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eig() function

From: Habibie Sumargo
Subject: eig() function
Date: Thu, 9 Oct 1997 09:38:34 -0400 (EDT)

Dr. Eaton,

While working with the eig() function in octave (all version that I am
awared of), I noticed that the computed eigenvalues and their
corresponding eigenvectors are sorted in an ascending order.  This also
happens with MatLab.  I am wondering if it is possible to add a routine in
the eig() function to return the eigenvalues and the corresponding
eigenvectors in a descending order, for the next release of octave.  To
make this function compatible with MatLab's eig function, make the
descending order feature as part of the OPT switch, i.e. when OPT=d, the
returned eigenvalues and their corresponding eigenvectors are sorted in a
descending order.

I know that sorting can be done after one has computed the eigenvalues and
their corresponding eigenvectors.  As a matter of fact, I usually sort the
eigenvalues and their respective eigenvectors in a descending order with
the following macros: 

        function [v,d] = eigsort (x)
                length  = is_square (x);
                if !(length)
                        error ('Input argument is not a square matrix!\n');

                [v,d]   = eig (x);
                [dd,ix] = sort (-diag (d));
                for i=1:length
                        d(i,i)  = - dd(i);
                v       = v(:,ix)

If sorting the eigenvalues and their corresponding eigenvectors in the
eig() function is not complicated and can speed up the process, wouldn't
it be better to add the sorting algorithm as an additional OPT feature in
the eig function instead of the above macros?

Thank you very much for your understanding.


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