How do I export the results of mixed formulation?

[lilac]

Hi.
How do I export the results of mixed formulation with fenics-dolfinx v0.8.0?
The essence of my code is :

mesh = dolfinx.mesh.create_rectangle(MPI_COMM_WORLD,
                                     np.array([[0.0, 0.0], [width, height]]),
                                     [nx, ny],
                                     cell_type=dolfinx.mesh.CellType.quadrilateral)

Q2 = basix.ufl.element("CG", mesh.basix_cell(), 2, shape=(mesh.geometry.dim,), dtype=dolfinx.default_real_type)
Q1 = basix.ufl.element("CG", mesh.basix_cell(), 1, dtype=dolfinx.default_real_type)
MixedSpace = dolfinx.fem.functionspace(mesh,basix.ufl.mixed_element([Q2,Q1]))
V, dofV = MixedSpace.sub(0).collapse()
Q, dofQ = MixedSpace.sub(1).collapse()

# X={U,P} : solution of the problem
X = dolfinx.fem.Function(MixedSpace) 
solver.solve(X)
U = dolfinx.fem.Function(V)
U.x.array[:] = X.x.array[dofV]
P = dolfinx.fem.Function(Q)
P.x.array[:] = X.x.array[dofQ]

ofile = dolfinx.io.XDMFFile(mesh.comm,"result.xdmf","w")
ofile.write_mesh(mesh)
ofile.write_function(U)
ofile.write_function(P)

and this code causes error like :

Degree of output Function must be same as mesh degree. Maybe the Function needs to be interpolated?

Thank you.

[dokken]

If you search for this error on the forum

@lilac

Degree of output Function must be same as mesh degree. Maybe the Function needs to be interpolated?

You get the following posts where I explain what is going on

TypeError: BoundingBoxTree.__init__() takes 2 positional arguments but 3 were given

Meshtags now require the geometry as second input The decision of having to explicitly interpolate into a supported XDMF-file function space (will be a continuous lagrange space of same order as the mesh geometry) was made to highlight that this makes a loss of accuracy and can be an expensive operation. If you are using higher order Lagrange/DG elements, you should use VTKFile or VTXWriter, as they support this kind of output. If you are using other spaces, such as RT, Nedelec etc, they sho…

[lilac]

Hi @dokken .
I solved my problem because of you!
I appreciate your awnser!