[Dune] STL vector and CGSolver

[Arne Rekdal]

Hello all.

I am currently working on implementing a simple FEM solver with DUNE.  
I am solving a problem with only Dirichlet boundaries. Currently I  
mark the vertices which is part of the boundary, and the unknowns are  
all the vertices minus the boundary vertices.

The plan next was to create a linear operator (A*x) so that the global  
matrix is not needed to be formed explicitly, and solve the equation  
system with conjugate gradients method.

I am encountering problems with this approach. The first problem is  
that I try to use vectors of type FieldVector<Type,Size>. In this  
class it is needed to know the size at compilation, but this is not  
available without finding the number of boundary vertices first. I  
have also tried to use std::vector<Type> representation, but this  
class does not have the ::field_type type definition which is used in  
CGSolver. I also see that a preconditioner is required, but I am not  
sure how to create one.

I am looking for explanations for how/why use fixed size vectors. Is  
the FieldVector intended for this use? Any help would be greatly  
appreciated.


Best regards
Arne Rekdal