MATLAB: Operations along the first dimension of a 3D array

Question

I have a 3D array M(d*d,m,n). For each d*d vector of M (i.e. vectors of the first dimension), I split it into d parts and take the sum of each part to form a new vector (of size d). For example, if u is a vector along the first dimension of M, then it will be replaced by the vector v, computed by:

    v = sum(reshape(u,d,d))';

For the moment I use a loop as following, but I think there should be a much faster way to do it.

    N = zeros(d,m,n)
    for i=1:m
    for j=1:n
         N(:,i,j) = sum(reshape(M(:,i,j),d,d))';
    end
    end

Thank you so much for any suggestions!

Answer 1

Looks like M should have been a 4D array from the beginning,

M=reshape(M,d,d,m,n);
N=sum(M,2);