Question

I have two numpy array (2 dimensional) e.g.

a1 = array([["a","b"],["a","c"],["b","b"],["a","b"]])
a2 = array([["a","b"],["b","b"],["c","a"],["a","c"]])

What is the most elegant way of getting a matrix like this:

array([[1,0,0,0],
       [0,0,0,1],
       [0,1,0,0],
       [1,0,0,0]])

Where element (i,j) is 1 if all(a1[i,:] == a2[j,:]) and otherwise 0

(everything involving two for loops I don’t consider elegant)

Answer 1

>>> (a1[:,numpy.newaxis] == a2).all(axis=2)
array([[ True, False, False, False],
       [False, False, False,  True],
       [False,  True, False, False],
       [ True, False, False, False]], dtype=bool)

If you really need integers, convert to int as last step:

>>> (a1[:,numpy.newaxis] == a2).all(axis=2).astype(int)
array([[1, 0, 0, 0],
       [0, 0, 0, 1],
       [0, 1, 0, 0],
       [1, 0, 0, 0]])