dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Saturday 06 December 2008 15:43:13 Anders Logg wrote:
> On Sat, Dec 06, 2008 at 03:34:40PM +0100, Johan Hake wrote:
> > On Saturday 06 December 2008 14:56:59 Anders Logg wrote:
> > > On Sat, Dec 06, 2008 at 12:05:41PM +0100, DOLFIN wrote:
> > > > One or more new changesets pushed to the primary dolfin repository.
> > > > A short summary of the last three changesets is included below.
> > > >
> > > > changeset:   5262:0e349fbe09ce4179252652fe5c1d58725951bc3f
> > > > tag:         tip
> > > > user:        "Johan Hake <hake@xxxxxxxxx>"
> > > > date:        Sat Dec 06 12:05:42 2008 +0100
> > > > files:       demo/pde/stokes/taylor-hood/python/demo.py
> > > > dolfin/function/SpecialFunctions.h dolfin/swig/dolfin_function_pre.i
> > > > site-packages/dolfin/function.py description:
> > > > Added support for f.split()
> > > >  - Renamed operator[] to f._sub instead of f.sub
> > > >  - f.sub(i) now returns an instantiated sub function
> > > >  - f.split() uses f.sub() to return a tuple of all sub functions
> > > >  - stoke/taylor-hood demo now runs.
> > >
> > > Excellent!
> > >
> > > It runs now but the solution looks completely crazy. The problem is
> > > that the boundary conditions are not set correctly since we use V and
> > > Q to set the boundary conditions for the sub systems and they don't
> > > know the offsets (DofMap::offset()).
> >
> > The problem here is that these spaces need to be SubSpaces?
>
> Yes.
>
> > If that is the case, we could extract the subspaces after a
> > MixedFunctionSpace is created and then store these in the spaces
> > attribute, either as pure cpp.SubSpaces or add another python class,
> > SubSpace, which is a cpp.SubSpace and stores the original ffc.element
> > too, or something?
>
> I was thinking something like this:
>
> W = VectorFunctionSpace(mesh, "triangle", 2) + FunctionSpace(mesh,
> "triangle", 1) V, Q = W.split()

Doesn't look too nice, and it will create a non intuitive work flow. Combining 
two interfaces aren't easy!

> The split function needs to both create SubSpaces (which will lead to
> DofMaps with correct offsets) and set the element correctly, which can
> be done by looking at the spaces attribute in MixedFunctionSpace.
>
> It's a bit weird since what we really do is
>
> V = VectorFunctionSpace(mesh, "triangle", 2)
> Q = FunctionSpace(mesh, "triangle", 1)
> W = V + Q
> V, Q = W.split()
>
> First, we put V and Q together to create a mixed function space. Then
> we split W again into V and Q. After the split V and Q know that they
> are part of the bigger space (at least they know the offset into the
> bigger space).

Yes I see this. Instead, or in addition, of using split, we could implement 
W.sub(i) that returns a subspace, which is then used when setting the bc, and 
other relevant places.

> The other option would be to let + have a side effect on V and Q but
> that does not seem to be a good solution.

I thought of this too. But I cannot se how we could do it with the present 
implementation of FunctionSpace/Subspace. 

If we could just call a member function in a FunctionSpace, let say 
V.make_subspace(W,i), and this would then create the needed stuff in V, to 
make it SubSpaceable. But now SubSpace is a subclass which effectively 
prevents this approach.

This approach is abit intrucive too. But what difference would it make for the 
user, i.e., if V in addition to be a FunctionSpace now also is a subspace of 
W?

Johan