MATLAB: Multiply an array of scalars by a 3D matrix

matrix multiplicationscalar product

I have a vector 10×1 and a 3D matrix NxNx10, and what I want to do is to multiply every dimension of the matrix (the 10 of them) by every scalar of the vector. I mean every of the 10 dimensions by a different scalar of the vector. Do you know if it exists a way to do it without using fors?

Best Answer

If you are using a recent version of MATLAB with implicit expansion enabled

A; % 10x1 array
M; % NxNx10 array
M_new = M.*reshape(A, 1, 1, []);

on older releases

M_new = bsxfun(@times, M, reshape(A, 1, 1, []))

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MATLAB: Reshape 3d to 2d array

squeeze()

transpose()

MATLAB: Compress down a 1000×1000 matrix into a 100×100 matrix

If you have image processing toolbox

M = rand(1000, 1000);
M_new = blockproc(M, [10 10], @(x) mean(x.data, 'all'))

Alternative Solution 1: (surprisingly the fastest)

M_new = conv2(M, ones(10)/100, 'valid');
M_new = M_new(1:10:end, 1:10:end);

Alternative Solution 2:

M_C = mat2cell(M, 10*ones(1,100), 10*ones(1,100));
M_new = cellfun(@(x) mean(x, 'all'), M_C);

Alternative Solution 3:

M_new = zeros(size(M)/10);
for i=1:100
    for j=1:100
        M_new(i, j) = mean(M(10*(i-1)+1:10*i,10*(j-1)+1:10*j), 'all');
    end
end

Best Answer

Related Solutions

MATLAB: Reshape 3d to 2d array

MATLAB: Compress down a 1000×1000 matrix into a 100×100 matrix

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