[petsc-users] determining dof at runtime

[Matthew Knepley]

On Mon, Apr 16, 2012 at 5:11 PM, Juha Jäykkä <juhaj at iki.fi> wrote:

> Hi list!
>
> I am trying to create a 3D DA such that I can use DMDAVecGetArray() and
> access
> it using array indexing. I know how to get several dof in the DA and how
> using
> a struct to access the members works, when the dof is determined at
> compile-
> time (so accesses like array[1][2][3].member1[2] are computed correctly.
>
> However, I would like to determine dof at run-time. Creating a struct with
> pointers (even of the type member[]) of course gives me a struct whose size
> depends on the *pointer* size, not my dof. So I was wondering if I could
> work
> around this somehow, but do not seem to be able to.
>

http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/DM/DMDAVecGetArrayDOF.html

   Matt


> My DA is created by
>
> DMDACreate3d(PETSC_COMM_WORLD,
>             DMDA_BOUNDARY_GHOSTED, DMDA_BOUNDARY_GHOSTED,
>             DMDA_BOUNDARY_GHOSTED, DMDA_STENCIL_BOX,
>             2, 2, 2,
>             PETSC_DECIDE, PETSC_DECIDE, PETSC_DECIDE,
>             ndof, NGHOSTS,
>             PETSC_NULL, PETSC_NULL, PETSC_NULL,
>             &da);
>
> so my Vec does have the correct dof, but I cannot seem to fool C to address
> this as array[z][y][x][n] or something of the like, where the last index is
> the index to ndof. (Usually I would index that as array[z][y][x].member[n],
> but now I do not know how to make that happen since I cannot define a
> struct
> whose storage size is not known to the compiler.)
>
> Any ideas? Or am I trying to accomplish something which is not possible?
> (If
> so, why would that be? The vector is of the correct size.)
>
> Cheers,
> -Juha
>
> --
>                 -----------------------------------------------
>                | Juha Jäykkä, juhaj at iki.fi                     |
>                 -----------------------------------------------
>



-- 
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
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