calculations or sum all diagonal elements

hi. to sum all diagonal elements i used

sum(diag(a))

know; if i want to sum first 170 elements or last 80 elements, how can i do this.

Answer 1

[Walter Roberson]

FirstN = @(V,N) V(1:N);
LastN = @(V,N) V(end-N+1:end);

sum(FirstN(diag(a), 170))
sum(LastN(diag(a), 80))

[mohammed abdul wadood]

thank sir, thank you so much