How to sort the matrix quickly?

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Xianjie 2012-12-10

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Dear all, How to sort the matrix quickly? Now i use this code to do it.

[V,DD]=eig(KK,MM);
DD=abs(DD);               
mp=3*(Ms+1)*(2*Ns+1)+P1*(2*Ns+1)+2*P2*(2*Ns+1); % the total dimension of the matrix
for p=1:mp              
    for q=p+1:mp
        if DD(p,p)>= DD(q,q)
           d=DD(p,p);
           dd=V(:,p);
           DD(p,p)=DD(q,q);
           V(:,p)=V(:,q);
           DD(q,q)=d;
           V(:,q)=dd;
        end
    end
end

Thank you very much.

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Answer 1

Image Analyst 2012-12-10

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And what's wrong with the built-in sort() function? Why aren't you using that? Is this a homework exercise?

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Xianjie 2012-12-10

Hello, the higher and lower can find in the D matrix. The V is corresponding to the D each column. I compare the value of D, can move the D and V in the same. sort only can sort the D? the D is the diag matrix

Image Analyst 2012-12-10

Uh, not quite sure I follow that. Anyway, Roger did it below using sort() and he's highly respected and always or nearly always right, so I believe using the built-in sort() function is the way to go.

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Answer 2

Roger Stafford 2012-12-10

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在 MATLAB Online 中打开

The following should be a much faster way of sorting V and DD by the absolute values of the eigenvalues in DD:
 [V,DD] = eig(KK,MM);
 D = diag(DD);
 [~,p] = sort(abs(D));
 V = V(:,p);
 DD = diag(D(p));

Roger Stafford

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Xianjie 2012-12-10

Thank you for your advice. Can you guid me for the usage of lapack for the eigenvalue problem. This is the lapack. But i am the first time to use the lapack. How to use this script to solve the eigenvalue problem? if the matrices is large, like 4000*4000,it will take a long time to solve this problem. Thank you very much.

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