splitting mixed vectors

On 30 August 2012 21:15, Jaroslav Hron
<email address hidden> wrote:
> New question #207313 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/207313
>
> Hi. If w is a function from a MixedSpace, what is the exact difference between (v,p)=w.split() and (v,p)=split(w)?

The difference is a bit tricky. The Python interface is a synthesis of
UFL (the form language) and DOLFIN (the solver environment). The
'split' version

    (v,p)=split(w)

is a UFL function. UFL Functions and Coefficients are abstract in that
they don't have any data (e.g. no dof map, no dof vector). The 'split'
version

    (v,p)=w.split()

is a DOLFIN function that works with a concrete Function w (i.e., one
that does have data). Generally, you'll want to use split(w) when
constructing forms but before solving, and split() after a solve.

What's awkward is that in the Python interface a Function is a
subclass of both a UFL Function and a DOLFIN Function. It's something
that we'll look at in the future to clean up.

Garth

> JH
>
> Example:
>
> mesh = UnitSquare(32,32)
> V = VectorFunctionSpace(mesh, "CG", 2)
> P = FunctionSpace(mesh, "CG",1)
> W = MixedFunctionSpace([V,P])
>
> (u, q) = TestFunctions(W)
>
> w = Function(W)
> (v, p) = w.split() # <- this doesn't work, gives Exception: Form (<empty Form>) seems to be zero: cannot compile it.
>
> #(v, p) = (as_vector((w[0], w[1])), w[2]) # <- this is ok
> #(v,p) = split(w) # <- this is ok
>
> F = -p*div(u)*dx + inner(grad(v),grad(u))*dx + div(v)*q*dx
> solve(F==0, w , bcs=bcs)
>
>
>
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