Simpler way without for loops

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];

Answer 1

[Sean de Wolski]

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)

[Meghana Dinesh]

Wouldn't the for loops decrease speed? This is why I wanted to change it. I will be working with large matrices.

[Meghana Dinesh]

When I give

 idx = [1,2;  % elements in 1st row, 2nd column of Mat_A_3D
        3,1;  % elements in 3rd row, 1st column of Mat_A_3D 
        3,3]  % elements in 3rd row, 3rd column of Mat_A_3D

I expect the output:

[2, 2, 2; 
 7, 7, 7; 
 9, 9, 9]

This is not what I am getting with

% Size
sz = size(Mat_A_3D);
% First page only
idx = sub2ind(sz,idx(:,1),idx(:,1));
% Add second and third pages:
idx3 = bsxfun(@plus,idx,(0:sz(3)-1)*sz(1)*sz(2));
MatNX3 = Mat_A_3D(idx3)

[Andrei Bobrov]

corrected

Answer 2

[Thorsten]

sz = size(Mat_A_3D);
ind = sub2ind(sz(1:2), idx(:,1), idx(:, 2));
offset = repmat(cumsum(repmat(prod(sz(1:2)), [1 sz(3)-1])), [size(ind) 1]);
ind = [ind  repmat(ind, [1 size(offset, 2) 1]) + offset];

Mat_NX3 = Mat_A_3D(ind)

Answer 3

[Andrei Bobrov]

n = size(M);
x = zeros(n);
x(sub2ind(n,idx(:,1),idx(:,2))) = 1;
out = reshape(M(cumsum(x,3) > 0),size(idx,1),[]);