dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

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

Then if someone wants to reset the initial guess between
> solves, it's enough to do
>
>   x.zero();
>
> before calling solve().
>
>
>
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