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["Garth N. Wells" <g.n.wells@xxxxxxxxxx>]

OK. It's taking me a while to get into the new structure.

So, for a 2D mesh, mesh.init(1, 2) computes which cells share an edge
and mesh.init(2, 1) computes which edges belong to a cell?  Inside
mesh.init(2, 1) would therefore be the place to compute local edge ID's?

Garth


Anders Logg wrote:
> On Tue, Oct 17, 2006 at 12:16:40PM +0200, Garth N. Wells wrote:
>> I'm running into a few problems trying to understand the new mesh
>> structure. If I do
>>
>>     UnitSquare mesh(1,1);
>>
>>     for (FacetIterator facet(mesh); !facet.end(); ++facet)
>>       cout << "Number of connected cell "
>>           << (*facet).numConnections(mesh.topology().dim()) << endl;
>>
>> I get the error
>>
>>     *** Assertion (entity < num_entities) failed
> 
> This is correct behaviour, but perhaps the error message should be
> more informative.
> 
> When you read a mesh from file or create a simple mesh like a mesh of
> the unit square or unit cube, the only connectivity that exists is
> 
>     mesh -> vertices
>     mesh -> cells
> 
>     cell -> vertices
> 
> In the above example, you use the connectivity
> 
>     cell -> facets (edges in this case)
> 
> This is possible since the iterators automatically generate the
> requested connectivity the first time they are used. So when you
> create a FacetIterator over the mesh, the facets of the mesh are
> created. 

The next time you iterate over facets, the facets don't need
> to be created.
> 
> But then the problem comes when you want to use the connectivity
> 
>     facet -> cells
> 
> This does not exist and you don't create an iterator that
> automatically computes it. So in this case you need to build the
> connectivity. This can be done by
> 
>     mesh.init(1, 2); // facet --> cell
> 
> If you call this first, then the above example should work.
> 
> The reason this worked for the old mesh is that then all the
> connectivity was computed at startup. Now, only the connectivity that
> is actually used is computed.
> 
> Any ideas for how to make this more user-friendly are welcome. My idea
> is that as long as you work with the iterators, everything gets
> generated as needed and otherwise you need to explicitly compute the
> data you need.
> 
>> but it I create a boundary mesh first,
>>
>>     UnitSquare mesh(1,1);
>>     MeshFunction<dolfin::uint> vertex_map;
>>     MeshFunction<dolfin::uint> cell_map;
>>     BoundaryMesh boundary(mesh, vertex_map, cell_map);
>>
>>     for (FacetIterator facet(mesh); !facet.end(); ++facet)
>>       cout << "Number of connected cell "
>>           << (*facet).numConnections(mesh.topology().dim()) << endl;
>>
>> everything works as expected. I guess the mesh is not building the list
>> of facets, and this is being done when creating a boundary mesh. Is this
>> a bug?
>>
>> Garth
> 
> This works because to compute the boundary, the connectivity facet -> cells
> is needed. So during the computation of the boundary, this gets
> computed and then exists when you ask for it.
> 
> The above example should work also with just a simple
> 
>   BoundaryMesh boundary(mesh);
> 
> The vertex and cell maps are don't needed for this.
> 
> /Anders
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