[Octave-bug-tracker] [bug #47236] output of eigs() with 'sa' option sorted differently from Matlab

[Rik]

Follow-up Comment #13, bug #47236 (project octave):

It turns out this is very specific to the type of matrix being solved.  The
code for eigs is eventually located in liboctave/numeric/eigs-base.cc.  The
solvers are templates based on qualities of the matrix.  It turns out that
only two of the matrix types are re-ordering the eigenvalues.  One of these
cases is the real symmetric which is what matrix A is.

The code is around line 927.


      octave_idx_type k2 = k / 2;
      if (typ != "SM" && typ != "BE")
        {
          for (octave_idx_type i = 0; i < k2; i++)
            {
              double dtmp = d[i];
              d[i] = d[k - i - 1];
              d[k - i - 1] = dtmp;
            }
        }


If I alter the code to exclude the "SA" option


if (typ != "SM" && typ != "SA" && typ != "BE")


then the results returned are in the same order as eig.

I'm attaching the quick patch I made (eigs_sa.patch) and the test script
tst_eigs.m which depends on the file yaledata.

I think what should happen is a script gets written that does the full
combinatorial test of all matrix types against all solver options ('lm', 'sm',
'sa', 'la' etc.).  Run that script on Octave and Matlab and see where the
differences are.




(file #36496, file #36497)
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Additional Item Attachment:

File name: eigs_sa.patch                  Size:0 KB
File name: tst_eigs.m                     Size:0 KB


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