Getting the indexes of a numpy array slice

Question

How can I get an array of indexes from a slice of a 2d numpy array? For example:

array = np.arange(20).reshape(5,4)
section = array[1:,1:]

section.shape is (4, 3). I want an array indexes of dimensions (2, 4, 3), with the first axis being the indexes of each value from section in array.

indexes should look like:

array([[[1, 1, 1],
        [2, 2, 2],
        [3, 3, 3],
        [4, 4, 4]], 

       [[1, 2, 3],
        [1, 2, 3],
        [1, 2, 3],
        [1, 2, 3]]])

The indexes are needed so that I can modify it, then index array with array[indexes[0], indexes[1]].

I think this could be solved by slicing np.mgrid... but I'm not sure how this would be done. Or is there a better solution?

Paul Panzer · Accepted Answer · 2020-07-03 08:06:19Z

1

Using np.mgrid:

>>> i,j = array.shape
>>> np.mgrid[1:i,1:j]
array([[[1, 1, 1],
        [2, 2, 2],
        [3, 3, 3],
        [4, 4, 4]],

       [[1, 2, 3],
        [1, 2, 3],
        [1, 2, 3],
        [1, 2, 3]]])

answered Jul 3, 2020 at 8:06

Paul Panzer

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