dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Sep 25, 2008 at 04:18:23PM +0100, Garth N. Wells wrote:
> 
> 
> Anders Logg wrote:
> > On Thu, Sep 25, 2008 at 03:53:01PM +0100, Garth N. Wells wrote:
> >>
> >> Anders Logg wrote:
> >>> On Thu, Sep 25, 2008 at 03:11:27PM +0100, Garth N. Wells wrote:
> >>>> Anders Logg wrote:
> >>>>> On Thu, Sep 25, 2008 at 12:41:33PM +0100, Garth N. Wells wrote:
> >>>>>> Nuno David Lopes wrote:
> >>>>>>> Is there a simple way of setting an initial guess for an Iterative 
> >>>>>>> LinearSolver? 
> >>>>>>> In Umfpack and PETSc the default initial guess is the zero vector right?
> >>>>>> At the moment, yes (note the UMFPACK is an LU solver, so an initial 
> >>>>>> guess doesn't do anything).
> >>>>>>
> >>>>>> It's very simple, and I've been meaning to add an option for using an 
> >>>>>> initial guess. It's also useful for Newton solvers. I'll add something 
> >>>>>> in the next few days.
> >>>>>>
> >>>>>> Garth
> >>>>> It would be natural to let the x argument always be the initial
> >>>>> guess. I thought we already did this.
> >>>>>
> >>>>> Would it be enough to make sure that Vector::init() does not reset the
> >>>>> values to zero?
> >>>>>
> >>>> Yes.
> >>>>
> >>>> The danger is if someone sends an uninitialised vector to the solver.
> >>>>
> >>>> Garth
> >>> We can just put something like this in the init() functions:
> >>>
> >>>   if (x && size() == N)
> >>>     return;
> >>>
> >>> I think we had this a while back but at some point VecZeroEntries was
> >>> inserted.
> >>>
> >> I think that we should have an option whether or not to use an initial 
> >> guess. The default can be to use the guess (as in your above code extract).
> >>
> >> Garth
> > 
> > I think it would work nicely if the solver calls x.init() and that
> > will initialize x to a zero vector only if it has not already been
> > initialized. 
> 
> Sounds a bit dangerous to me that Vector::init() sometimes zeroes a the 
> vector, and sometimes it doesn't. Should we get rid of Vector::init() 
> and use Vector::resize() and Vector::zero() instead? No ambiguity then.
> 
> Garth

That sounds good.

-- 
Anders

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