Summing Diagonal Elements of Matrix in Octave

Question

Given a square matrix M, how can you find the sum of the elements on the diagonal? There must be an easier method than this:

sum(sum(diag(diag(M), 0)))

Answer 1

Actually, what I was looking for was the trace:

1> M = reshape(1:9, 3, 3)
M =

   1   4   7
   2   5   8
   3   6   9

2> trace(M)
ans =  15