Function in CNS_derive.cpp

[wazedxwxx]

I am learning the example in GPU->CNS. But the function

void cns_derpres (const Box& bx, FArrayBox& pfab, int dcomp, int /ncomp/,
const FArrayBox& rhoefab, const Geometry& /geomdata/,
Real /time/, const int* /bcrec/, int /level/)
{
auto const rhoe = rhoefab.array();
auto p = pfab.array();
Parm const* parm = CNS::d_parm;
amrex::ParallelFor(bx,
[=] AMREX_GPU_DEVICE (int i, int j, int k) noexcept
{
p(i,j,k,dcomp) = (parm->eos_gamma-1.)*rhoe(i,j,k);
});
}

void cns_dervel (const Box& bx, FArrayBox& velfab, int dcomp, int /ncomp/,
const FArrayBox& datfab, const Geometry& /geomdata/,
Real /time/, const int* /bcrec/, int /level/)
{
auto const dat = datfab.array();
auto vel = velfab.array();
amrex::ParallelFor(bx,
[=] AMREX_GPU_DEVICE (int i, int j, int k) noexcept
{
vel(i,j,k,dcomp) = dat(i,j,k,1)/dat(i,j,k,0);
});
}

in the file CNS_derive.cpp makes me unable to understand.
I think the operations in the function body of these two functions do not seem to be executed. Even if I delete the code inside the function body, I can still get the results complied by nvcc that look correct. I want to figure out what these two functions do？

[WeiqunZhang]

These functions are used to compute so-called derived variables. You don't seem to see a difference by deleting the function body, because they are only used in writing plotfiles. They compute pressure and velocity based on conservative variables. I assume what p = (gamma-1)*rhoe does is obvious. In cns_dervel, dat(i,j,k,1) is rho*vx and dat(i,j,k,0) is rho as specified in CNS_setup.cpp

    // Velocities                                                                                                                                                                                          
    derive_lst.add("velocity",IndexType::TheCellType(),AMREX_SPACEDIM,
                   {AMREX_D_DECL("x_velocity","y_velocity","z_velocity")},
                   cns_dervel,the_same_box);
    derive_lst.addComponent("velocity",desc_lst,State_Type,Density,1+AMREX_SPACEDIM);

So vel(i,j,k,dcomp) = dat(i,j,k,1)/dat(i,j,k,0); is just vx = rho*vx / rho.