How to sum functions?

Question

Hello all.
If i have two functions in FEniCS: f1 and f2, that defined on the same mesh how can i apply mathematical operators via C++ interface to them? I want to sum or divide them. (f3= f1+f2; f4=f1/f2, f2!=0).

Mesh mesh("./mesh.xml");
Function f1 (mesh,"./f1.xml");
Function f2 (mesh,"./f2.xml");
Function f3 (mesh);
Function f4 (mesh);
//f3=f1+f2;
//f4=f1/f2;

Answer 1

With the present development version of DOLFIN you should be able to do simple linear combinations, like your first example:

f3=a*f1+f2/b;

where a and b are scalars. If that does not work you should report an issue at bitbucket.

There are no support for Function division. Then you need to declare a projection form in UFL and solve a linear system.

Comments

Thanks for answer!
Is there any way to do it with current stable version (1.2.0)?

Answer 2

It is not quite clear what do you mean by division of two finite element functions. But if you think of DOF-wise division this could be done using numpy interface (as Vector interface supports only division by scalar). This should then done the job

f4.vector()[:] = f1.vector().array()/f2.vector().array()

assuming all the DOFs in f2.vector() are non-zero. Note that this might be quite slow for large vectors.