Convert from a mesh tag to a mesh field

tagging.cc

#include <gmi_mesh.h>
#include <apfMDS.h>
#include <apfMesh2.h>
#include <apf.h>
#include <PCU.h>
#include <cassert>

apf::MeshTag* create_my_tag(apf::Mesh* m) {
  apf::MeshTag* tag = m->createDoubleTag("my_tag", 1);
  apf::MeshEntity* vtx;
  apf::MeshIterator* it = m->begin(0);
  double vals[1];
  vals[0] = 1.0;
  while ((vtx = m->iterate(it)))
    m->setDoubleTag(vtx, tag, vals);
  m->end(it);
  return tag;
}

apf::Field* convert_my_tag(apf::Mesh* m, apf::MeshTag* t) {
  apf::MeshEntity* vtx;
  apf::MeshIterator* it = m->begin(0);
  apf::Field* f = apf::createLagrangeField(m, "my_field", apf::SCALAR, 1);
  double vals[1];
  while ((vtx = m->iterate(it))) {
    m->getDoubleTag(vtx, t, vals);
    apf::setComponents(f, vtx, 0, vals);
  }
  m->end(it);
  return f;
}

int main(int argc, char** argv) {
  assert(argc==3);
  MPI_Init(&argc, &argv);
  PCU_Comm_Init();
  gmi_register_mesh();
  apf::Mesh2* m = apf::loadMdsMesh(argv[1],argv[2]);
  m->verify();
  apf::MeshTag* my_tag = create_my_tag(m);
  convert_my_tag(m, my_tag);
  apf::writeVtkFiles("output", m);
  m->destroyNative();
  apf::destroyMesh(m);
  PCU_Comm_Free();
  MPI_Finalize();
}