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

[Tonio Herrmann]

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. 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;
}
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