Invalid value rank UFL/C++

Question

Dear all,

I have a question: I'm porting a python project to UFL/C++, but I am encountering errors. All the code I am posting here is inspired by the undocumented/dg-poisson/cpp/ demo.

The error is the following:

*** Error:   Unable to assemble form.
*** Reason:  Invalid value rank for coefficient 1 (got 0 but expecting 1). You might have forgotten to specify the value rank correctly in an Expression subclass.
*** Where:   This error was encountered inside AssemblerBase.cpp.

The test is easy: a plate clamped at the bottom and pulled at the top, so one Dirichlet and one Neumann BCs. In my UFL I have (here only the relevant parts, no bulk forces):

cell    = triangle
element = VectorElement("Lagrange", cell, 2)

u = TrialFunction(element)
v = TestFunction(element)
force_vector = Coefficient(element)

# Known term
L = inner(force_vector, v) * ds(1)

My C++ main file is this:

        // Neumann BCs
        EdgeForceExpression force;

        // Boundary markers set by default to 0 (MAYBE!)
        FacetFunction<std::size_t> boundary_markers(mesh, 0);

        // Mark the edge with load as 1
        top.mark(boundary_markers, 1);

        // Set loads RHS
        L.f            = f_bulk;
//        L.force_vector = f_surf;
        L.force_vector = force;
        L.ds           = boundary_markers;

You can see that force_vector is assigned a force object, this is an instance of the following class:

// Force on an edge
class EdgeForceExpression : public Expression
{
    void eval(Array<double>& values, const Array<double>& x) const
    {
        values[0] = 1.0e6;
        values[1] = 0.0;
    }
};

As you can see, in the main file there's a commented assignment for the force to f_surf, defined as Constant f_surf(1.0e6, 0.0);. If I use this, the program runs fine, and produces correct results.

Why can't I use the force as defined by eval in the class? I cannot see how, but I'm a newbie in this...

Thanks!

Answer 1

Hi, despite the eval method your code constructs a scalar valued expression and that's why you get rank 0 in the error message. You need to add a proper constructor to make the expression vector valued (rank 1). This demo shows how to go about it.

Comments

Perfectly clear, I mistakenly assumed that expressions were implicitly 3D.

Thanks!