[petsc-users] VecGetValues and VecGetArray return nothing but zeros

[Barry Smith]

> On Sep 8, 2016, at 10:43 AM, Tonio Herrmann <tonioherrmann at gmail.com> wrote:
> 
> Hello,
> during my attempts to learn PETSc, I am trying to understand the relation between global and local vectors on a DMDA. There is a problem in my code below, which only returns zeros when I try to access the local values, although VecView shows that the values are correctly transferred from the global vector g to the local vector l.
> Is it obvious, what I am doing wrong?
> 
> Another question, I was expecting that "VecView(l,PETSC_VIEWER_STDOUT_WORLD);" would print l synchronized from each process, but only l from process #0 is shown.

   This will not work; when you do a view the MPI communicator of the object must be the same as the MPI communicator in the viewer. 

    I don't really recommend ever looking at all the subdomains local vectors but if you really want to you can use

PetscViewerGetSubViewer() to get a sub viewer for each MPI process one at a time and call the VecView() one at at time with each viewer and local vector.

  Barry



> How can I see the other parts of l?
> 
> Thank you
> Herrmann
> 
> The example code:
> 
> #include "petsc.h"
> 
> int main(int argc,char **argv){
>   PetscInt sz;
>   int ierr,rank;
>   Vec g,l;
>   DM da;
>   
>   ierr=PetscInitialize(&argc, &argv, NULL, NULL);CHKERRQ(ierr);
>   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);
>   int nx=2,ny=4;
>   //create DA
>   ierr=DMDACreate2d(PETSC_COMM_WORLD,DM_BOUNDARY_GHOSTED,DM_BOUNDARY_GHOSTED,DMDA_STENCIL_BOX,nx,ny,PETSC_DECIDE,PETSC_DECIDE,1,1,NULL,NULL,&da);CHKERRQ(ierr);
>   //create global vector and fill with numbers 101,102,...
>   ierr=DMCreateGlobalVector(da,&g);CHKERRQ(ierr);
>   ierr=VecGetSize(g,&sz);CHKERRQ(ierr);
>   for (int k=0;k<sz;k++){
>     VecSetValue(g,k,100+k,INSERT_VALUES);
>   }
>   VecAssemblyBegin(g);
>   VecAssemblyEnd(g);
>   //create local vector and transfer global values into local
>   ierr=DMCreateLocalVector(da,&l);CHKERRQ(ierr);
>   ierr=VecGetSize(l,&sz);CHKERRQ(ierr);
>   DMGlobalToLocalBegin(da,g,INSERT_VALUES,l);
>   DMGlobalToLocalEnd(da,g,INSERT_VALUES,l);
>   VecAssemblyBegin(l);
>   VecAssemblyEnd(l);
> 
>   //view local vector, ok
>   VecView(l,PETSC_VIEWER_STDOUT_WORLD);
> 
>   //access local values by VecGetValues, fails (only zeros!)
>   double *data=(double*)malloc(100*sizeof(double));
>   PetscInt ix[100];
>   ierr=VecGetSize(l,&sz);CHKERRQ(ierr);
>   for (int k=0;k<sz;k++) ix[k]=k;
>   ierr=VecGetValues(l,sz,ix,data);CHKERRQ(ierr);
> 
>   PetscSynchronizedPrintf(PETSC_COMM_WORLD,"VecGetValues on rank %d:\n",rank);
>   for (int k=0;k<sz;k++) PetscSynchronizedPrintf(PETSC_COMM_WORLD,"%d ",data[k]);
>   PetscSynchronizedPrintf(PETSC_COMM_WORLD,"\n");
> 
>   //access local values by VecGetArrays, fails (only zeros!)
>   ierr=VecGetArray(l,&data);CHKERRQ(ierr);
>   PetscSynchronizedPrintf(PETSC_COMM_WORLD,"VecGetArray on rank %d:\n",rank);
>   for (int k=0;k<sz;k++){
>     PetscSynchronizedPrintf(PETSC_COMM_WORLD,"%d ",data[k]);
>   }
>   PetscSynchronizedPrintf(PETSC_COMM_WORLD,"\n");
>   PetscSynchronizedFlush(PETSC_COMM_WORLD,NULL);
>   VecRestoreArray(l,&data);CHKERRQ(ierr);
>   
>   ierr = PetscFinalize();CHKERRQ(ierr);
>   return ierr;
> }