Python编程：从入门到实践(5-7)
5. If 语句
cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
if car == 'bmw':
print(car.upper())
else:
print(car.title())
输出：
Audi
BMW
Subaru
Toyota
requested_topping = 'mushrooms'
if requested_topping != 'anchovies':
print("Hold the anchovies!")
输出：
Hold the anchovies!
>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 and age_1 >= 21
False
>>> age_1 = 22
>>> age_0 >= 21 and age_1 >= 21
True>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 or age_1 >= 21
True
>>> age_0 = 18
>>> age_0 >= 21 or age_1 >= 21
False>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'pepperoni' in requested_toppings
Falsebanned_users = ['andrew', 'carolina', 'david']
user = 'marie'
if user not in banned_users:
print(user.title() + ", you can post a response if you wish.")
输出：
Marie, you can post a response if you wish.
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!")
输出：
Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!
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.")
输出：
Your admission cost is $5.age = 12
if age < 4:
price = 0
elif age < 18:
price = 5
elif age < 65:
price = 10
elif age >= 65:
price = 5
print("Your admission cost is $" + str(price) + ".")requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
print("Adding " + requested_topping + ".")
print("\nFinished making your pizza!")
输出：
Adding mushrooms.
Adding green peppers.
Adding extra cheese.
Finished making your pizza!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!")
输出：
Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.
Finished making your pizza!requested_toppings = []
if requested_toppings:
for requested_topping in requested_toppings:
print("Adding " + requested_topping + ".")
print("\nFinished making your pizza!")
else:
print("Are you sure you want a plain pizza?")
输出：
Are you sure you want a plain pizza?available_toppings = ['mushrooms', 'olives', 'green peppers', 'pepperoni', 'pineapple', 'extra cheese']
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']
for requested_topping in requested_toppings:
if requested_topping in available_toppings:
print("Adding " + requested_topping + ".")
else:
print("Sorry, we don't have " + requested_topping + ".")
print("\nFinished making your pizza!")
输出：
Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.
Finished making your pizza!6. 字典
alien_0 = {'color': 'green', 'points': 5}
print(alien_0['color'])
print(alien_0['points'])
输出：
green
5字典是一种动态结构，可随时在其中添加键—值对。要添加键—值对，可依次指定字典名、用方括号括起的键和相关联的值。
Python不关心键—值对的添加顺序，而只关心键和值之间的关联关系。
alien_0 = {'color': 'green', 'points': 5}
print(alien_0)
alien_0['x_position'] = 0
alien_0['y_position'] = 25
print(alien_0)
输出：
{'color': 'green', 'points': 5}
{'color': 'green', 'points': 5, 'y_position': 25, 'x_position': 0}alien_0 = {}
alien_0['color'] = 'green'
alien_0['points'] = 5
print(alien_0)
输出：
{'color': 'green', 'points': 5}alien_0 = {'color': 'green'}
print("The alien is " + alien_0['color'] + ".")
alien_0['color'] = 'yellow'
print("The alien is now " + alien_0['color'] + ".")
输出：
The alien is green.
The alien is now yellow.alien_0 = {'color': 'green', 'points': 5}
print(alien_0)
del alien_0['points']
print(alien_0)
输出：
{'color': 'green', 'points': 5}
{'color': 'green'}favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
print("Sarah's favorite language is " +
favorite_languages['sarah'].title() + ".")
输出：
Sarah's favorite language is C.user_0 = {
'username': 'efermi',
'first': 'enrico',
'last': 'fermi',
}
for key, value in user_0.items():
print("\nKey: " + key)
print("Value: " + value)
输出：
Key: last
Value: fermi
Key: first
Value: enrico
Key: username
Value: efermifavorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
for name, language in favorite_languages.items():
print(name.title() + "'s favorite language is " + language.title() + ".")
输出：
Jen's favorite language is Python.
Sarah's favorite language is C.
Phil's favorite language is Python.
Edward's favorite language is Ruby.favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
for name in favorite_languages.keys():
print(name.title())
输出：
Jen
Sarah
Phil
Edwardfavorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
friends = ['phil', 'sarah']
for name in favorite_languages.keys():
print(name.title())
if name in friends:
print(" Hi " + name.title() + ", I see your favorite language is " +
favorite_languages[name].title() + "!")
输出：
Edward
Phil
Hi Phil, I see your favorite language is Python!
Sarah
Hi Sarah, I see your favorite language is C!
Jenfavorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
for name in sorted(favorite_languages.keys()):
print(name.title() + ", thank you for taking the poll.")
输出：
Edward, thank you for taking the poll.
Jen, thank you for taking the poll.
Phil, thank you for taking the poll.
Sarah, thank you for taking the poll.favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
print("The following languages have been mentioned:")
for language in favorite_languages.values():
print(language.title())
输出：
The following languages have been mentioned:
Python
C
Python
Rubyfavorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
print("The following languages have been mentioned:")
for language in set(favorite_languages.values()):
print(language.title())
输出：
The following languages have been mentioned:
Python
C
Rubyalien_0 = {'color': 'green', 'points': 5}
alien_1 = {'color': 'yellow', 'points': 10}
alien_2 = {'color': 'red', 'points': 15}
aliens = [alien_0, alien_1, alien_2]
for alien in aliens:
print(alien)
输出：
{'color': 'green', 'points': 5}
{'color': 'yellow', 'points': 10}
{'color': 'red', 'points': 15}# 创建一个用于存储外星人的空列表
aliens = []
# 创建30个绿色的外星人
for alien_number in range(30):
new_alien = {'color': 'green', 'points': 5, 'speed': 'slow'}
aliens.append(new_alien)
# 显示前五个外星人
for alien in aliens[:5]:
print(alien)
print("...")
# 显示创建了多少个外星人
print("Total number of aliens: " + str(len(aliens)))
输出：
{'speed': 'slow', 'color': 'green', 'points': 5}
{'speed': 'slow', 'color': 'green', 'points': 5}
{'speed': 'slow', 'color': 'green', 'points': 5}
{'speed': 'slow', 'color': 'green', 'points': 5}
{'speed': 'slow', 'color': 'green', 'points': 5}
...
Total number of aliens: 30# 存储所点比萨的信息
pizza = {
'crust': 'thick',
'toppings': ['mushrooms', 'extra cheese'],
}
# 概述所点的比萨
print("You ordered a " + pizza['crust'] + "-crust pizza " + "with the following toppings:")
for topping in pizza['toppings']:
print("\t" + topping)
输出：
You ordered a thick-crust pizza with the following toppings:
mushrooms
extra cheeseusers = {
'aeinstein': {
'first': 'albert',
'last': 'einstein',
'location': 'princeton',
},
'mcurie': {
'first': 'marie',
'last': 'curie',
'location': 'paris',
},
}
for username, user_info in users.items():
print("\nUsername: " + username)
full_name = user_info['first'] + " " + user_info['last']
location = user_info['location']
print("\tFull name: " + full_name.title())
print("\tLocation: " + location.title())
输出：
Username: aeinstein
Full name: Albert Einstein
Location: Princeton
Username: mcurie
Full name: Marie Curie
Location: Paris7. 用户输入和while循环
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。
message = input("Tell me something, and I will repeat it back to you: ")
print(message)
输出：
Tell me something, and I will repeat it back to you: Hello everyone!
Hello everyone!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 + "!")>>> age = input("How old are you? ")
How old are you? 21
>>> age = int(age)
>>> age >= 18
True>>> 4 % 3
1
>>> 5 % 3
2
>>> 6 % 3
0
>>> 7 % 3
1
你可以使用while 循环来数数，例如，下面的while 循环从1数到5：
current_number = 1
while current_number <= 5:
print(current_number)
current_number += 1
输出：
1
2
3
4
5
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)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)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() + "!")current_number = 0
while current_number < 10:
current_number += 1
if current_number % 2 == 0:
continue
print(current_number)x = 1
while x <= 5:
print(x)
x += 1
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())
输出：
Verifying user: Candace
Verifying user: Brian
Verifying user: Alice
The following users have been confirmed:
Candace
Brian
Alicepets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
pets.remove('cat')
print(pets)
输出：
['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
['dog', 'dog', 'goldfish', 'rabbit']
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 + ".")
输出：
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.