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[James Avery <question156272@xxxxxxxxxxxxxxxxxxxxx>]

New question #156272 on DOLFIN:
https://answers.launchpad.net/dolfin/+question/156272

Hi all,

I have a very simple question that, for some reason, I'm having a hell
of a time figuring out. I need to evaluate the integral of some
function on each cell -- in order to determine whether or not to
subdivide the cell in question. With Deal.II, I would do something like 
the following:

Let 'density' be the function that should be integrated on the cell.
Then

  fe_values.reinit (cell); 	
  quadrature_points = fe_values.get_quadrature_points(); 

  // Compute the function value in the cell's quadrature points
  density.value_list(quadrature_points,function_values); 

  // Integral is function values dotted with the Jacobian-times-weights.
  double cell_density = 0;
  for(size_t q=0;q<N_q;q++)
    cell_density += function_values[q]*fe_values.JxW(q);

computes the cell-integral, where the types above are as follows:

  Function<dim> density;	// This is a continuous function, like the FEniCS 'Expression' class.
  FEValues<dim> fe_values(fe, quadrature_formula, update_quadrature_points | update_JxW_values);
  vector< Point<dim> > quadrature_points(N_q);

I've been searching both the documentation and the source code for a
long time, and I still haven't figured out how to do this with
Dolfin. It seems I'm completely blind; a bit of help crossing the
street would be much appreciated!

Many thanks in advance.

Cheers, 
James

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