dolfin team mailing list archive

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

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.

> 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.

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.

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