Lots of ways. You need to start learning them, as that understanding of the tools in MATLAB is necessary for success.
A simple one:
or
diag(diag(A,-1),-1) + diag(diag(A)) + diag(diag(A,1),1)
Ok, so those are pretty simple tricks. But they represent ideas that you need to learn and understand.
If you are working with diagonal matrices, you need to learn to use tools like sparse, spy, spdiags, sub2ind, ind2sub, etc. Here, for example, I'll extract a sparse pentadiagonal part of the 10x10 matrix A.
Apenta = spdiags(spdiags(A,-2:2),-2:2,10,10);
Lots of other ways too.
Best Answer