ")
party_size = int(party_size)
if party_size > 8:
    print("I'm sorry, you'll have to wait for a table.")
else:
    print("Your table is ready.")

运行结果：

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How many people are in your dinner party tonight? 7
Your table is ready.

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How many people are in your dinner party tonight? 12
I'm sorry, you'll have to wait for a table.

练习 7.3 10 的整数倍

让用户输入一个数，并指出这个数是否是 10 的整数倍。

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number = input("input a number:")
number = int(number)

if number % 10 == 0:
    print(f"{number} is a multiple of 10.")
else:
    print(f"{number} is not a multiple of 10.")

运行结果：

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input a number:20
20 is a multiple of 10.

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input a number:12
12 is not a multiple of 10.

7.2    while循环

在 Python 中，‘while’ 循环用于重复执行代码块，直到条件不再满足。

基本语法

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while 条件:
    # 循环体代码
else:
    # 循环正常结束后执行（非 break 退出时触发）

示例

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count = 0
while count < 5:
    print(f"当前计数: {count}")
    count += 1
else:
    print("循环正常结束")

运行结果

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当前计数: 0
当前计数: 1
当前计数: 2
当前计数: 3
当前计数: 4
循环正常结束

 核心特点
条件驱动：只要条件为 ’True‘，循环持续执行。
灵活性：适用于不确定循环次数的场景（如用户输入验证、实时监控）。
手动控制：需在循环体内更新条件变量，避免无限循环。

 常见应用场景
(1) 用户输入验证

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while True:
    user_input = input("请输入 'yes' 退出：")
    if user_input.lower() == "yes":
        break
    print("输入无效，请重试！")

(2) 处理动态数据

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data = [1, 2, 3, 4, 5]
while data:
    item = data.pop()
    print(f"处理元素: {item}")

‘break’和‘continue’

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num = 0
while num < 10:
    num += 1
    if num == 3:
        continue  # 跳过本次循环剩余代码
    if num == 7:
        break     # 退出循环
    print(num)
# 输出: 1 2 4 5 6

避免无限循环

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# 条件变量未更新
count = 0
while count < 5:
    print("卡在循环中！")  # count 始终为 0

正确写法

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count = 0
while count < 5:
    print("正常执行")
    count += 1  # 必须更新条件变量

 性能优化技巧：使用标志变量，简化复杂退出条件

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running = True
while running:
    if error_occurred:
        running = False

练习 7.4 比萨配料

编写一个循环，提示用户输入一系列比萨配料，并在用户输入'quit'时结束循环。每当用户输入一种配料后，都打印一条消息，指出要在比萨中添加这种配料。

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prompt = "\nWhat topping would you like on your pizza?"
prompt += "\nEnter 'quit' when you are finished: "
while True:
    topping = input(prompt)
    if topping != 'quit':
       print(f" I'll add {topping} to your pizza.")
    else:
        break

运行结果：

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What topping would you like on your pizza?
Enter 'quit' when you are finished: 2
 I'll add 2 to your pizza.

What topping would you like on your pizza?
Enter 'quit' when you are finished: QUIT
 I'll add QUIT to your pizza.

What topping would you like on your pizza?
Enter 'quit' when you are finished: quit

练习 7.5 电影票

有家电影院根据观众的年龄收取不同的票价：不到3岁的观众免费；3（含）～12岁的观众收费10美元；年满12岁的观众收费15美元。请编写一个循环，在其中询问用户的年龄，并指出

其票价。

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prompt = "\nHow old are you?"
prompt += "\nEnter 'quit' when you are finished. "
while True:
    age = input(prompt)
    if age == 'quit':
        break
    age = int(age)
    if age < 3:
        print(" You get in free!")
    elif age < 12:
        print(" Your ticket is $10.")
    else:
        print(" Your ticket is $15.")

运行结果：

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How old are you?
Enter 'quit' when you are finished. 12
 Your ticket is $15.

How old are you?
Enter 'quit' when you are finished. 4
 Your ticket is $10.

How old are you?
Enter 'quit' when you are finished. 0
 You get in free!

How old are you?
Enter 'quit' when you are finished. quit

7.3    使用while循环处理列表和字典

在 Python 中，`while` 循环可以灵活处理列表和字典，尤其适用于需要动态修改容器内容或条件复杂的场景。

1. 处理列表
(1) 遍历列表

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fruits = ["apple", "banana", "cherry"]
index = 0
while index < len(fruits):
    print(fruits[index])
    index += 1
# 输出: apple banana cherry

(2) 动态修改列表

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# 删除所有偶数
numbers = [1, 2, 3, 4, 5, 6]
i = 0
while i < len(numbers):
    if numbers[i] % 2 == 0:
        del numbers[i]  # 删除元素后，后续元素前移，索引不需递增
    else:
        i += 1          # 未删除时递增索引
print(numbers)  # 输出: [1, 3, 5]

(3) 条件筛选

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# 从后向前遍历（避免索引错位）
data = [10, 20, 30, 40, 50]
i = len(data) - 1
while i >= 0:
    if data[i] > 25:
        data[i] = data[i] * 2
    i -= 1
print(data)  # 输出: [10, 20, 60, 80, 100]

2. 处理字典
(1) 遍历字典键

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person = {"name": "Alice", "age": 30, "city": "New York"}
keys = list(person.keys())  # 转换为列表避免遍历时修改错误
index = 0
while index < len(keys):
    key = keys[index]
    print(f"{key}: {person[key]}")
    index += 1
# 输出: name: Alice \n age:30 \n city:New York

(2) 动态删除键值对

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# 删除值为空的键
user_data = {"name": "Bob", "email": "", "age": 25}
keys = list(user_data.keys())  # 先固定键列表
index = 0
while index < len(keys):
    key = keys[index]
    if not user_data[key]:  # 值为空则删除
        del user_data[key]
    index += 1
print(user_data)  # 输出: {'name': 'Bob', 'age': 25}

(3) 条件更新字典

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# 将所有数值类型的值翻倍
data = {"a": 10, "b": "hello", "c": 3.14, "d": [1, 2]}
keys = list(data.keys())
index = 0
while index < len(keys):
    key = keys[index]
    if isinstance(data[key], (int, float)):
        data[key] *= 2
    index += 1
print(data)  # 输出: {'a':20, 'b':'hello', 'c':6.28, 'd':[1,2]}

练习 7.8 熟食店

创建一个名为sandwich_orders的列表，其中包含各种三明治的名字，再创建一个名为finished_sandwiches的空列表。遍历列表sandwich_orders，对于其中的每种三明治，都打印一条消息，如“I made your tuna sandwich.”，并将其移到列表finished_sandwiches中。当所有三明治都制作好后，打印一条消息，将这些三明治列出来。

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sandwich_orders = ['a','b','c']
finished_sandwiches = []
while sandwich_orders:
    sandwich = sandwich_orders.pop()
    finished_sandwiches.append(sandwich)
    print(f'I made your {sandwich} sandwich')

print("\n")
for sandwich in finished_sandwiches:
    print(f"I made a {sandwich} sandwich.")

运行结果

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I made your c sandwich
I made your b sandwich
I made your a sandwich


I made a c sandwich.
I made a b sandwich.
I made a a sandwich.

练习 7.9 五香烟熏牛肉卖完了

使用为练习7.8创建的列表sandwich_orders，并确保'pastrami'在其中至少出现了三次。在程序开头附近添加这样的代码：先打印一条消息，指出熟食店的五香烟熏牛肉（pastrami）卖完了；再使用一个while循环将列表sandwich_orders 中的'pastrami'都删除。确认最终的列表finished_sandwiches中未包含'pastrami'。

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sandwich_orders = ['a','b','c','pastrami','pastrami','pastrami']
finished_sandwiches = []
print("I'm sorry, we're all out of pastrami today.")
while 'pastrami' in sandwich_orders:
    sandwich_orders.remove('pastrami')

while sandwich_orders:
    sandwich = sandwich_orders.pop()
    finished_sandwiches.append(sandwich)
    print(f'I made your {sandwich} sandwich')

print("\n")
for sandwich in finished_sandwiches:
    print(f"I made a {sandwich} sandwich.")

练习 7.10 梦想中的度假胜地

编写一个程序，调查用户梦想中的度假胜地。使用类似于“If you could visit one place in the world, where would you go?”的提示，并编写一个打印调查结果的代码块。

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name_prompt = "\nWhat's your name?"
place_prompt = "If you could visit one place in the world, where would it be? "
continue_prompt = "Would you like to let someone else respond? (yes/no) "
response = {}

while True:
    name = input(name_prompt)
    place = input(place_prompt)
    response[name] = place

    repeat = input(continue_prompt)
    if repeat == 'no':
        break

print('\n')
for name,place in response.items():
    print(f"{name.title()} would like to visit {place.title()}.")

运行结果

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What's your name?zzy
If you could visit one place in the world, where would it be? hongkong
Would you like to let someone else respond? (yes/no) yes

What's your name?zzyy
If you could visit one place in the world, where would it be?  shenyang
Would you like to let someone else respond? (yes/no) no 


Zzy would like to visit Hongkong.
Zzyy would like to visit  Shenyang.

 

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