python - Numpy sum elements in array based on its value

Question

I have unsorted array of indexes:

i = np.array([1,5,2,6,4,3,6,7,4,3,2])

I also have an array of values of the same length:

v = np.array([2,5,2,3,4,1,2,1,6,4,2])

I have array with zeros of desired values:

d = np.zeros(10)

Now I want to add to elements in d values of v based on it's index in i.

If I do it in plain python I would do it like this:

for index,value in enumerate(v):
    idx = i[index]
    d[idx] += v[index]

It is ugly and inefficient. How can I change it?

Answer 1

np.add.at(d, i, v)

You'd think d[i] += v would work, but if you try to do multiple additions to the same cell that way, one of them overrides the others. The ufunc.at method avoids those problems.