[petsc-users] accessing individual elements of a global vector created from dmda

[Bishesh Khanal]

On Tue, Sep 17, 2013 at 6:15 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   Bishesh,
>
>     Do you need off process values? That is, might one process ask for a
> value that is not in the local part of the vector and belongs on a
> different process? If you know that the values accessed are always local
> then you can just use sol[k][j][i].vx   the k,j,i are the global indices
> over the entire mesh.
>
>     If any process might ask for any value on the entire mesh then you
> need to use DMDAGlobalToNaturalBegin/End() followed by
> VecScatterCreateToAll() and scatter all the values to all processes then
> you access them locally.  Note this is not scalable.
>
> Thanks Barry. Any process might ask for any value on the entire mesh since
this is a function I need to provide to the part of the code that "sees" my
solver as an external object and that part does not use petsc explicity and
not necessarily parallel. I did as you suggested and it works, thanks.



>
>    Barry
>
> Note that though with sol[k][j][i].vx it looks like you can access any
> value for k,j,i if you use a k, j, i that do not correspond to a value that
> belongs to this process you will get a memory error.
>
>
>
>
> On Sep 17, 2013, at 11:06 AM, Bishesh Khanal <bisheshkh at gmail.com> wrote:
>
> > Dear all,
> > I have a vector say sol which is a vector created from a dmda with dof =
> 3.
> > Now I need to provide an interface to the part of my code which does not
> use petsc but needs access to the values of sol.
> >
> > I was thinking of providing an interface such as:
> > double solver::getSolutionAtPosition(int x, int y, int z, int field);
> > which should return a value of the vector sol at the grid position
> (x,y,z) and dof = field.
> >
> > I know how to get the array and loop through all the values such as:
> > DMDALocalInfo info;
> > ierr = DMDAGetLocalInfo(da,&info);CHKERRQ(ierr);
> >
> > Field ***sol;
> > ierr = DMDAVecGetArray(da,b,&rhs);CHKERRQ(ierr);
> >
> >     for (PetscInt k = info.zs; k<info.zs+info.zm; ++k) {
> >         for (PetscInt j = info.ys; j < info.ys+info.ym; ++j) {
> >             for (PetscInt i = info.xs; i < info.xs+info.xm; ++i) {
> >                 //access sol[k][j][i].vx   or sol[k][j][i].vy or
> sol[k][j][i].vz depending on field
> >
> >             }
> >         }
> >     }
> >
> > But if I have a particular (x,y,z) based on global co-ordinate system,
> how do I get the value for the given field ?
> >
> > Thanks,
> > Bishesh
>
>
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