Fem-fenics: Difference between revisions

* All the objects are managed using boost::shared_ptr <>. It means that '''the same resource can be shared by more objects''' and useless copies should be avoided. For example, if we have two different functional spaces in the same problem, like with Navier-Stokes for the velocity and the pressure, the mesh is shared between them and no one has its own copy.

* The '''essential BC are imposed directly to the matrix''' with the command '''assemble()''', which sets to zero all the off diagonal elements in the corresponding line, sets to 1 the diagonal element and sets to the exact value the rhs. This means that we could loose the symmetry of the matrix, if any. To avoid this problem and preserve the symmetry of the system it is available the '''assemble_system()''' command which builds at once the lhs and the rhs.

* The '''coefficient of the variational problem''' can be specified using either an ''Expression()'', a ''Constant()'' or a ''Function()''. They are different objects which behave in different ways: an ''Expression'' or a ''Constant'' object overloads the eval() method of the  dolfin::Expression class and it is evaluated at run time using the octave function feval (). A ''Function'' instead doesn't need to be evaluated because it is assembled copying element-by-element the values contained in the input vector.