Python 使用列表一部分（切片）

使用列表的一部分（切片）

• 处理列表的部分元素

切片

• 指定第一个元素的索引和最后一个元素索引加1
• 列表名[索引：索引+1] 索引加1：列表中第索引个元素 （左包括右不包括）
• 未指定索引 列表名[:] 提取列表所有元素（即复制列表）
• 未指定第一个索引：默认是从列表开头开始提取元素
• 未指定末尾元素：默认是提取到列表最后一个元素
• 负数索引返回离列表末尾相应距离的元素

# 切片案例
players = ['charles','martina','michael','florence','eli']
print(players[:])
print(players[0:3])
print(players[:3])
print(players[3:])
print(players[-3:])

['charles', 'martina', 'michael', 'florence', 'eli']
['charles', 'martina', 'michael']
['charles', 'martina', 'michael']
['florence', 'eli']
['michael', 'florence', 'eli']

遍历切片

• 遍历切片，在for循环中使用切片

# 遍历切片案例
players = ['charles','martina','michael','florence','eli']

for player in players[:3]:
    print(player)

charles
martina
michael

复制列表

• 既有列表，又要创建全新的列表
• 省略起始索引和终止索引[ : ],让Python创建一个始于第一个元素，终止于最后一个元素的切片

# 复制列表案例1：
my_foods = ['pizza','falafel','carrot cake']
friend_foods = my_foods[:]

print("my favorite foods are :")
print(my_foods)
print("my friend favorite foods are :")
print(friend_foods)

my favorite foods are :
['pizza', 'falafel', 'carrot cake']
my friend favorite foods are :
['pizza', 'falafel', 'carrot cake']

# 复制列表案例2：
my_foods = ['pizza','falafel','carrot cake']
friend_foods = my_foods[:]
my_foods.append('cannoli')
friend_foods.append('ice cream')

print("my favorite foods are :")
print(my_foods)
print("my friend favorite foods are :")
print(friend_foods)

my favorite foods are :
['pizza', 'falafel', 'carrot cake', 'cannoli']
my friend favorite foods are :
['pizza', 'falafel', 'carrot cake', 'ice cream']

# 简单地赋值列表，未用切片
# 不能得到两个列表
my_foods = ['pizza','falafel','carrot cake']
# 将friend_foods 关联到包含在my_foods中的列表，两个变量都只想同一个列表
friend_foods = my_foods
my_foods.append('cannoli')
friend_foods.append('ice cream')

print("my favorite foods are :")
print(my_foods)
print("my friend favorite foods are :")
print(friend_foods)

my favorite foods are :
['pizza', 'falafel', 'carrot cake', 'cannoli', 'ice cream']
my friend favorite foods are :
['pizza', 'falafel', 'carrot cake', 'cannoli', 'ice cream']

练习

# 1.切片
# 列表，使用切片打印前3个元素，打印中间三个元素，打印末尾三个元素
numbers = list(range(1,10))
print(numbers[:])
print(numbers[0:3])
print(numbers[3:6])
print(numbers[6:9])

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]

# 2.比萨
# 想出至少三种喜欢的比萨，将其名称存储在一个列表中，再使用for循环将每种比萨的名称都打印出来
# 修改这个for循环，使其打印包含比萨名称的句子，对于每种比萨都显示一行输出，如“I like pizza”
# 在程序末尾添加一行代码，不在for循环里，指出你有多喜欢比萨，输出应包含对每种比萨的消息，还有一个总结性句子，如“I reall love pizza”

pizzas = ['chess pizza','beef pizza','corn pizza','pepperoni pizza']
for pizza in pizzas:
    print(pizza)
print("#" * 20)
for pizza in pizzas:
    print("I like " + pizza.title() + "!")
print("\nI reall love pizza！")
##################################################
print("#" *50)
print("#" *50)
#################################################

# 创建比萨饼的副本，并将其存储到变量friend_pizza中，
# 在原来的比萨列表中添加一种比萨
# 在列表friend_pizzas中添加另一种比萨
# 核实两个不同的列表，为此打印消息“My favorite pizzas are:”,使用for循环打印第一个列表，打印消息“My friend's favorite pizzas are:”,在使用for循环
#打印第二个列表。
friend_pizza = pizzas[:]
pizzas.append('dog pizza')
friend_pizza.append('pig pizza')
print(pizzas)
print(friend_pizza)
print("My favorite pizzas are:")
for pizza in pizzas:
    print(pizza)
print("My friend's favorite pizzas are:")
for friend in friend_pizza:
    print(friend)

chess pizza
beef pizza
corn pizza
pepperoni pizza
####################
I like Chess Pizza!
I like Beef Pizza!
I like Corn Pizza!
I like Pepperoni Pizza!

I reall love pizza！
##################################################
##################################################
['chess pizza', 'beef pizza', 'corn pizza', 'pepperoni pizza', 'dog pizza']
['chess pizza', 'beef pizza', 'corn pizza', 'pepperoni pizza', 'pig pizza']
My favorite pizzas are:
chess pizza
beef pizza
corn pizza
pepperoni pizza
dog pizza
My friend's favorite pizzas are:
chess pizza
beef pizza
corn pizza
pepperoni pizza
pig pizza

# 3.使用多个循环
# 打印复制列表案例2中的元素
my_foods = ['pizza','falafel','carrot cake']
friend_foods = my_foods[:]
my_foods.append('cannoli')
friend_foods.append('ice cream')

print("my favorite foods are :")
for my_food in my_foods:
    print(my_food)
print("my friend favorite foods are :")
for friend_food in friend_foods:
    print(friend_food)

my favorite foods are :
pizza
falafel
carrot cake
cannoli
my friend favorite foods are :
pizza
falafel
carrot cake
ice cream