Try this:
A = [1 2 3; 4 5 6; 7 8 9];
Af = fliplr(A); % Flip Matrix
ND = size(A,1)-1; % Number of Diagonals
D = ND:-1:-ND; % Diagonal Offset Vector
for k = 1:numel(D)
Dsum(k) = sum(diag(Af,D(k))); % Sum Each Diagonal
end
