MATLAB: Top N values for multidimensional matrix with different N at each grid cell

Question

Hello all,

I have been looking for an efficient way to execute the following, but haven't come across anything in the forum. I would like to find the top N values in a multidimensional array at each grid cell of Matrix B (144x90x1877). I have found the following approach in the forum, but it gives the top N values for the entire multidimensional matrix.

    n= 2; % any number specified
    [a,ix] = sort(A(:),'descend');
    a = a(1:n);
    ix = ix(1:n);

I would like to have the top N values for each grid cell be defined by another matrix(Matrix A, 2D matrix). For example, I would like to start with cell 1×1 matrix A which specifies 5 top N values so I go to Matrix B 1x1x8766 and sort the values in descending order and extract the top 5 values to be average and stored in another matrix. Continue with each cell.

I would appreciate any guidance on how to proceed with this. Thank you

Answer 1

Let A and B:

A= [0 1 5 10 12
 12 3 4 8 9
 4 6 8 9 0]
B = randi([3 8],3,5,15);

solution:

    t = A > 0;
    [m,n,k] = size(B);
    C = zeros(m,n);
    C0 = cumsum(sort(B,3,'descend'),3)./reshape(1:k,1,1,[]);
    [ii,jj] = ndgrid(1:m,1:n);
    C(t) = C0(sub2ind([m,n,k],ii(t),jj(t),A(t)));