Sum array by number in numpy

Question

Assuming I have a numpy array like: [1,2,3,4,5,6] and another array: [0,0,1,2,2,1] I want to sum the items in the first array by group (the second array) and obtain n-groups

Answer 1

A pure python implementation:

l = [1,2,3,4,5,6]
g = [0,0,1,2,2,1]

from itertools import izip
from operator import itemgetter
from collections import defaultdict

def group_sum(l, g):
    groups = defaultdict(int)
    for li, gi in izip(l, g):
        groups[gi] += li
    return map(itemgetter(1), sorted(groups.iteritems()))

print group_sum(l, g)

[3, 9, 9]