为什么我在 Python 中的负切片不起作用? [英] Why does my negative slicing in Python not work?

问题描述

我是 Python 新手，已经阅读了一些关于切片的教程，但是我在空闲时运行的示例似乎没有返回我期望的结果.例如，我已将以下列表分配给变量 a

I am new to Python and have read several tutorials on slicing however the examples I run in idle don't seem to return what I expect it to. For example I have assigned the follow list to the variable a

a=[0,1,2,3,4,5,6,7,8,9]

现在我将切片理解为 [number I want to include:number up to and don't want to include:step]

因此，如果我执行 a[1]，我会期望 1.如果我做 a[1:3]，它会是 1,2

Hence if I do a[1], I would expect 1. If I do a[1:3], it would be 1,2

现在如果我做 a[-1]，我得到 9 但是如果我做 a[-1:-5]，我什么也得不到.我看到的只是[].这是为什么?我希望看到 9,8,7,6

Now if I do a[-1], I get 9 BUT if I do a[-1:-5], I get nothing. All I see is []. why is that? I would expect to see 9,8,7,6

我在 Windows 7 Professional 上运行 Python 2.7

I am running Python 2.7 on Windows 7 Professional

推荐答案

在这种情况下，您需要添加 step 参数以获得您想要的:

In this case, you would need to add in the step argument in order to get what you want:

In [1]: a=[0,1,2,3,4,5,6,7,8,9]

In [2]: a[-1:-5:-1]
Out[2]: [9, 8, 7, 6]

如果你想节省更多的空间，你可以省略第一个参数:

And if you want to save a bit more space, you can omit the first argument:

In [3]: a[:-5:-1]
Out[3]: [9, 8, 7, 6]

Python 处理负"切片的方式是将对象的 len 添加到负数.因此，当您说 In a[-1:-5] 时，它基本上是在说 a[(-1+10):(-5+10)]，即等于 a[9:5]，并且由于 start/end 指的是两者之间的所有字符(在列表中向前"移动)，它不会返回任何内容(因此是您的空白列表).您可以通过执行以下操作来查看这一点:

The way that Python handles the 'negative' slices is that it adds then len of the object to the negative number. So when you say In a[-1:-5], it is basically saying a[(-1+10):(-5+10)], which equals a[9:5], and since start/end refers to all characters between the two (moving 'forward' through the list), it doesn't return anything (hence your blank list). You can see this by doing something like:

In [5]: a[-5:9]
Out[5]: [5, 6, 7, 8]

In [6]: a[5:9]
Out[6]: [5, 6, 7, 8]

使用负索引和正索引得到相同的结果，因为 -5 + 10 = 5.

You get the same result with the negative and positive indices, since -5 + 10 = 5.

提供 -1 step 参数告诉它从第一个元素开始，但从开始位置向后移动.

Providing the -1 step argument tells it to start at the first element but move backwards from the start position.

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