Re: norm of sparse matrices

[Marco Caliari]

On Sat, 26 Nov 2011, Jordi Gutiérrez Hermoso wrote:

> 2011/11/25 Jordi Gutiérrez Hermoso <address@hidden>:
> > 2011/11/25 Marco Caliari <address@hidden>:
>
> > > I gave a look at the paper and at Higham's code pnorm.m, but I don't see any
> > > random initial guess. On the other hand, if look at this run
> > >
> > > octave:1> format long e
> > > octave:2> S=sprand(5,5,0.5);
> > > octave:3> norm(S)
> > > ans =  1.25336637903553e+00
> > > octave:4> norm(S)
> > > ans =  1.25336637903553e+00
> > > octave:5> norm(S)
> > > ans =  1.25336638124303e+00
> > > octave:6> norm(S)
> > > ans =  1.25336638124303e+00
> > > octave:7> norm(S)
> > > ans =  1.25336638124303e+00
> > > octave:8> norm(S)
> > > ans =  1.25336638124303e+00
> > > octave:9> norm(S)
> > > ans =  1.25336638124303e+00
> > > octave:10> norm(S)
> > > ans =  1.25336638124303e+00
> > >
> > > you see that the norm changes at the third computation and then it remains
> > > constant. This is not a random behaviour. Can anybody reproduce it? I can
> > > *almost* always reproduce it.
> >
> > Yes, I was just able to reproduce that, and you're right, there's no
> > randomisation in the algorithm. This may point to a deeper error,
> > perhaps UB. I'm investigating with the debugger now...
>
> I think I found the bug. There was some uninitialised memory getting
> read. Can you test to see if this patch fixes the problem?
>
>    http://hg.savannah.gnu.org/hgweb/octave/rev/d672edef956e
>
> Thanks,
> - Jordi G. H.

Yes, thank you very much.

Marco