python切片的奇怪行为 [英] Strange behavior with python slicing

问题描述

假设我们有以下列表:

>>> a = [x for x in range(10)]
>>> print(a)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

分别地，两种分割方法都可以按预期进行:

Separately, both ways to slice work as expected:

>>> a[3:8]
[3, 4, 5, 6, 7]

>>> a[::-1]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

但是，当结合使用时:

>>> a[3:8:-1]
[]

我希望它是[7, 6, 5 ,4, 3]或[6, 5, 4, 3, 2](如果首先发生反转). 考虑未传递start或stop参数时会发生什么，也很有趣:

I would expect it to be [7, 6, 5 ,4, 3] or perhaps [6, 5, 4, 3, 2] (if reversing happened first). It is also interesting to consider what happens when either start or stop parameters are not passed:

>>> a[:5:-1]
[9, 8, 7, 6]

这几乎是我所期望的，只有短一点.用numpy对此进行了测试，它的行为似乎与此相同.

This is almost what I would expect, only its one item short. Tested this with numpy and it seems to behave in the same way.

这是怎么回事?

推荐答案

使用

a[3:8:-1]

切片的开始和停止位置未根据步长进行调整.负数表示您使它从3向后退，但是没有索引在3到8范围内的元素从3倒数，因此您将获得一个空列表.

The start and stop positions of the slice aren't adjusted based on the step. With a negative step, you're having it go backwards from 3, but there are no elements with indices in the range 3 to 8 counting back from 3, so you get an empty list.

您需要相应地设置开始和停止位置:

You need to set the start and stop accordingly:

a[8:3:-1]

从8倒数到4.

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