dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

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.

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
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