为什么扩展切片不能反转列表? [英] Why does extended slicing not reverse the list?

问题描述

我正在用python切片列表，无法解释某些结果. 以下两个对我来说似乎很自然:

I'm slicing lists in python and can't explain some results. Both of the following seem natural to me:

>>>[0,1,2,3,4,5][1:4:1]
[1, 2, 3]

>>>[0,1,2,3,4,5]
[::-1] == [5,4,3,2,1,0]

但是

>>>[0,1,2,3,4,5][1:4:-1]
[]

以为我希望它是[3,2,1].为什么会产生[]?为什么不反转列表?首先在python，步骤或切片内部发生什么?

thought I expected it to be [3,2,1]. Why does it produce [ ]? Why does it not reverse the list? What happens first inside python, the step or the slicing?

我也发现

>>>[0,1,2,3,4,5][-3:-6:-1]
[3,2,1]

推荐答案

如果要对此切片，则切片将类似于此[start:end:step].为此:

If you are slicing this then slicing will look like this [start:end:step]. For this one:

>>> [0,1,2,3,4,5][1:4:1]
[1, 2, 3]

它从索引1到索引3凝视着，因为它一次淘汰了索引4一步. 在第二个列表中您将获得一个空列表，因为您是从第一个索引开始以-1步进.因此，这将是解决方案.

It is staring from index 1 to index 3 because it exlcudes the index 4 with 1 step at a time. You are getting an empty list in the second one because you are stepping -1 from 1st index. So this will be the solution.

>>> [0,1,2,3,4,5][4:1:-1]
[4, 3, 2]

之所以起作用，是因为您将索引从4向前索引为-1，而前进了-1.

This works because you are taking an index from 4 to one with -1 step forward.

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