Project expression to subfunction

Question

I want to project an expression to a 0th-degree DG subfunction (i.e. function of a "subspace" of a mixed space). Below is my attempt. However, it seems something goes wrong, perhaps the order of the DOFs is handled incorrectly. That is suggested by the plot of the resulting function. I have read about similar issues on the forums and tried FunctionAssigner, but without success.

from dolfin import *

mesh = UnitSquareMesh(10, 10)

RT    = FunctionSpace(mesh, "RT", 1)
P     = FunctionSpace(mesh, "DG", 0)
mixed = MixedFunctionSpace([RT, P, P])

psi0  = Function(mixed);

f = Expression("x[0]")

assign(psi0.sub(2), project(f, mixed.sub(2).collapse()))

plot(psi0.sub(2))

interactive()

I tried this with version 1.3.0 of Fenics.

Answer 1

This should be fixed in the development version of DOLFIN.

Answer 2

I guess this is related to Issue #93, because if I replace the RT-space with the P-space, it seems to work. This is however not desirable for me. Is there some workaround that I could use to accomplish the same thing?