How to add a new field output variable

This page describes the basic steps required to add a new variable to the field output.

This example adds a variable pressure to the field output for the multi-material PDE system:

Determine if the new variable is available from the primitive variable vector, or the conserved solution vector. If conserved, skip this step. If primitive, add corresponding entry to the primitive() function in src/Control/Inciter/OutVar.hpp.
Add the "outvar-function" corresponding to the variable to be extracted for output, in src/PDE/MultiMat/Problem/FieldOutput.hpp inside the section titled "Functions that compute physics variables from the numerical solution". Replace the MultiMat in the above path by the appropriate PDE system name.
```
//! Compute bulk pressure for output to file
//! \note Must follow the signature in tk::GetVarFn
//! \param[in] U Numerical solution
//! \param[in] rdof Number of reconstructed solution DOFs
//! \return Bulk pressure ready to be output to file
static tk::GetVarFn::result_type
bulkPressureOutVar( const tk::Fields& U, std::size_t rdof )
{
  using tk::operator+=;
  auto nmat = g_inputdeck.get< tag::multimat, tag::nmat >();
  auto p = U.extract_comp( pressureDofIdx(nmat,0,rdof,0) );
  for (std::size_t k=1; k<nmat; ++k)
    p += U.extract_comp( pressureDofIdx(nmat,k,rdof,0) );
  return p;
}
```
When adding this function, a few things to remember:
- Ensure that the newly added function follows the function signature of tk::GetVarFn declared in src/PDE/FunctionPrototypes.hpp.
- When the primitive flag has been set in step 1, the first argument of this function will be the primitive vector. Otherwise, it will be the conserved solution vector. In this example, the multimat PDE system stores material pressures in the primitive vector. Therefore, the input arg U is the primitive vector.
- Access the primitive/solution vector appropriately and store the values into a std::vector< tk::real >. Data::extract_comp() can be used for this purpose. This newly allocated vector should be returned by the function.
Link the newly defined outvar-function to the keyword in MultiMatOutVarFn() in src/PDE/MultiMat/Problem/FieldOutput.cpp as: OutFnMap["pressure"] = multimat::bulkPressureOutVar;