MATLAB: Using accumarray to do sum with duplicate indicies

Question

Hi folks, Let f and A be arrays with size(f) = [N,1] and size(A) = [M,1] with N > M. Let J be a list of indices for f with size(J) = size(A). I wish to compute the for loop

for k = 1:M,
  f(J(k)) = f(J(k)) + A(k);
end

However, due to time constraints I need to do this computation without the for loop. If all the indices J were unique I would simply say f(J) = f(J) + A. However, I will need to consider the case where there are many duplicate indices in J. I suspect this can be accomplished with accumarray, but its not clear to me how to do this. Does anybody have any suggestions?

Thanks, Paul

Answer 1

Yes, it is a direct application of accumarray,

   f=f+accumarray(J,A,size(f));