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[Johan Hake <question125106@xxxxxxxxxxxxxxxxxxxxx>]

Question #125106 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/125106

Johan Hake posted a new comment:
On Sunday September 12 2010 09:27:26 Anders Logg wrote:
> Question #125106 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/125106
> 
> Anders Logg posted a new comment:
> I think your problem is the way you call the Expression constructor:
> 
>   Expression(1)
> 
> This creates a vector-valued expression (with only one component).
> 
> Try with Expression() and it should work.

It looked like this helped. But why are a Vector expression with only one 
value different from a scalar expression?

Johan
 
> --
> Anders
> 
> On Sun, Sep 12, 2010 at 10:46:36AM -0000, Øystein Sørensen wrote:
> > New question #125106 on DOLFIN:
> > https://answers.launchpad.net/dolfin/+question/125106
> > 
> > I have a problem in which I need to define an Expression using
> > "complicated" C++ code. Before making the real code, I am trying to test
> > it, with the following trivial code:
> > 
> > -----------------------------------------------------------
> > 
> > from dolfin import *
> > 
> > code = '''
> > class SomeExpr : public Expression
> > {
> > 
> > public:
> >     SomeExpr() : Expression(1) {}
> >     
> >     void eval(Array<double>& values, const Data& data) const
> > 
> > {
> > 
> >     values[0] = data.x[0];
> > 
> > }
> > 
> > };
> > '''
> > 
> > mesh = UnitInterval(10)
> > V = FunctionSpace(mesh, 'CG', 2)
> > 
> > u = TrialFunction(V)
> > v = TestFunction(V)
> > f = Expression(code)
> > a = f*inner(grad(u),grad(v))*dx
> > L = Constant(0)*v*dx
> > 
> > -----------------------------------------------------------
> > 
> > When running it, I get the error message:
> > Trying to integrate expression of rank 1 with free indices ().
> > 
> > On the other hand, if I just write
> > f = Expression('x[0]'),
> > which is what I try to do in the above code, no error messages are
> > issued.
> 
> --
> Anders

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