Any way to retrieve subdomain ID (Python)?

mesh
1

Hi,

I want to extract the unique set of subdomain IDs, is there any easy interface of doing that?

I am currently doing it in a rather sloppy way. For instance:

cell_domains = MeshFunction('size_t', mesh, DIM)
cell_domains.set_all(100)
subdomains = set(cell_domains.array())

Thanks in advance,
Victor

2

I don’t think this is sloppy. You might get the result slightly faster with numpy.unique.

3

These methods will not work correctly in parallel.

4

Here is the parallel solution

from dolfin import *
import numpy as np

mesh = UnitSquareMesh(32, 32)
comm = mesh.mpi_comm()

f = MeshFunction('size_t', mesh, 2)
f.array()[:] = comm.rank

# Local unique
my_unique = np.unique(f.array())
all_unique = np.unique(np.hstack(comm.allgather(my_unique)))

print(all_unique)
5

Thanks for the reply, yes Allgather (uppercase as I use mpi4py with an older version of dolfin) is the way to go

Another question I have right now is Meshunction('size_t', mesh, dim-1) extracts all the subdomain IDs including those for both internal faces and boundary faces. I am wondering if you know a way to only extract boundary faces.

Thanks
Victor

6

Consider

from dolfin import *
import numpy as np

mesh = UnitSquareMesh(32, 32)
comm = mesh.mpi_comm()

f = MeshFunction('size_t', mesh, mesh.topology().dim()-1)
# Fake data
f.array()[:] = comm.rank

# Auxiliary bdry selector
bdry_f = MeshFunction('size_t', mesh, mesh.topology().dim()-1)
DomainBoundary().mark(bdry_f, 1)

# Local unique 
my_unique = np.unique(f.array()[bdry_f.array() == 1])
all_unique = np.unique(np.hstack(comm.allgather(my_unique)))

print(all_unique)
7

So basically, there’s not a direct way to discriminate boundary faces from internal faces without preset a subdomain IDs for boundary?