How to rewrite the nested for loop to modify matrix elments when matrix size is huge

Hi I have the below matrix MN which is already pre-defined and I want to modify its elements without using for loop. Here I just set N=2 but in reality N is around 500. f(m) and g(n) are just user-defined functions. Is there a better way to rewrite it avoiding nested for loops?

N=2;
NN=N^2;
MN=sparse(NN*6,NN*6);
tic;
for m=1:N
    for n=1:N
        m_offset = (m-1)*N+n;
        n_offset = (m-1)*N+n;
        MN(m_offset,n_offset)    =  f(m);
    	MN(m_offset,n_offset+NN) =  g(n);
    end
end
toc;

Answer 1

[Matt J]

To vectorize this, you must vectorize f() and g().

Also, building a sparse matrix this way is a bad idea. You should use the syntax,

MM=sparse(I,J,S,NN*6,NN*6)