Numpy sum of values in subarrays between pairs of indices

Question

Suppose I have an array A. I have a series of index pairs (a1, b1), (a2, b2) ... (an, bn)

I want to obtain all the sums of the elements between those pairs. i.e.

Answer 1

Here's another way:

a = np.random.rand(3000)
indices = np.array([[0,3], [9,20], [5,30], [9,33]])
sums = np.add.reduceat(a, indices.ravel())[::2]

assert np.all(sums == np.array([a[i:j].sum() for i,j in indices]))

The cumsum one above is probably more efficient if there are many indices.