MATLAB: What are efficient ways to extract lower dimensional slices from a high-dimensional array

array subsref sub2indMATLAB

I am looking for an efficient way to extract sub-arrays from a larger array. There are (at least) two standard solutions both having certain disadvantages.
First solution: Use a loop Disadvantage: Might be slow if nidx (see code below) is large
Second solution: use sub2ind Disadvantage: Interim indices created are large if nidx * ndim3 is large
Any other (especially better) ideas?
Kind regards, gg
   %create 3dim testarray
   ndim1 = 3;
   ndim2 = 4;
   ndim3 = 5;
   testarr = rand([ndim1,ndim2,ndim3]);
   %pick some indices into the first two dimensions
   nidx = 10;
   idx1 = randi([1,ndim1],[nidx,1]);
   idx2 = randi([1,ndim2],[nidx,1]);
   %now extract slices from testarr according to the indices
   %First solution: by loop
   newarr = zeros(nidx, ndim3);
   for kidx = 1:nidx
      newarr(kidx,:) = testarr(idx1(kidx), idx2(kidx), :);
   end
   %Second solution using sub2ind
   refidx1 = repmat(idx1, [ndim3,1]);
   refidx2 = repmat(idx2, [ndim3,1]);
   tmp = repmat((1:ndim3),[nidx, 1]);
   refidx3 = tmp(:);
   lidx = sub2ind(size(testarr), refidx1, refidx2, refidx3);
   newarr_alt = reshape(testarr(lidx), nidx, ndim3);
   %are they equal this time?
   isequal(newarr, newarr_alt)

Best Answer

s = size(testarr);
m = prod(s(1:2));
out = testarr(bsxfun(@plus,idx2*(s(1)-1)+idx1,0:m:(s(3)-1)*m))
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