Python：切片多维数组
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Python: slicing a multi-dimensional array
提问by SlightlyCuban
I'm new to Python and numpy. I've figured out how to slice 1 dimensional sequence: arr[start:end], and access an element in the array: el = arr[row][col].
我是 Python 和 numpy 的新手。我已经想出了如何对一维序列进行切片：arr[start:end]，并访问数组中的元素：el = arr[row][col]。
Trying something like slice = arr[0:2][0:2](where arris a numpy array) doesn't give me the first 2 rows and columns, but repeats the first 2 rows. What did I just do, and how do I slice along another dimension?
尝试类似slice = arr[0:2][0:2](where arris a numpy array) 不会给我前 2 行和列，而是重复前 2 行。我刚刚做了什么，我如何沿着另一个维度切片？
采纳答案by mgilson
If you use numpy, this is easy:
如果您使用numpy，这很容易：
slice = arr[:2,:2]
or if you want the 0's,
或者如果你想要0，
slice = arr[0:2,0:2]
You'll get the same result.
你会得到同样的结果。
*note that sliceis actually the name of a builtin-type. Generally, I would advise giving your object a different "name".
*请注意，这slice实际上是内置类型的名称。一般来说，我会建议给你的对象一个不同的“名称”。
Another way, if you're working with lists of lists*:
另一种方式，如果您正在使用列表列表*：
slice = [arr[i][0:2] for i in range(0,2)]
(Note that the 0's here are unnecessary: [arr[i][:2] for i in range(2)]would also work.).
（请注意，这里的 0 是不必要的： [arr[i][:2] for i in range(2)]也可以。）。
What I did here is that I take each desired row 1 at a time (arr[i]). I then slice the columns I want out of that row and add it to the list that I'm building.
我在这里所做的是一次获取每个所需的第 1 行 ( arr[i])。然后我从该行中切出我想要的列并将其添加到我正在构建的列表中。
If you naively try: arr[0:2]You get the first 2 rows which if you then slice again arr[0:2][0:2], you're just slicing the first two rows over again.
如果你天真地尝试：arr[0:2]你得到前两行，如果你再次切片arr[0:2][0:2]，你只是再次切片前两行。
*This actually works for numpy arrays too, but it will be slow compared to the "native" solution I posted above.
*这实际上也适用于 numpy 数组，但与我上面发布的“本机”解决方案相比，它会很慢。
