python编程：从入门到实践 （第一版） 第五章学习笔记

第五章：if 语句

示例

并以首字母大写的方式打印其中的汽车名，但对于汽车名’bmw’ ，以全大写的方式打印

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

检查特定值是否包含在列表中

>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'pepperoni' in requested_toppings
False

检查特定值是否不包含在列表中

>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'pepperoni' not in requested_toppings
True

if-elif-else 结构

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

省略else 代码块

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) + ".")

注意： else 是一条包罗万象的语句，只要不满足任何if 或elif 中的条件测试，其中的代码就会执行，这可能会引入无效甚至恶意的数据。如果知道最终要测试的条件，应考虑使用一个elif 代码块来代替else 代码块。这样，你就可以肯定，仅当满足相应的条件时，你的代码才会执行。

确定列表不是空的

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 。

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?")

使用多个列表

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!

习题

5-2

car = 'Audi'
if car == 'Audi':
	print("This is an Audi car.")
elif car == 'bmw':
	print("This is not an Audi car.")
print("---------------------------------------------")
print(car == 'audi')
print(car.lower() == 'audi')
print("---------------------------------------------")
number = 50
print(number == 35)
print(number != 35)
print(number > 35)
print(number >= 35)
print(number < 35)
print(number <= 35)
print(number > 35 and number < 40)
print(number > 35 or number < 35)
print("--------------------------------------------")
cars = ['audi', 'bmw', 'toyota', 'subaru']
if 'honda' in cars:
	print("本田在列表中")
if 'honda' not in cars:
	print("本田不在列表中")

5-3

#版本1 测试通过
alien_color = 'green'
if alien_color == 'green':
	print("You get five points")

#版本2 测试未通过
alien_color = 'red'
if alien_color == 'green':
	print("You get five points")

5-4

#版本1 仅使用if
alien_color = 'yellow'
if alien_color == 'green':
	print("You get five points")
if alien_color == 'red':
	print("You get ten points")
if alien_color == 'yellow':
	print("You get ten points")

#版本2 使用if-else
alien_color = 'yellow'
if alien_color == 'green':
	print("You get five points")
else:
	print("You get ten points")

5-5

alien_color = 'red'
if alien_color == 'green':
	print("You get five points")
elif alien_color == 'yellow':
	print("You get ten points")
else :
	print("You get fifteen points")

5-6

age = 65
if age < 2:
    print("婴儿")
elif age < 4:
    print("蹒跚学步")
elif age < 13:
    print("儿童")
elif age < 20:
    print("青少年")
elif age < 65:
    print("成年人")
else:
    print("老年人")

5-7

favorite_fruits = ['apple', 'peach', 'banana']
test_fruits = ['apple', 'pear', 'strawberry', 'peach', 'banana']
for test_fruit in test_fruits:
    if test_fruit in favorite_fruits:
        print("You really like " + test_fruit + "!")

5-8

users = ['admin', 'Tom', 'Eric', 'Jack', 'Bruce']
for user in users:
    if user == 'admin':
        print("Hello " + user + ",would you liketo see a status report?")
    else:
        print("Hello " + user + ",thank you for logging in again.")

5-9

users = []
if users:
    for user in users:
        if user == 'admin':
            print("Hello " + user + ",would you liketo see a status report?")
        else:
            print("Hello " + user + ",thank you for logging in again.")
else:
    print("We need to find some users!")

5-10

current_users = ['Tom', 'JACK', 'Bruce', 'JERRY', 'Sam']
new_users = ['david', 'Jack', 'jerry', 'Smith','John']
current_users2=[]
for current_user in current_users:
    current_users2.append(current_user.title())
for new_user in new_users:
    if new_user.title() in current_users2:
        print(new_user.title() + " has been used!You need to input other usernames")
    else:
        print(new_user.title() + " has not been used.")

5-11

numbers = list(range(1, 11))
for number in numbers:
    print(number)
print("-------------------------------")
for number in numbers:
    if number == 1:
        print(str(number) + "st")
    elif number == 2:
        print(str(number) + "nd")
    elif number == 3:
        print(str(number) + "rd")
    else:
        print(str(number) + "th")