Matlab summation to form matrix

Question

I am looking for some help with a fast loop to form a bunch of values. Given a 30-vector, x and another 30-vector which is the expected value of such data expx . I want to be able to quickly sum 30*30=90 values to form a symmetric 30x30 matrix. Here is how the (k,l)-entry of my 30x30 matrix is:

so x forms the entries on the left part of the bracket, x_i and expx is the right part, ie < x_i > for i=1,2,...,30. You don't need to worry about what values makes up these vectors, I've already determined them. Does anyone know how I should form such elements of the matrix and put them into my 30x30 matrix.

I guess I'd start off with:

M=30;
C = zeros(M); 

I'm struggling to get the summation though.

Answer 1

why not this:

M=30;
C = zeros(M);
for k = 1:30
    for l = 1:30
        for i = 1:30
            C(k,l) = (x(k-i+1) - expx(k-i+1)) * (x(l-i+1) - x(l-i+1));
        end
    end
end