# How to sort the matrix quickly?

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Xianjie le 10 Déc 2012
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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### Réponses (2)

Image Analyst le 10 Déc 2012
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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Image Analyst le 10 Déc 2012
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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Roger Stafford le 10 Déc 2012
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 le 10 Déc 2012
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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