How to obtain total sum by group

Hi, Could someone explain how to compute the total sum by group (without using for loop)?

A example is: groupid = [1; 1; 1; 2; 2; 3; 3;3]; value=[1;2;3;4;5;6;7;8]. the desired result is: runsum = [6;6;6;9;9; 21;21;21].

I am trying to get the result without using loop. Can "accumarray" do this?

Thanks a lot

Siying

Answer 1

[Geoff Hayes]

Siying - yes, accumarray can be used to sum elements of the same group. For example, we can do

 groupSums = accumarray(groupid,value);

which gives us

 groupSums =
     6
     9
    21

with your runsum as

 runsum = groupSums(groupid);

[Siying Liu]

Thank you very much! I did know how to obtain groupSums, but I didn't know how to expand that according to the groupid. The last line of the code is very clean and helpful!

[Siying Liu]

I have one more following up question. The last line seems to work only if the groupSums is a vector, is there a way I can expand this method to a matrix? Thank you in advance!

[Geoff Hayes]

Which is the matrix: the group ids or values, or both? Please provide an example.

Answer 2

[jgg]

Yes. This should work; it's probably not the shortest way to do it, but it avoids looping.

 key = unique(groupid);
 tsum = accumarray(groupid,value);
 [~,idx] = ismember(groupid,key);
 runsum = tsum(idx);

Answer 3

[Walter Roberson]

group_sum = accumarray(groupid, value);
runsum = group_sum(A);

Your question reminds me of the similar http://uk.mathworks.com/matlabcentral/answers/260501-unique-and-min-by-index

[Siying Liu]

Yes! I did read that thread! The problem I have is don't know how to covert the group sum to the matrix I need without using a loop.

Thanks!