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

I tried scripts from everyone and my considerations are:

Joe: Will only work if you have few groups.

kevpie: Too slow because of loops (this is not pythonic way)

Bi_Rico and Sven: perform good, but will only work for Int32 (if the sum goes over 2^32/2 it will fail)

Alex: is the fastest one, good for sum.

But if you want more flexibility and the possibility to group by other statistics use SciPy:

from scipy import ndimage

data = np.arange(10000000)
groups = np.arange(1000).repeat(10000)
ndimage.sum(data, groups, range(1000))

This is good because you have many statistics to group (sum, mean, variance, ...).