Looping through multi-dimensional array and filtering based on a condition

Question

I believe similar questions have been asked but none which deal with the problem I am facing.

I have an array of shape (H,W,L) - I must loop through each instance of the array to filter out values (their x,y location) that meet a particular criteria. (say val > t_r and val < t_c) - I must repeat this for each of the K values.

For eg: If we have an array of shape (2,3,4)

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

suppose the first criteria is val > 2 and the second criteria is val < 6 and store the (row, col) value in a N x 3 array. Where the first 2 values are the 'row','col' and the last one corresponds to the layer / third dimension. then the expected output of the operation should be something like -

output = [[0,2,0],[1,0,0],[1,0,0],[1,1,0],[1,2,0]....] this would correspond to the values filtered from A[:,:,0]

One approach I have thought of is - using 3 for loops - i,j,k to loop over each of the elements, but I am unable to figure out the exact implementation. I would also like to implement vectorization wherever possible. I could use some guidance.

Just a general remark: you almost never need loops with NumPy. — Matt Hall
– Matt Hall, Commented Oct 3, 2019 at 11:49

rafaelc · Accepted Answer · 2019-10-03 04:12:55Z

1

You may use np.nonzero and vectorize your comparisons.

a = np.asarray(A)
res = np.vstack(np.nonzero((a>2)&(a<6))).T

array([[0, 0, 2],
       [0, 1, 0],
       [0, 1, 1],
       [0, 1, 2],
       [1, 1, 1],
       [1, 1, 2],
       [2, 0, 0],
       [2, 1, 1],
       [2, 1, 2],
       [3, 0, 2],
       [3, 1, 0]], dtype=int64)

You can always reorder the columns to your liking e.g.:

res[:, [1,2,0]]

array([[0, 2, 0],
       [1, 0, 0],
       [1, 1, 0],
       [1, 2, 0],
       [1, 1, 1],
       [1, 2, 1],
       [0, 0, 2],
       [1, 1, 2],
       [1, 2, 2],
       [0, 2, 3],
       [1, 0, 3]], dtype=int64)

answered Oct 3, 2019 at 4:12

rafaelc

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