Fredrik P asked . 2024-01-16

How can I speed up this function?

I'm struggling to speed up the code inside the version1 function. The only way to vectorize it that I can seem to figure out is version2, but that actually makes the function slower.

n1 = 400;
n2 = 3;
n3 = 7;
n4 = 2;

A = rand(n1, n2, n3);
B = randi(n2, [n4, n1, n3]);
disp(timeit(@() version1(A, B, C, n1, n2, n3, n4)));
disp(timeit(@() version2(A, B, C, n1, n2, n3, n4)));

function C = version1(A, B, n1, n2, n3, n4)
    C = zeros(n4, n1, n3);
    for ii = 1:n3
        for jj = 1:n1
            for kk = 1:n4
                C(kk, jj, ii) = A( ...
                    jj, ...
                    B(kk, jj, ii), ...
                    ii ...
                );
            end
        end
    end
end

function C = version2(A, B, n1, n2, n3, n4)
    C = zeros(n4, n1, n3);
    for ii = 1:n3
        for jj = 1:n1
            C(:, jj, ii) = A( ...
                jj, ...
                B(:, jj, ii), ...
                ii ...
            );
        end
    end
end

performance , vectorization , MATLAB , Programming