Cumulative summation of a numpy array by index

Question

Assume you have an array of values that will need to be summed together

d = [1,1,1,1,1]

and a second array specifying which elements need to be

Answer 1

If I understand the question correctly, there is a fast function for this (as long as the data array is 1d)

>>> i = np.array([0,0,1,2,2])
>>> d = np.array([0,1,2,3,4])
>>> np.bincount(i, weights=d)
array([ 1.,  2.,  7.])

np.bincount returns an array for all integers range(max(i)), even if some counts are zero