Vectorize a double loop

Hi everyone

I'm tryng to vectorize the folowing piece of code:

n=30;
d=3;
a=1;
b=2;
cell=3;

for ki = 2:n-1
    for kj = 2:n-1
       M((cell-1)*n^2+(ki-1)*n+kj,(cell-1)*n^2+(ki-1)*n+kj-1) = 1/d^2;       
       M((cell-1)*n^2+(ki-1)*n+kj,(cell-1)*n^2+(ki-1)*n+kj) = -4/d^2-a;    
       M((cell-1)*n^2+(ki-1)*n+kj,(cell-1)*n^2+(ki-1)*n+kj+1) = 1/d^2;       
       M((cell-1)*n^2+(ki-1)*n+kj,(cell-1)*n^2+(ki-1)*n+kj-n) = 1/d^2;
       M((cell-1)*n^2+(ki-1)*n+kj,(cell-1)*n^2+(ki-1)*n+kj+n) = 1/d^2;       
       C((cell-1)*n^2+(ki-1)*n+kj,1) = b;            
    end
end

How can I do it? I will appreciate any help! Thanks!

[Walter Roberson]

What are we to presume about the datatype and size of cell ? It does not appear to be a cell array.

[Walter Roberson]

(cell-1)*n^2+(ki-1)*n+kj

You are faking 4 dimensional indexing. You should switch to actual 4D indexing. reshape() before and after if you need to.

[Walter Roberson]

@Ana Carla Buchini

If you feel that your post is unclear, then since you are the one who wrote it, you should clarify it.

Answer 1

[Bob Thompson]

Does this work? I haven't been able to test it.

       M((cell-1)*n^2+(1:n-2)*n+(2:n-1),(cell-1)*n^2+(1:n-2)*n+(2:n-1)-1) = 1/d^2;       
       M((cell-1)*n^2+(1:n-2)*n+(2:n-1),(cell-1)*n^2+(1:n-2)*n+(2:n-1)) = -4/d^2-a;    
       M((cell-1)*n^2+(1:n-2)*n+(2:n-1),(cell-1)*n^2+(1:n-2)*n+(2:n-1)+1) = 1/d^2;       
       M((cell-1)*n^2+(1:n-2)*n+(2:n-1),(cell-1)*n^2+(1:n-2)*n+(2:n-1)-n) = 1/d^2;
       M((cell-1)*n^2+(1:n-2)*n+(2:n-1)j,(cell-1)*n^2+(1:n-2)*n+(2:n-1)+n) = 1/d^2;       
       C((cell-1)*n^2+(1:n-2)*n+(2:n-1),1) = b; 

[AC]

Thank you for your answer! I tried this, but didn't work. When using the double for loop, I get (n-2)*(n-2) combinations of ki's and kj's. But (cell-1)*n^2+(1:n-2)*n+(2:n-1) is a vector of length (n-2). I need a vector of length (n-2)*(n-2)