How to include a function of independent variable x into weak form?

General Discussion
1

Hi all, what’s the best way to include a function of spatial variable x into the weak form? I know that freefem has x and y built in, but is it still necessary to interpolate them onto the mesh or can I simply enter them as they are? E.g:

mesh dom4;
dom4=buildmesh(…);

fespace Vh(dom4, P2);
Vh u, v;
…
Vh xx=x; <----- is this neccessary to include a function of x in the weak form of my problem?

problem(u,v)
=int2d(dom4)(… f(xx)… ) <---- Here, f is just an arbitrary function.
…
;

OR, can I write it as follows without interpolating x onto the mesh:

problem(u,v)
=int2d(dom4)(… f(x)… )
…
;