Sum elements over indices in cell array

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Lennart Sinjorgo 2022-6-10

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https://ww2.mathworks.cn/matlabcentral/answers/1737730-sum-elements-over-indices-in-cell-array

评论： Matt J 2022-6-11

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

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Rik 2022-6-10

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 2022-6-11

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.

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Answer 1

Matt J 2022-6-10

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https://ww2.mathworks.cn/matlabcentral/answers/1737730-sum-elements-over-indices-in-cell-array#answer_982845

编辑：Matt J 2022-6-10

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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