[petsc-users] DMCreateGlobalVector in fortran.

[Jed Brown]

Manuel Valera <mvalera at mail.sdsu.edu> writes:

> Ok, let me try to clarify a little bit,
>
> I have a u0 3d array with the mentioned indices (starting from -1), im
> creating a dmda with the sizes in the constructor, no problem with that.
>
> Next, im trying to distribute this u0 array into global and local vectors,
> apparently i can do that since no error comes out.
>
> Now, when i get the array back with VecGetArrayF90(), the indices start now
> from 0, is that right? so the whole array have been shifted in its indices.

See FormFunction and FormFunctionLocal.

  https://www.mcs.anl.gov/petsc/petsc-dev/src/snes/examples/tutorials/ex5f90.F90.html

Of course you can just define FormFunctionLocal to use an index range
that is shifted from

  PetscScalar  x(user%gxs:user%gxe,user%gys:user%gye)
  PetscScalar  f(user%xs:user%xe,user%ys:user%ye)

(this is just Fortran array passing; nothing to do with PETSc).

> This changes things for me because my idea was to use the previous serial
> code to operate on the arrays, but this is written with indices starting
> from -1. There are dozens of arrays with the same structure, but they also
> operate with other arrays with indices starting from 1, at the same time,
> usually in triple nested loops which code the stencil used within them, to
> update and correct the velocities after solving for pressure, per example.
>
> So it would be much easier to shift the indices of these arrays by one,
> rather than changing all the code to match the new indices starting from
> zero,

Maybe, but note that you need to enforce boundary conditions and
different processes will have a different range so preserving the index
bounds might cause more confusion than it's worth.
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