Re: [Libmesh-users] Get the n_nodes for a certain bid?

[John P. <jwp...@gm...>]

On Mon, Nov 12, 2018 at 4:31 PM Stogner, Roy H <roy...@ic...>
wrote:

>
> On Mon, 12 Nov 2018, Yuxiang Wang wrote:
>
> > Quick question: how do I get the n_nodes for a certain boundary set
> (given
> > bid)? I know I can do:
> >
> > MeshBase::const_node_iterator nodeit =
> > mesh.bid_nodes_begin(NODE_BOUNDARY_ID);
> > const MeshBase::const_node_iterator node_end =
> > mesh.bid_nodes_end(NODE_BOUNDARY_ID);
> >
> > to loop through all the nodes, but would there be a way to find out how
> > many nodes are there in the same set?
>
> Just std::distance, I believe.
>

This might be a bit slow, depending on what else you are doing. Another
possibility would be to call:

std::vector<std::tuple<dof_id_type, boundary_id_type>> vec
  = mesh.get_boundary_info().build_node_list();

which returns a vector of _all_ the (node_id, boundary_id) pairs in the
mesh (they will be sorted by node_id). You could then count up the number
of entries with a specific boundary_id as needed...

-- 
John