Converting Function to an Expression

1

Hello,

I’m trying to create a tensor function on the mesh and I’m following the “Tensor-weighted Poisson” tutorial.
https://fenicsproject.org/docs/dolfin/1.3.0/python/demo/documented/tensor-weighted-poisson/python/documentation.html

In the tutorial, a Mesh Function is created for each entry of the tensor and then these functions are converted to Expressions.

However, when I run the supplied C++ code for creating an Expression from the functions, I get an error.

That is, when I run:

# Code for C++ evaluation of conductivity
conductivity_code = """

class Conductivity : public Expression
{
public:

  // Create expression with 3 components
  Conductivity() : Expression(3) {}

  // Function for evaluating expression on each cell
  void eval(Array<double>& values, const Array<double>& x, const ufc::cell& cell) const
  {
    const uint D = cell.topological_dimension;
    const uint cell_index = cell.index;
    values[0] = (*c00)[cell_index];
    values[1] = (*c01)[cell_index];
    values[2] = (*c11)[cell_index];
  }

  // The data stored in mesh functions
  boost::shared_ptr<MeshFunction<double> > c00;
  boost::shared_ptr<MeshFunction<double> > c01;
  boost::shared_ptr<MeshFunction<double> > c11;

};
"""

# Define conductivity expression and matrix
c00 = MeshFunction("double", mesh, "c00.xml.gz")
c01 = MeshFunction("double", mesh, "c01.xml.gz")
c11 = MeshFunction("double", mesh, "c11.xml.gz")

c = Expression(cppcode=conductivity_code)

I get the following error:

RuntimeError: Must supply C++ code to Expression. You may want to use UserExpression

Is there something I need to download to get the C++ code working correctly? Or is there something in this tutorial that is outdated?

Thanks for the help,
T

2

Hi, thats a really old dolfin version, 1.3, see the newest version (2018.1.0) here

3

That worked, thanks!

As a follow up question, do you know of a way to access the elements of UFL matrix objects.

Specifically, I am doing something like this:

C = as_matrix(((c[0], c[1]), (c[1], c[2])))

And I want to access the values of this matrix on each element (to check if they are correct).