Fill matrix of occurences from column/row arrays of indexes

Question

I\'m searching for an efficient way to create a matrix of occurrences from two arrays that contains indexes, one represents the row indexes in this matrix,

Answer 1

Use np.add.at, specifying a tuple of indices:

>>> np.add.at(new_matrix, (rows, columns), 1)
>>> new_matrix
array([[ 1.,  0.,  0.],
       [ 0.,  2.,  0.],
       [ 0.,  1.,  2.],
       [ 2.,  1.,  5.]])

np.add.at operates on the array in-place, adding 1 as many times to the indices as specified by the (row, columns) tuple.