Cumulative sum by group

Hi, Could someone explain how to compute the running sum by group (without using for loop)? A example is: groupid = [1; 1; 1; 2; 2; 3; 3;]; value=[1;2;3;4;5;6;7]. the desired result is: runsum = [1;3;6; 4;9; 6;13]. Thanks a lot. zppine

Answer 1

Using a for-loop here may be your best option. However, my unconditional love for accumarray forces me to present you with this:

groupid = [1; 1; 1; 2; 2; 3; 3;]; 
value=[1;2;3;4;5;6;7]; 
gcs = cell2mat(accumarray(groupid,value,[],@(x){cumsum(x)}))

Answer 2

Hi, Sean. Very good idea. Thanks a lot.