Question

Is there any built-in function in Python3/Numpy which filters an array and returns indices of the elements which are left? Something similar to numpy.argsort for sorting. The filter I have is setting both min and max thresholds – all values below/above min/max have to be filtered out.

I’ve seen Python’s function filter, but I don’t see a way to extract indices using it.

EDITED: Lots of useful information in the answers, thank you!

As @SvenMarnach pointed out, mask is enough:

mask = (min_value < a) & (a < max_value)

Now I have to apply this mask to other arrays of the same shape as a, but not sure what is the best way to do it…

Answer 1

You can get the indices of the elements in the one-dimensional array a that are greater than min_value and les than max_value with

indices = ((min_value < a) & (a < max_value)).nonzero()[0]

Usually you don’t need those indices, though, but you can work more efficiently with the mask

mask = (min_value < a) & (a < max_value)

This mask is an Boolean array with the same shape as a.

Edit: If you have an array b of the same shape as a, you can extract the elements of b corresponding to the True entries in mask with

b[mask]