NumPy k-th diagonal indices

Question

I\'d like to do arithmetics with k-th diagonal of a numpy.array. I need those indices. For example, something like:

>>> a = numpy.eye(2)
>>>         


        

Answer 1

The indices of the k'th diagonal of a can be computed with

def kth_diag_indices(a, k):
    rowidx, colidx = np.diag_indices_from(a)
    colidx = colidx.copy()  # rowidx and colidx share the same buffer

    if k > 0:
        colidx += k
    else:
        rowidx -= k
    k = np.abs(k)

    return rowidx[:-k], colidx[:-k]

Demo:

>>> a
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14],
       [15, 16, 17, 18, 19],
       [20, 21, 22, 23, 24]])
>>> a[kth_diag_indices(a, 1)]
array([ 1,  7, 13, 19])
>>> a[kth_diag_indices(a, 2)]
array([ 2,  8, 14])
>>> a[kth_diag_indices(a, -1)]
array([ 5, 11, 17, 23])