Re: [Libmesh-devel] DofObject::var_to_vg()

[Roy S. <roy...@ic...>]

On Wed, 20 Jun 2018, Derek Gaston wrote:

> Right now I have exactly _one_ var group (which makes this even more
> puzzling)...

No kidding.  How many variables in the group?

> and I'm trying to just pull out the first dof from that group
> using dof_indices().  I then do all the rest of my indexing off of
> that first dof for all the other dofs in the group.

You mean using dof_number()?  DofMap::dof_indices() will at minimum
return you all the dofs for a single variable.

> Maybe there's a better way to be doing that?  Is there a more
> straightforward way to get the first dof in a var_group?

No, but (if you're using dof_number()) it sounds like what you want is
a dof_number_group() call instead.  Then you can determine vg and vig
*once* rather than in each dof_number() call.

We could use the same thing in DofMap::_dof_indices and
DofMap::old_dof_indices if it works well.

> I think I wouldn't mind just storing a var_num->var_group vector
> somewhere that every DofObject has access to.  The variable numbers
> are always contiguous starting from 0 so we should just be able to
> index into it.

The trouble is "has access to".  I'd rather avoid adding another 8
bytes per node+elem if we can.

Wait, I'm being stupid.  We don't actually have to make a pointer to
it a member variable!  Every time we call dof_number() we're doing so
with a DofMap in hand, so it would be easy to cache lookups there and
then pass a DofMap * to the DofObject as an argument with which to
access that cache.
---
Roy