Mfem: Subtracting two gridfunctions?

[saisan7973]

Hi there,

How can I subtract one gridfunction from another?
The following code to calculate c=a-b does not produce errors when running, but it does not return a plot and gives an error in glvis.

Please help.

Sai

MFEM abort: input stream is not a FiniteElementSpace!
 ... in function: mfem::FiniteElementCollection* mfem::FiniteElementSpace::Load(mfem::Mesh*, std::istream&)
 ... in file: fem/fespace.cpp:2216

   GridFunction a(&fes_h1);
   GridFunction b(&fes_h1);
   //values calculated for a and b
   GridFunction c(&fes_h1);
   c=a-b;
   socketstream c_sock(vishost, visport);
   c_sock.precision(8);
   c_sock << "solution\n" << *mesh << c << "window_title 'GridFunction c'" <<flush;

[rw-anderson]

Try:

c = a;
c -= b;

[saisan7973]

That was unexpected. Thank you!

I didn't find -= or += operator in http://mfem.github.io/doxygen/html/classmfem_1_1GridFunction.html. How did you know that it would work? Please share.

[sshiraiwa]

GridFunction is Vector (subclass)
https://github.com/mfem/mfem/blob/master/linalg/vector.hpp#L250

[tzanio]

I'll close this now, feel free to reopen if you have further questions.