# Sum elements over indices in cell array

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Lennart Sinjorgo on 10 Jun 2022
Commented: Matt J on 11 Jun 2022
I have a cell array, in which each cell is a vector of integer numbers. These numbers are certain indices of another vector, say A. For each cell, I want to compute the sum of the elements in A, which are being indexed by my cell. For example, I can easily do this with a for loop (see code below). However, I was wondering if it is possible to vectorize this piece of code.
What I have done now, is artificially edit the cells so that they have the same length, by adding a dummy index. This dummy index then refers to a value 0 in vector A, so that I can easily sum over them by indexing A with the matrix induced by the cell array. However, this takes up a lot of memory.
Any ideas?
A = 1:15;
% some vector containging the values I need
myCell = {[1 3 4], [2 5 8 12]};
% the cell array generally contains arrays of different lengths
sumValues = zeros(1,2);
% I want to fill the array SumValues efficiently
for j = 1:2
sumValues(j) = sum(A(myCell{j}));
end
Rik on 10 Jun 2022
Loops can be surprisingly fast if there isn't a straightforward way to vectorize your code. Did you run any profiling to see if this is actually the bottleneck in your process?
Lennart Sinjorgo on 11 Jun 2022
Yes, its rather time consuming, although definitely not the bottleneck. That doesn't mean I shouldn't look for ways to speed it up though.

Matt J on 10 Jun 2022
Edited: Matt J on 10 Jun 2022
Once you've split things into cells, there is nothing faster than a loop. The better data organization would be to use group labels instead:
A = 1:15;
I = [1 3 4, 2 5 8 12]; %The indices as a vector
G= [1 1 1, 2 2 2 2]; %Group labels
sumValues=accumarray(G(:),A(I(:)))
sumValues = 2×1
8 27
Lennart Sinjorgo on 11 Jun 2022
Thank you! I did not know of this function! I'll have a look and see if it's faster than what I'm doing right now.
Matt J on 11 Jun 2022
You're welcome, but if you conclude that it works well for you, please Accept-click the answer.

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