Directly output exact solution.

src/driver.hh

@@ -46,6 +46,7 @@
 //== }
 #include"problem.hh"
 #include "gridoperator.hh"
+#include "vtkfunction.hh"
 
 template<typename GV>
 void driver (const GV& gv, Dune::ParameterTree& ptree)
@@ -151,13 +152,11 @@ void driver (const GV& gv, Dune::ParameterTree& ptree)
   newton.apply();
 
   // Write VTK output file
-  Z w(gfs); // Lagrange interpolation of exact solution
-  Dune::PDELab::interpolate(g,gfs,w);
   int subsampling = ptree.get("output.subsampling",(int)1);
   Dune::SubsamplingVTKWriter<GV> vtkwriter(gv,Dune::refinementIntervals(subsampling));
   using Function = Dune::P1VTKFunction<GV, typename Z::Container>;
   vtkwriter.addVertexData(std::make_shared<Function>(gv, *z.storage(),"fesol"));
-  vtkwriter.addVertexData(std::make_shared<Function>(gv, *w.storage(),"exact"));
+  vtkwriter.addVertexData(PPS::makeScalarVTKGridFunction<GV>(glambda, "exact"));
   vtkwriter.write(ptree.get("output.filename","output"),
                   Dune::VTK::appendedraw);
 }

src/vtkfunction.hh

+#ifndef PACXX_PROJECTSEMINAR_2019_SRC_VTKFUNCTION_HH
+#define PACXX_PROJECTSEMINAR_2019_SRC_VTKFUNCTION_HH
+
+#include <memory>
+#include <string>
+#include <utility>
+
+#include <dune/common/fvector.hh>
+
+#include <dune/grid/io/file/vtk/function.hh>
+
+namespace PPS {
+
+  template<class GridView, class Functor>
+  class ScalarVTKGridFunction :
+    public Dune::VTKFunction<GridView>
+  {
+    Functor functor_;
+    std::string name_;
+
+  public:
+    typedef typename GridView::ctype ctype;
+    enum { dim = GridView::dimension };
+    typedef typename GridView::template Codim< 0 >::Entity Entity;
+
+    ScalarVTKGridFunction(Functor functor, std::string name) :
+      functor_(std::move(functor)),
+      name_(std::move(name))
+    { }
+    int ncomps() const override { return 1; }
+    std::string name () const override { return name_; }
+    double evaluate(int comp, const Entity& e,
+                    const Dune::FieldVector<ctype,dim>& xi) const override
+    {
+      assert(comp == 0);
+      return functor_(e, xi);
+    }
+  };
+
+  template<class GridView, class Functor>
+  auto makeScalarVTKGridFunction(Functor functor, std::string name)
+  {
+    return std::make_unique<ScalarVTKGridFunction<GridView, Functor> >
+      (std::move(functor), std::move(name));
+  }
+
+} // namespace PPS
+
+#endif // PACXX_PROJECTSEMINAR_2019_SRC_VTKFUNCTION_HH