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

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Rida
Rida 2014-4-14
回答： Walter Roberson 2018-3-26
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!!

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采纳的回答

Mischa Kim
Mischa Kim 2014-4-14
Rida, use
sum(diag(A(1:3,1:3)))

更多回答（2 个）

Alberto
Alberto 2014-4-14
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.
Walter Roberson
Walter Roberson 2018-3-26
Without a loop:
r = size(A, 1);
sum(A([1, r+1, 2*(r+1)]))

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