[petsc-users] difference between DMDAVecGetArrayDOF and DMDAVecGetArray?

[Matthew Knepley]

On Sat, Feb 28, 2015 at 1:35 PM, Gideon Simpson <gideon.simpson at gmail.com>
wrote:

> I’m having some trouble understanding what the difference between these
> two routines are, though I am finding that there certainly is a
> difference.  I have the following monte carlo problem.  I am generating
> n_sample paths each of length n_points, and storing them in a 1D DA:
>
> DMDACreate1d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE, n_samples, n_points, 0,
> NULL, &da);
> DMCreateGlobalVector(da,&paths_vec);
>
> When I then go to access them,
>
> PetscScalar **u_array;
>
> I find that:
>
> DMDAVecGetArrayDOF(da, paths_vec, &u_array);
>
> works as exepected, in that u_array[i] is a pointer to the first index of
> the i-th sample path, but if I call:
>
> DMDAVecGetArray(da, paths_vec, &u_array);
>
> u_array[i] is something else, and my attempts to manipulate it result in
> segmentation faults, even though the code compiles and builds.


Suppose that you have 4 PetscScalar values at each vertex of the 1D DMDA.
If you use

  PetscScalar **u;

  DMDAVecGetArrayDOF(da, uVec, &u);

  u[i][2] /* refers to 3rd scalar on vertex i */

On the other hand you could use

typedef struct {
  PetscScalar a, b, c, d;
} Vals;

 Vals *u;

 DMDAVecGetArray(da, uVec, &u);

 u[i].c /* refers to the same value as above */

Basically the DOF version gives you an extra level of indirection for the
components.

  Thanks,

     Matt


>
> -gideon
>
>


-- 
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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