How to find certain index of a diagonal matrix , when a condition meets?

I have the following code:

covar = 1.0/m .* X' * X; % computing co-variance

[U, S, V] = svd(covar); % singular value decomposition % U contains principal components and V contains diagonal matrix

% %% % % ===== Dimensioanlity Reduction Finding K for projection======

s3=sum (diag(S));

sum1=0;

temp1=0;

for j=1:length(S)

    sum1=sum1+S(j,j);

    div_res=(sum1/s3);

    if(div_res==0.9)

    index=find(j) ; 

    end
end

All I want is, that value of "j" when the div_res=>0.9 but I am not getting the correct answer

Answer 1

[Azzi Abdelmalek]

n=size(A,1);
idx=find(A(sub2ind([n n],1:n,1:n))>=0.9)

[Azzi Abdelmalek]

That depends on how many times the condition is met