[petsc-users] difference between DMDAVecGetArrayDOF and DMDAVecGetArray?

[Gideon Simpson]

Supposing that I do not have a priori information as to the number of degrees of freedom per DA vertex; I want the number of mesh points along each sample path to be variable.  Hence, I can’t really use a statically defined structure as you suggest.  In that case, are the DOF routines the only option?

-gideon

> On Feb 28, 2015, at 2:42 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Sat, Feb 28, 2015 at 1:35 PM, Gideon Simpson <gideon.simpson at gmail.com <mailto: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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