Add multiple values to one numpy array index

Question

Simple Version: if I do this:

import numpy as np
a = np.zeros(2)
a[[1, 1]] += np.array([1, 1])

I get [0, 1] a

Answer 1

This is what the at method of NumPy ufuncs is for:

output = numpy.zeros(size)
numpy.add.at(output, a[a.cond].ix, a[a.cond].val)