Map from dof index to vertex index

[Kaushik Das]

Hello,

Is there a way to know which dof_index is located at which vertex. 

For example, I want to "map" a dof_index that I get from affine_constrains.get_constraint_entries(line);

to a vertex index that are in the range of triangulation.begin_vertex() and triangulation.end_vertex().

Thank you,

Kaushik 

[Wolfgang Bangerth]

Kaushik,
there is no such way in the library because, in general, DoFs need not be
located on vertices. That is only true for the Q1 element.

But if you look at the implementation of the
DoFTools::map_dofs_to_support_points(), you can probably come up with a way to
get what you are looking, assuming you really are only using Q1 elements.

Best
W.


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Wolfgang Bangerth email: bang...@colostate.edu
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[Jean-Paul Pelteret]

To add to what Wolfgang has already said, there’s this entry in our FAQ on this topic (well, the reverse mapping from vertex to DoF):

https://github.com/dealii/dealii/wiki/Frequently-Asked-Questions#how-do-i-get-the-degree-of-freedom-indices-at-vertices

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[Kaushik Das]

Thank you, Prof. Bangerth. 

Yes, I am using Q1 elements. 

I was able to do it following your hint. 

I am assuming that the ordering/sequence of dofs that we get from cell->get_dof_indices() will always be the same as the ordering/sequence of vertexes in a cell, that is the 'canonical' ordering given in GeometryInfo. Will that be true for linear hex Q1 elements always?

Thank you very much,

Kaushik 

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[Wolfgang Bangerth]

On 7/17/21 6:39 AM, Kaushik Das wrote:
> I am assuming that the ordering/sequence of dofs that we get
> from cell->get_dof_indices() will always be the same as the ordering/sequence
> of vertexes in a cell, that is the 'canonical' ordering given in GeometryInfo.
> Will that be true for linear hex Q1 elements always?

Yes.