MATLAB: How to populate 2d-array from 3d array with page indices

arrayindexingMATLAB

Hi!
Please see my example below. I start with a non-scalar structure, which I concatenate into a 3d-array. I then find the minimum values and their indices in the 3rd dimension using 'min'. Then, from a second array of exactly the same size, I want to extract a new array with entries corresponding to indices in the min value indices from the other array. I can do this perfectly well with the loop below, but doing it by vectorization would be twice as fun 🙂
The values in 'b' array is chosen to better illustrate the problem, as it is directly related to the 'pageIdx' array.
BR Andreas.
    s(1).f=rand(2);
    s(2).f=rand(2);
    s(3).f=rand(2);
    a=cat(3,s.f);
    b=ones(2,2,3);
    b(:,:,2)=2;
    b(:,:,3)=3;
    [minVals,pageIdx]= min(a,[],3)
    bSize = size(b); nRows = bSize(1); nCols = bSize(2);
    bMinVals = zeros(nRows,nCols);
    for iRow = 1:nRows
        for jCol = 1:nCols
            bMinVals(iRow,jCol)=b(iRow,jCol,pageIdx(iRow,jCol));
        end
    end
    bMinVals == pageIdx

Best Answer

a = rand(2, 2, 3);
b = ones(2, 2, 3); b(:, :, 2) = 2; b(:, :, 3) = 3; 
b= -b; %to differentiate from PageIdx
[minVals, PageIdx] = min(a, [], 3);
[r, c] = ndgrid(1:size(b, 1), 1:size(b, 2));
bMinVals = reshape(b(sub2ind(size(b), r(:), c(:), PageIdx(:))), size(b, 1), [])
isequal(bMinVals, -PageIdx)
Related Question