dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Sunday 07 December 2008 14:57:01 Martin Sandve Alnæs wrote:
> 2008/12/6 Johan Hake <hake@xxxxxxxxx>:
> > On Saturday 06 December 2008 20:54:34 Anders Logg wrote:
> >> On Sat, Dec 06, 2008 at 04:03:25PM +0100, Johan Hake wrote:
> >> > 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 can get make_subspace working as you suggest below, then it
> >> would be easy to modify __add__ for FunctionSpaceBase:
> >>
> >>   W = MixedFunctionSpace([self, other])
> >>   self.make_subspace(W, 0)
> >>   other.make_subspace(W, 1)
> >>   return W
> >
> > Yes something like that.
> >
> >> This would make the interface look nice but someone (you know who you
> >> are) might think we are insane... :-)
> >
> > *laugh*
> >
> > I am not very happy with it either, as the __add__ operator should not do
> > such a thing. What we are saying is that V and Q are individual
> > FunctionSpaces before we make make the add. After we have made it they
> > are suddenly subspaces of the result of an addition. Not very intuitive.
> >
> > One slution could be to remove the __add__ operator and instead force the
> > user to instantiate the Mixed space with a constructor, e.g.
> >
> >   W = MixedFunctionSpace(V,Q)
> >
> > With this it is easier to justify that we are doing somthing with V and
> > Q.
>
> No it isn't.
>
> This isn't a matter of taste, it just doesn't scale outside a minimal
> example.
>
> What will happen here?
>   W0 = MixedFunctionSpace(V0,V1) # or V0 + V1, same thing
>   W1 = MixedFunctionSpace(V1,V2)
>   W2 = MixedFunctionSpace(V2,V3)

He, he, thats true! This is probably a dead horse...

> >> > 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?
> >>
> >> The only reason we need this is to be able to set boundary conditions
> >> for sub systems. For that we need two things: the offset into the
> >> global system vector and the DofMap for the subsystem in question.
> >> There might be other ways to define the interface for DirichletBC,
> >> for example:
> >>
> >>   bc = DirichletBC(W, 0, ...)
> >
> > This is maybee the best one? The DirichletBC operates at the global
> > vector from the mixed space, and then it is more intuitive to actually
> > use the mixed space to set the BC.
>
> Agree.
>
> But we also need to assign a subfunction to a function and vice versa,
> perhaps like:
>
> u.assign(U.sub(0))
> p.assign(U.sub(1))
> U.sub(0).assign(u)
> U.sub(1).assign(p)

I think this is already taken care of. The cpp.SubFunction wont be seen in the 
python interface. It is hidden in f._sub(). The f.sub(0) will return a new 
Function, which is instantiated using the SubFunction.

Johan