How can I sum only few values in a diagonal matrix?

if I have a diagonal matrix, and I use sum(diag(A)) , it gives me the sum of entire diagonal, but I only want the sum of lets say 1st three values in the diagonal? How can I add them?

Thank you!!

Answer 1

[Mischa Kim]

Rida, use

 sum(diag(A(1:3,1:3)))

[mohammed abdul wadood]

a tired it with my data, it's work perfectly, thanks Mischa.

Answer 2

[Alberto]

Sum=0;

for k=1:3 % will sum 3 first diagonal elements Sum=Sum+A(k,k);

end

You can change the range k=1:3 with an array with the index of elements u want to sum.

[mohammed abdul wadood]

it's work to, thanks Alberto

Answer 3

[Walter Roberson]

Without a loop:

r = size(A, 1);
sum(A([1, r+1, 2*(r+1)]))