[petsc-users] How to get cell number from node number?

[Matthew Knepley]

On Fri, Nov 30, 2018 at 8:19 AM barry via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Sorry for the imprecisely description.
>
> I have an unstructured grid (DMPLEX) with triangle mesh.
>
> cell number (2 dim)={0};    line number (1 dim)={1, 2, 3};    node number
> (0 dim)={4, 5, 6}
>
> So, you have a 2D interpolated simplex mesh in DMPlex, which means it has
vertices, edges, and cells.
If you want the vertices on a given cell, you would get the closure and
then filter out any points which are
not vertices. For example,

PetscInt *closure = NULL;
PetscInt  clSize, cl;

ierr = DMPlexGetDepthStratum(dm, 0, &vStart, &vEnd);CHKERRQ(ierr);
ierr = DMPlexGetTransitiveClosure(dm, cell, PETSC_TRUE, &clSize,
&closure);CHKERRQ(ierr);
for (cl = 0; cl < clSize*2; cl += 2) {
  const PetscInt point = closure[cl];

  if (point >= vStart && point < vEnd) {
    <this is a vertex>
  }
}
ierr = DMPlexRestoreTransitiveClosure(dm, cell, PETSC_TRUE, &clSize,
&closure);CHKERRQ(ierr);

If you do not use edges, then its much easier. Just call DMPlexGetCone().
If you do use edges, but also
get the vertices all the time, you can make an index for this query, which
will speed it up greatly at the
cost of some memory.

  Thanks,

     Matt

> Thank you,
>
> Barry
> On 11/30/18 8:33 PM, Stefano Zampini wrote:
>
>
>
> Il giorno Ven 30 Nov 2018, 15:05 Tsung-Hsing Chen via petsc-users <
> petsc-users at mcs.anl.gov> ha scritto:
>
>> Hi,
>> Is there any function that can get cell number from the given node number
>> exist already?
>>
>
> Cell number of what? DMPLEX? And what is "node number"?
>
>
>> Thank you,
>> Barry
>>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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