MATLAB: Simpler way without for loops

matlab functionmatricesmatrix manipulation

How can I implement this without using for loops?
    for i = 1:i_rows
        Mat_NX3(i,1) = (Mat_A_3D  (idx(i,1), idx(i,2), 1)  );
        Mat_NX3(i,2) = (Mat_A_3D  (idx(i,1), idx(i,2), 2)  );
        Mat_NX3(i,3) = (Mat_A_3D  (idx(i,1), idx(i,2), 3)  );
    end
where:
  • Mat_NX3 is the output N X 3 matrix
  • Mat_A_3D is a 3D matrix
  • idx is a matrix with 2 columns, (index points) telling which co-ordinate values should be copied from Mat_A_3D into Mat_NX3. It has i_rows number of rows
Example if
Mat_A_3D (:,:,1) = [1   2   3
                    4   5   6
                    7   8   9];
Mat_A_3D (:,:,2) = [1   2   3
                    4   5   6
                    7   8   9];
Mat_A_3D (:,:,3) = [1   2   3
                    4   5   6
                    7   8   9];
and
idx = [1,1
       3,3];
output
Mat_NX3 = [1  1  1;
           9  9  9];

Best Answer

I would just keep the for-loops.
Alternatively, you could use sub2ind to build a linear index from the pieces and then index with it:
% Size
sz = size(Mat_A_3D);
% First page only
idx = sub2ind(sz,idx(:,1),idx(:,2));
% Add second and third pages:
idx3 = bsxfun(@plus,idx,(0:sz(3)-1)*sz(1)*sz(2));
MatNX3 = Mat_A_3D(idx3)
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