打印在Python

Question

pizzas = ["hawai","salame","vegetable","capriciosa","new york"] 
for pizza in pizzas: 
    print("I like " + pizza.title() + " pizza!") 
print("\n" + "The first three pizzas in the list are: " + str(pizzas[0:3])) 
print("\n" + "The last three pizzas in the list are: " + str(pizzas[-1:-3])) 

最后三个项目的列表中，我得到：

I like Hawai pizza! 
I like Salame pizza! 
I like Vegetable pizza! 
I like Capriciosa pizza! 
I like New York pizza! 

The first three pizzas in the list are: ['hawai', 'salame', 'vegetable'] 

The last three pizzas in the list are: [] 

，我很困惑。是不是-1表示列表中的最后一个元素？我索引[start:stop]所以不应该打印我的最后3个项目？我究竟做错了什么 ？

Answer 1

您应该使用pizzas[-3:]来代替，以便您的start是列表末尾的第三个元素，而您的end是列表的最后一个。

pizzas = ["hawai","salame","vegetable","capriciosa","new york"] 

for pizza in pizzas: 
    print("I like " + pizza.title() + " pizza!") 

print("\nThe first three pizzas in the list are: " + str(pizzas[:3])) 
print("\nThe last three pizzas in the list are: " + str(pizzas[-3:])) 

Answer 2

默认步骤是1，所以从-1到-3，步骤为1返回空片。你可以明确地指示步骤为-1，但这种反转项目的顺序：

>>> pizzas[-1:-3:-1] 
['new york', 'capriciosa'] # -3 excluded 

但是，要获得最后三个项目，你要的是pizza[-3:]