Question

Lets say I have 2 vectors:

a=[0 1 0 1 1 0 1 0 0 0 1 1 1];
b=[1 1 1 1 1 1 2 2 2 3 3 3 3];

For every group of numbers in b I want to cumsum, so that the result should look like that:

c=[1 3;2 1;3 3]

That means that I have for the ones in b 3 ones in a, for group two in b I have only one one in a etc.

Answer 1

If you’re looking for a solution where b can be anything, then a combination of hist and unique will help:

num = unique(b(logical(a))); %# identify the numbers in b with non-zero counts
cts = hist(b(logical(a)),num); %# count
c = [num(:),cts(:)]; %# combine. 

If you want the first column of c to go from 1 to the maximum of b, then you can rewrite the first line as num=1:max(b), and you’ll also get rows in c where the counts are zero.