Determine arguments where two numpy arrays intersect in Python

Question

I have two arrays, say:

a, b = np.array([13., 14., 15., 32., 33.]), np.array([15., 16., 17., 33., 34., 47.])

I need to find the indices of all the elements in a that are not present in b. In the above example the result would be:

[0, 1, 3]

Because a[0], a[1] and a[3] are 13., 14. and 32., which are not present in b. Notice that I don't care to know the actual values of 13., 14. and 32. (I could have used set(a).difference(set(b)), in that case). I am genuinely interested in the indices only.

If possible the answer should be "vectorized", i.e. not using a for loop.

Answer 1

You could use np.in1d:

>>> np.arange(a.shape[0])[~np.in1d(a,b)].tolist()
  [0, 1, 3]

Answer 2

It is quite easy, use numpy.intersect1d for calculating elements shared between a and b, then check which of those elements are not in a using numpy.in1d and finally get their position in the array using numpy.argwhere.

>>> import numpy as np
>>> a, b = np.array([13., 14., 15., 32., 33.]), np.array([15., 16., 17., 33., 34., 47.])
>>> np.argwhere(np.in1d(a, np.intersect1d(a,b)) == False)
array([[0],
   [1],
   [3]])

If you prefer a list just add .flatten to convert the matrix to a vector and then apply .tolist to get the list:

>>> np.argwhere(np.in1d(a, np.intersect1d(a,b)) == False).flatten().tolist()
 [0, 1, 3]

Answer 3

Fairly straight forward if you use loops:

def difference_indices(a, b):

    # Set to put the unique indices in
    indices = []

    # So we know the index of the element of a that we're looking at
    a_index = 0

    for elem_a in a:

        found_in_b = False
        b_index = 0

        # Loop until we find a match. If we reach the end of b without a match, the current 
        # a index should go in the indices list
        while not found_in_b and b_index < len(b):
            if elem_a == b[b_index]: found_in_b = True
            b_index = b_index + 1

        if not found_in_b: indices.append(a_index)
        a_index = a_index + 1

    return indices

This should work with lists containing any one type, as long as they are the same type, and the __eq__ function is defined for that type.

Doing this without loops would require a knowledge of python greater than mine! Hope this is useful for you.