# [SOLVED] NumPy – Remove subarrays containing identical elements

## Issue

Suppose I have the following 4 by 3 by 3 array,

``````array([[[-2, -2, -2],
[-2, -2, -2],
[-2, -2, -2]],

[[-2, -2, -2],
[-2, -2, -2],
[-2, -2, -2]],

[[-2, -2, 71],
[-1, -1, -1],
[71, -1, 52]],

[[-2, -2, -2],
[-2, -2, -2],
[-2, -2, -2]]])
``````

I would like to filter such array by the following standard:

Treat each 3 by 3 array as a block. If all elements in this block are equal to -2, we should cut the whole block, so the target array would look like this (1 by 3 by 3):

``````array([[[-2, -2, 71],
[-1, -1, -1],
[71, -1, 52]]])
``````

I could only come up with a brute force solution with an explicit `if` condition and a `for` loop, but it does not work. Can anyone share a better method?

You can recreate the original array by the following commands

``````array = np.array([[-2,-2,-2,-2,-2,-2,-2,-2,-2],
[-2,-2,-2,-2,-2,-2,-2,-2,-2],
[-2,-2,71,-1,-1,-1,71,-1,52],
[-2,-2,-2,-2,-2,-2,-2,-2,-2]])
newarr = array.reshape(4,3,3)
``````

## Solution

The other answers are nice if you expect your arrays along axis 0 to always be a specific value, like -2, or 872385, etc.

In case you want something more general where you want to filter out any arrays which contain a single value, you can filter arrays by rank.

Since any matrix of a single value will have rank 1, you can filter by `rank != 1`:

``````In [2]: x[np.linalg.matrix_rank(x) != 1]
Out[2]:
array([[[-2, -2, 71],
[-1, -1, -1],
[71, -1, 52]]])
``````

This will work for any matrix along axis 0 which is filled with the same values. Another example:

``````In [4]: x
Out[4]:
array([[[ 5,  5,  5],
[ 5,  5,  5],
[ 5,  5,  5]],

[[-2, -2, -2],
[-2, -2, -2],
[-2, -2, -2]],

[[-2, -2, 71],
[-1, -1, -1],
[71, -1, 52]],

[[99, 99, 99],
[99, 99, 99],
[99, 99, 99]]])

In [5]: x[np.linalg.matrix_rank(x) != 1]
Out[5]:
array([[[-2, -2, 71],
[-1, -1, -1],
[71, -1, 52]]])
``````