dolfin team mailing list archive

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

On Fri, 2010-08-06 at 10:06 -0700, Johan Hake wrote:
> On Friday August 6 2010 09:52:20 Garth N. Wells wrote:
> > We have an issue with MixedFunctionSpace which makes incorrect usage
> > very easy. For example, if I have
> > 
> >     V = VectorFunctionSpace(mesh, "CG", 2)
> >     Q = FunctionSpace(mesh, "CG", 1)
> >     W = V * Q
> > 
> > The dof maps for Q and W.sub(1) are not the same. I've seen a number of
> > users using V or Q in setting boundary conditions rather than W.sub(0)
> > and W.sub(1).
> 
> I have done this mistake myself :P
> 
> I guess this problem only excist in POyDOLFIN?
> 

Yes.

> > When using
> > 
> >     W = MixedFunctionSpace([ V, Q])
> > 
> > could we create W and then inside the MixedFunctionSpace constructor set
> > 
> >    V = W.sub(0)
> >    Q = W.sub(1)
> 
> No as you are then just changing the local references of V and Q inside the 
> __init__ method. 

I realise this, but then at least Q and W.sub(1) will point to the same
space. It may be confusing the creating a mixed space changes V and Q.

> What you need is a method of FunctionSpace that transform it 
> to a SubSpace. Not sure that is possible?
> 
> Is there a user case where V and Q are used for what they are (not subspaces), 
> after a declaration of a mixed space? We will screw such a use if we turn V 
> and Q into a subspaces.
> 
> If this is not a consern, we might be able to tag V and Q as beeing used to 
> create a mixed space inside the __init__ of mixedfunctionspace and issue some 
> kind of warning (or exception!) whenever these are used to create a Function 
> or someother FunctionSpace dependent object. This can be done purely in the 
> Python layer. 
> 

I'm really not sure what the best approach is.

Garth

> Johan
> 
> > ?
> > 
> > Garth
> > 
> > 
> > 
> > 
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