[anja]

This time I compiled:

vector Uav = vector::zero;
labelfixedPatchID = mesh.boundaryMesh().findPatchID("inlet");
if (fixedPatchID != -1)
{
forAll(U.boundaryField(), patchI)
{
if (U.boundaryField()[patchI])
{
Uav += average(U.boundaryField()[patchI]);
}
}
mesh.boundaryMesh()[fixedPatchID];
}
Info << "Inlet velocity: " << Uav << endl;

scalar pav = 0.0;
if (fixedPatchID != -1)
{
forAll(p.boundaryField(), patchI)
{
if (p.boundaryField()[patchI])
{
pav += average(p.boundaryField()[patchI]);
}
}
mesh.boundaryMesh()[fixedPatchID];
}
Info << "p : " << pav << endl;

To leave out the fixes value, as I only want the results for the inlet.
Error:
could not convert '(+ U. Foam::GeometricField<type,>::boundaryField [with Type = Foam::vector, PatchField = Foam::fvPatchField, GeoMesh = Foam::volMesh]())->Foam::GeometricField<foam::vector,>::GeometricBou ndaryField:: <anonymous>.Foam::FieldField<foam::fvpatchfield,>: :<anonymous>.Foam::PtrList<t>: :operator[] [with T = Foam::fvPatchField<foam::vector>](patchI)' to 'bool'