Python（2.x）列表/子列表选择-1怪异

发布于 2024-09-13 12:32:22 字数 521 浏览 11 评论 0原文

所以我一直在玩Python并注意到一些看起来有点奇怪的事情。从列表中选择时 -1 的语义似乎不一致。

所以我有一个数字列表

ls = range(1000)

列表的最后一个元素当然是 ls[-1] 但如果我采用它的子列表，这样我就可以得到从中点到末尾的所有内容这样做

ls[500:-1]

，但这并没有给我一个包含列表中最后一个元素的列表，而是一个包含直到最后一个元素的所有内容的列表。但是，如果我这样做，

ls[0:10]

我会得到一个还包含第十个元素的列表（因此选择器应该包含在内），那么为什么它不适用于 -1。

我当然可以做 ls[500:] 或 ls[500:len(ls)] （这很愚蠢）。我只是想知道 -1 的处理是什么，我意识到我在那里不需要它。

原文

So I've been playing around with python and noticed something that seems a bit odd. The semantics of -1 in selecting from a list don't seem to be consistent.

So I have a list of numbers

ls = range(1000)

The last element of the list if of course ls[-1] but if I take a sublist of that so that I get everything from say the midpoint to the end I would do

ls[500:-1]

but this does not give me a list containing the last element in the list, but instead a list containing everything UP TO the last element. However if I do

ls[0:10]

I get a list containing also the tenth element (so the selector ought to be inclusive), why then does it not work for -1.

I can of course do ls[500:] or ls[500:len(ls)] (which would be silly). I was just wondering what the deal with -1 was, I realise that I don't need it there.

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破晓 2024-09-20 12:32:22

在 list[first:last] 中，不包含 last。

第 10 个元素是 ls[9]，ls[0:10] 中没有 ls[10]。

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情话难免假 2024-09-20 12:32:22

如果您想获取包含最后一个元素的子列表，请在冒号后留空：

>>> ll=range(10)
>>> ll
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> ll[5:]
[5, 6, 7, 8, 9]
>>> ll[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If you want to get a sub list including the last element, you leave blank after colon:

>>> ll=range(10)
>>> ll
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> ll[5:]
[5, 6, 7, 8, 9]
>>> ll[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

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倾听心声的旋律 2024-09-20 12:32:22

对于这两个实例，我得到了一致的行为：

>>> ls[0:10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> ls[10:-1]
[10, 11, 12, 13, 14, 15, 16, 17, 18]

但请注意，列表的第十个元素位于索引 9 处，因为列表是 0 索引的。这可能就是你的困扰所在。

换句话说，[0:10] 并不是从索引 0-10 开始，它实际上是从 0 到第十个元素（这会得到索引 0-9，因为 10 不包含在内）在切片的末尾）。

I get consistent behaviour for both instances:

>>> ls[0:10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> ls[10:-1]
[10, 11, 12, 13, 14, 15, 16, 17, 18]

Note, though, that tenth element of the list is at index 9, since the list is 0-indexed. That might be where your hang-up is.

In other words, [0:10] doesn't go from index 0-10, it effectively goes from 0 to the tenth element (which gets you indexes 0-9, since the 10 is not inclusive at the end of the slice).

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他不在意 2024-09-20 12:32:22

对我来说这似乎非常一致；正指数也不包含在内。我认为你做错了。请记住 range() 也是非包容性的，并且 Python 数组是从 0 索引的，下面是一个示例 Python 会话来说明：

>>> d = range(10)
>>> d
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> d[9]
9
>>> d[-1]
9
>>> d[0:9]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> d[0:-1]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> len(d)
10

It seems pretty consistent to me; positive indices are also non-inclusive. I think you're doing it wrong. Remembering that range() is also non-inclusive, and that Python arrays are 0-indexed, here's a sample python session to illustrate:

>>> d = range(10)
>>> d
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> d[9]
9
>>> d[-1]
9
>>> d[0:9]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> d[0:-1]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> len(d)
10

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