dolfin team mailing list archive

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

Anders Logg wrote:
> On Sat, Oct 17, 2009 at 05:41:54PM +0100, Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Sat, Oct 17, 2009 at 01:54:57PM +0100, Garth N. Wells wrote:
> > > > Johan Hake wrote:
> > > > > On Saturday 17 October 2009 12:32:54 Garth N. Wells wrote:
> > > > > > Will
> > > > > >
> > > > > >      uh = U[1]
> > > > > >
> > > > > > return a deep or shallow copy of the sub-Function?
> > > > > To avoid confusion with the ufl interface we have limited the interface for
> > > > > SubFunctions in PyDOLFIN to split. split returns a shallow copy by default.
> > > > > pass True to split and it will return a deepcopy. In your case it would be:
> > > > >
> > > > >   uh = U.split()[0]
> > > > >
> > > > > and
> > > > >
> > > > >   uh = U.split(True)[0]
> > > > >
> > > > > operator[] is used when you define forms. We have not yet managed to merge the
> > > > > two operations into one.
> > > > OK. Do we still have the function 'sub'?
> > > >
> > > >
> > > > > > Where is the operator [] define for sub-Functions?
> > > > > The operator you are using is the ufl one, which is defined in
> > > > > ufl.exproperators.py.
> > > > OK, but it's not clear to me then what's happing with this extract of my
> > > > code:
> > > >
> > > >   problem = VariationalProblem(a, L, bcs)
> > > >   Uh = problem.solve()
> > > >
> > > >   u = Expression("epx(x[0])", V = Vexact)
> > > >
> > > >   uh = Uh[1]
> > > >   M = (uh-u)*(uh-u)*dx
> > > >   error = sqrt(assemble(M, mesh=mesh))
> > > This should work fine since uh will be a component of the coefficient
> > > Uh in the UFL form.
> > >
> > > Same as when you write say inner(v, u)*dx or anything else that
> > > accesses components.
> > >
> > > So in summary [] can be used for any purpose in forms. It can also be
> > > used for plotting, but extracting the vector etc will not work.
> > It works as expected, but what's going on behind the scenes? Who is
> > creating the dof map and is the vector being copied?
>
> There's nothing out of the ordinary going on behind the scenes.

There must be something going on - either a reference to or a copy of 
the underlying vector, and a dof map. Where are these coming from?

Garth

> When you write
>
>   uh = Uh[1]
>
> it's no different from doing
>
>   a = uh[1]*v*dx
>
> or doing
>
>   gradu = grad(u)
>   a = dot(grad(v), gradu)*dx
>
> gradu is not a new Function, it's a UFL expression involving the grad
> operator and the Function u. gradu can be used in forms and plotted
> (since the plot will project it to a Function), but you can't do
> gradu.vector().
>
> --
> Anders
>
>
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