[petsc-users] Global DA index using DAGetCorners

[Jed Brown]

On Thu, Nov 25, 2010 at 10:51, khalid ashraf <khalid_eee at yahoo.com> wrote:

> Sorry for the spam. Just to illustrate the problem. The following code
> produces the
> following output when run on 4 processors.
>
> Code:
> ierr=DAGetCorners(appctx->da,&xs,&ys,&zs,&xm,&ym,&zm);CHKERRQ(ierr);
>  for (k=zs; k<zs+zm; k++) {
>   for (j=ys; j<ys+ym; j++) {
>   for (i=xs; i<xs+xm; i++) {
>     {
>     Vec_A[k][j][i] = (PetscReal)(k*my*mx+j*mx+i);
>     }}}
>
> Output:
> Vec_A (mx=16 X my=16 X mz=4):
>

You have not shown how you produce this output.


> 0  1  2  3 4 5 6 7
> 16 17 18 19 20 21 22 23
> 32 33 34 35 36 37 38 39
> 48 49 and so on
>
> I don't understand why does the array value jump to 16 after 7. Meaning
> why is the 8th element of the vector is 16. Thanks.
>

You are seeing the "PETSc ordering" instead of the "Natural ordering", see
Figure 9 of the Users Manual

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manual.pdf#page=50

DAVecGetArray sets up the indexing so you can write your code in terms of
the natural ordering even though the actual data are not stored that way.

Jed
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