How to calculate sum of specific array elements

Let's say you have two arrays of the same size: one with the data (x) and one carrying some indices that characterize the data (q)

For example: x = [ 3 5 2 10 6 4] q = [ 0 0 1 2 3 3]

This means the first two elements of (x) belong to the same category indicated by 0 and so on.

How to calculate the sums of elements in (x) which belong to the same category i.e. the same integer number as specified by (q).

The result for the above example should be: r = [ 8 2 10 10 ]

Is this possible without using a loop?

Answer 1

[Kye Taylor]

Try

x = [ 3 5 2 10 6 4]';
q = [ 0 0 1 2 3 3]';

% note first input must be column vector with positive integers (indices)
yourSums = accumarray(q+1,x);