How to obtain dofs of FunctionSpace in dolfinx?

dolfinx
1

Hello all.

If this question sounds basic, it’s because it is.

I’m looking for the number of degrees of freedom of a given function space V. I expect a very large integer as output. My understanding from this documentation is that there should be a property of V called dim which will give me what I need.

However, with my dolfinx Docker installation in complex mode I only obtain AttributeError: 'FunctionSpace' object has no attribute 'dim'.

This arises even in the simplest test case :

import dolfinx
from mpi4py.MPI import COMM_WORLD
mesh=dolfinx.UnitSquareMesh(COMM_WORLD,2,2)
import ufl
FE=ufl.FiniteElement("Lagrange",mesh.ufl_cell(),1)
V=dolfinx.FunctionSpace(mesh,FE)
V.dim

I suppose there could be a way of hacking around this with a locate_dofs_geometrical but it sounds ugly. Any ideas ?

2

Consider:

import dolfinx
from mpi4py.MPI import COMM_WORLD
mesh=dolfinx.UnitSquareMesh(COMM_WORLD,2,2)
import ufl
FE=ufl.FiniteElement("Lagrange",mesh.ufl_cell(),1)
V=dolfinx.FunctionSpace(mesh,FE)

num_dofs_local = (V.dofmap.index_map.size_local) * V.dofmap.index_map_bs
num_dofs_global = V.dofmap.index_map.size_global * V.dofmap.index_map_bs

print(f"Number of dofs (owned) by rank {COMM_WORLD.rank}: {num_dofs_local}")
if COMM_WORLD.rank ==0:
    print(f"Number of dofs global: {num_dofs_global}")

Yielding:

root@81aa206a058d:~/shared# mpirun -n 1 python3 num_dofs.py 
Number of dofs (owned) by rank 0: 9
Number of dofs global: 9

root@81aa206a058d:~/shared# mpirun -n 2 python3 num_dofs.py 
Number of dofs (owned) by rank 0: 6
Number of dofs global: 9
Number of dofs (owned) by rank 1: 3
2 Likes
3

Thank you kindly @dokken, reactive and helpful as ever !

I still think it is a shame that the documentation (in the top 10 google searches for “dolfinx Function Space degrees of freedom”) has it wrong

4

can you provide a link to the documentation which has the error?

5

I believe I have.

But for reference : https://fenicsproject.org/olddocs/dolfinx/dev/python/generated/dolfinx.function.html#dolfinx.function.FunctionSpace

1 Like
6

Hm, didn’t know we had an olddocs for Dolfinx. @mscroggs will have a look at it.

2 Likes
7

It would be a good idea to delete the link to the old docs.