dolfin team mailing list archive

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

Anders Logg wrote:
> On Thu, Sep 11, 2008 at 03:02:36PM +0100, Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Thu, Sep 11, 2008 at 02:51:41PM +0100, Garth N. Wells wrote:
> > > > Martin Sandve Alnæs wrote:
> > > > > I don't agree, isn't the point that several DiscreteFunctions can
> > > > > share a FunctionSpace?
> > > > Yes, but a dof map doesn't define a function space.
> > > >
> > > > Garth
> > > I think the dof map should be in the FunctionSpace, that way several
> > > functions may share the dof map (which may take time to compute if we
> > > want to do some reordering).
> > >
> > > Two functions in the same function space share the mesh, the element
> > > and the dof map (these three define the space) but each function has
> > > its own vector.
> > My point isn't that a DofMap won't shared, it's that it's not part of 
> > the definition of a function space.
>
> It is part of the function space if we say that a function space is
> defined by a particular basis:
>
>   V = span{\phi_i}
>
> To define the basis, we need the dof map.

The dof map is needed to tie the finite element basis + domain (mesh) to 
a matrix/vector.

> > It always possible that two discrete functions which are equivalent can 
> > share a mesh and finite element (which defines the function space), but 
> > have different vectors and dof maps.
>
> Is that something we will encounter much? 

Probably not, but it shows why the dof map doesn't belong in FunctionSpace.

On reflection, FunctionSpace is pretty simple, so I don't actually see 
why we need it. If we want to stress that a group of objects belong 
together in the definition of a discrete function, why not just use a 
tuple in DiscreteFunction?

Garth

It would simplify matters if
> we said that a FunctionSpace is a function space with a particular
> basis. We do that anyway if we include the finite element. Otherwise,
> we could say that (P_1)^d and BDM_1 are the same function space, but
> we think of them as different.
>
> > > I added the FiniteElement class to avoid exposing ufc:: classes
> > > directly. We do the same for ufc::dof_map (but then we have a reason).
> > > It might not be necessary, but it looked more uniform to have
> > >
> > >   (Mesh, FiniteElement, DofMap)
> > >
> > > instead of
> > >
> > >   (Mesh, ufc::finite_element, DofMap)
> > Sure. I just can't find where FiniteElement is defined?
> >
> > Garth
>
> I thought I had pushed it. It should be pushed now.
>
>
>
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