Delete 2d subarray from 3d array in numpy

Question

In numpy I have a 3d array and I would ike to remove some of the 2d subarrays. Think about it like this:

r = range(27)
arr = np.reshape(r, (3,3,3))

del = [[0,1,2],[0,0,2]]
flatSeam = np.ravel_multi_index(del, arr.shape)
arr = np.delete(arr, flatSeam)

So at the end I would like to have an array of the shape (3,2,3) without the elements 00, 10, 22 from the original array. My problem is that I acn not use ravel_multi_index for this, because my indices are 2d and the array shape is 3d, so the wrong indices are calculated (the code above also does not execute because the indices array and the shape have to be the same size).

Do you have any ideas how I can achieve this?

So, the first row of del would always be range(arr.shape[1]), like you have [0,1,2], here ? — Divakar
– Divakar, Commented Nov 15, 2016 at 13:51
The first row of del should be range(arr.shape[0]). So first row are the row indices and the second row are the column indices. — Cilenco
– Cilenco, Commented Nov 15, 2016 at 13:55

Divakar · Accepted Answer · 2016-11-15 14:36:31Z

3

Here's an approach using advanced-indexing -

# arr: Input array, rm_idx : 2-row list/array of indices to be removed
m,n,p = arr.shape
mask = np.asarray(rm_idx[1])[:,None] != np.arange(n)
out = arr[np.arange(m)[:,None],np.where(mask)[1].reshape(m,-1)]

Alternatively, with boolean-indexing -

out = arr.reshape(-1,p)[mask.ravel()].reshape(m,-1,p)

A bit less memory-intensive approach as we try to avoid creating 2D mask -

vmask = ~np.in1d(np.arange(m*n),rm_idx[1] + n*np.arange(m))
out = arr.reshape(-1,p)[vmask].reshape(m,-1,p)

edited Nov 15, 2016 at 14:36

answered Nov 15, 2016 at 14:07

Divakar

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9 Comments

Cilenco Over a year ago

Thank you for this! From which namespaces comes rm_idx? Oh sorry this is my del array right?

Divakar Over a year ago

@Cilenco rm_idx would be your del. I used a different variable name as there's del operator in Python, so that might cause conflict.

Cilenco Over a year ago

Ah okay good point! But with this I get following error: TypeError: list indices must be integers or slices, not tuple in the mask line

Divakar Over a year ago

@Cilenco I was assuming it to be an array. Edited to accept both arrays and list.

Cilenco Over a year ago

Ah yes okay. Works great now :) Thanks for your help!

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