dolfin team mailing list archive

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

Johan Hake wrote:
> On Monday 17 August 2009 14:03:55 Garth N. Wells wrote:
> > Johan Hake wrote:
> > > On Monday 17 August 2009 13:33:21 Garth N. Wells wrote:
> > > > Please send any comments on how a SubFunction should work as I'm going
> > > > to work on it soon (the Cahn-Hilliard demo runs in parallel(!) ;) with
> > > > the exception of the extraction of sub-Functions for output). I'm
> > > > inlined to change a sub function such that for
> > > >
> > > >    Function u0 = u[0]
> > > >
> > > > the Function u0 will only point the vector belonging to u rather than
> > > > creating a new vector. The advantages are:
> > > >
> > > > - No need to copy a vector
> > > > - DofMap will not require 'is_view'
> > > > - Possible to do things like
> > > >     u0  = 0.0;
> > > >     u1  += v;
> > > >     u0.interpolate();
> > > What would Function::vector() for a SubFunction then return? The original
> > > full Vector?
> > Yes.
>
> Ok.
>
> > > It would be cool to add a view of the original Vector that only
> > > represents the values of the dofs in the SubFunction, without coping
> > > data. I fiddled around with this when adding slicing for the PyDOLFIN la
> > > interface, but realized that it would be too difficult.
> > This is more or less what I plan to do, although internally. A user
> > wouldn't see the vector, but operations like interpolate would only
> > involve part of the vector.
>
> I see. Other direct Vector operations would then operate directly on the 
> shared Vector, like get, set, aso.
>
> > We could add a class like
> >
> >    VectorView(GenericVector& x, DofMap& dof),
> >
> > which could derive from GenericVector, to provide views. It isn't a
> > priority for me though.
>
> Yes, I also thought along these lines, however I did not think of doing it 
> using a DofMap, which really is the natural thing. I will also not have 
> possibility to priorities it for now.

On second thought,

  VectorView(GenericVector& x, std::vector<uint> map& map)

would be better. The DofMap could produce the map,

  std::vector<uint> map = dofmap.view();

Garth


> Johan
>
> > Garth
> >
> > > Johan
> > >
> > > > Disadvantages are:
> > > >
> > > > - It may be unclear that u0 and u share data.
> > > > - If u goes out of scope, u0 is will keep the vector belonging to u
> > > > alive (only an issue if the vector is very large compared to that
> > > > required by u0).
> > > >
> > > > Garth
> > > > _______________________________________________
> > > > DOLFIN-dev mailing list
> > > > DOLFIN-dev@xxxxxxxxxx
> > > > http://www.fenics.org/mailman/listinfo/dolfin-dev