# Error in tutorial related to fenicsx

**URL:** <https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192>\
**Category:** Uncategorized\
**Created:** [22 May 2023 13:06 UTC](https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192 "2023-05-22T13:06:34Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Mk2020](https://avatars.discourse-cdn.com/v4/letter/m/a5b964/32.png) [@Mk2020](https://bempp.discourse.group/u/Mk2020)\
**Post date:** [22 May 2023 13:06 UTC](https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192/1 "2023-05-22T13:06:34Z")

</div>

Hi everyone,

I want to use Fenicsx together with Bempp to calculate a double-layer potential. When running the tutorial “Simple FEM-BEM coupling for the Helmholtz equation with FEniCSx” I get an error. How I can fix that?

Thanks

```auto
import dolfinx
from dolfinx.fem import FunctionSpace, Function
from dolfinx.mesh import create_unit_cube
import dolfinx.geometry
import ufl
from mpi4py import MPI
import bempp.api
import numpy as np

mesh = create_unit_cube(MPI.COMM_WORLD, 10, 10, 10)

from bempp.api.external import fenicsx

fenics_space = FunctionSpace(mesh, ("CG", 1))
trace_space, trace_matrix = \
    fenicsx.fenics_to_bempp_trace_data(fenics_space)
bempp_space = bempp.api.function_space(trace_space.grid, "DP", 0)

fem_size = fenics_space.dofmap.index_map.size_global
bem_size = bempp_space.global_dof_count

print("FEM dofs: {0}".format(fem_size))
print("BEM dofs: {0}".format(bem_size))

```

The error is:

```auto
TypeError Traceback (most recent call last)
Input In [6], in <cell line: 16>()
     12 from bempp.api.external import fenicsx
     14 fenics_space = FunctionSpace(mesh, ("CG", 1))
     15 trace_space, trace_matrix = \
---> 16 fenicsx.fenics_to_bempp_trace_data(fenics_space)
     17 bempp_space = bempp.api.function_space(trace_space.grid, "DP", 0)
     19 fem_size = fenics_space.dofmap.index_map.size_global

File ~/miniconda3/envs/fenics/lib/python3.8/site-packages/bempp/api/external/fenicsx.py:61, in fenics_to_bempp_trace_data(fenics_space)
     59 if family in ["Lagrange", "P"]:
     60 if degree == 1:
---> 61 return p1_trace(fenics_space)
     62 else:
     63 raise NotImplementedError()

File ~/miniconda3/envs/fenics/lib/python3.8/site-packages/bempp/api/external/fenicsx.py:117, in p1_trace(fenics_space)
    114 except AttributeError:
    115 pass
--> 117 for tet, cell_verts in enumerate(tets):
    118 cell_dofs = fenics_space.dofmap.cell_dofs(tet)
    119 for v in range(4):

TypeError: 'dolfinx.cpp.graph.AdjacencyList_int32' object is not iterable

```

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<div class="post-metadata">

**Author:** ![mscroggs](https://yyz2.discourse-cdn.com/free1/user_avatar/bempp.discourse.group/mscroggs/32/3_2.png) [@mscroggs](https://bempp.discourse.group/u/mscroggs)\
**Post date:** [22 May 2023 13:18 UTC](https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192/2 "2023-05-22T13:18:02Z")

</div>

I’ll look into this. Which version of FEniCSx and Bempp are you using?

---

<div class="post-metadata">

**Author:** ![Mk2020](https://avatars.discourse-cdn.com/v4/letter/m/a5b964/32.png) [@Mk2020](https://bempp.discourse.group/u/Mk2020)\
**Post date:** [22 May 2023 14:30 UTC](https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192/3 "2023-05-22T14:30:39Z")

</div>

Hi, I have installed Bempp CL with anaconda (conda install -c conda-forge bempp-cl), The version is v0.3.0. The version of FEniCSx that I use is 0.6.

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<div class="post-metadata">

**Author:** ![mscroggs](https://yyz2.discourse-cdn.com/free1/user_avatar/bempp.discourse.group/mscroggs/32/3_2.png) [@mscroggs](https://bempp.discourse.group/u/mscroggs)\
**Post date:** [20 July 2023 13:56 UTC](https://bempp.discourse.group/t/error-in-tutorial-related-to-fenicsx/192/4 "2023-07-20T13:56:29Z")

</div>

I think this error should be fixed in [Update to work with latest FEniCSx by mscroggs · Pull Request #200 · bempp/bempp-cl · GitHub](https://github.com/bempp/bempp-cl/pull/200). I’ll make a new release in the next fews days once I’ve merged this fix.