Cannot create 1d mesh

[dokken]

The whole point is that DOLFIN doesn’t care about if the facets are of dimension 1 or 2.
ds is an integration measure for entities of one degree lower dimension than the cells of the mesh (i.e. 1 for 2D meshes, 2 for 3D meshes).

@mekong

addition to the test for ds_l and ds_r that I have been asking about, the following test for the full line ds also fails: I generate line_mesh.xdmf and line_mesh.h5 with your code and then integrate the f(x) = x^2f(x)=x2f(x) = x^2 over the line

As I have now stated a few times, please be incredibly careful when mixing integration measures and mesh functions.

1. You have a mesh of lines (i.e. topological dimension 0).
2. To read in data regarding facets (the information that should be in ds integrals, you need to read in data about the vertices, as done in:

@dokken

mesh = Mesh()
xdmf = XDMFFile(mesh.mpi_comm(), "mesh.xdmf")
xdmf.read(mesh)
mvc = MeshValueCollection("size_t", mesh, mesh.topology().dim())
with XDMFFile("mesh.xdmf") as infile:
    infile.read(mvc, "name_to_read")
cf = cpp.mesh.MeshFunctionSizet(mesh, mvc)
xdmf.close()

mvc = MeshValueCollection("size_t", mesh, mesh.topology().dim()-1)
with XDMFFile("surface.xdmf") as infile:
    infile.read(mvc, "name_to_read")
sf = cpp.mesh.MeshFunctionSizet(mesh, mvc)
xdmf.close()