[petsc-users] How to access Vector data properly

[Chu Songtao]

Hello,
    I am just a beginner. Now I get confused on how to correctly use Vec and DMDA.
    I set 1D DMDA global vector with natural values for a test. Then use VecView and DMDAVecGetArray to view the data, but the results are different. I don't know why.

this is the code:

PetscMPIInt     rank;
PetscErrorCode  ierr;
Vec             global,local,natural;
DM              da;
PetscReal       *xnatural,*val;
PetscInt        i,start,end,x,xm;


ierr = PetscInitialize(&argc,&argv,(char*)0,help);if (ierr) return ierr;
ierr = MPI_Comm_rank(PETSC_COMM_WORLD,&rank);CHKERRQ(ierr);

ierr = DMDACreate1d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,5,1,1,NULL,&da);CHKERRQ(ierr);
ierr = DMSetUp(da);CHKERRQ(ierr);
ierr = DMCreateGlobalVector(da,&global);CHKERRQ(ierr);

ierr = DMDACreateNaturalVector(da,&natural);CHKERRQ(ierr);
ierr = VecGetOwnershipRange(natural,&start,&end);CHKERRQ(ierr);
ierr = VecGetArray(natural,&xnatural);CHKERRQ(ierr);
for (i=start; i<end; i++) {
    xnatural[i-start] = i;
}
ierr = VecRestoreArray(natural,&xnatural);CHKERRQ(ierr);
ierr = DMDANaturalToGlobalBegin(da,natural,INSERT_VALUES,global);CHKERRQ(ierr);
ierr = DMDANaturalToGlobalEnd(da,natural,INSERT_VALUES,global);CHKERRQ(ierr);
ierr = VecDestroy(&natural);CHKERRQ(ierr);

VecView(global,PETSC_VIEWER_STDOUT_WORLD);

ierr = DMDAGetCorners(da,&x,NULL,NULL,&xm,NULL,NULL);CHKERRQ(ierr);
ierr = DMDAVecGetArray(da, global, &val);CHKERRQ(ierr);
ierr = PetscSynchronizedPrintf(PETSC_COMM_SELF, "Rank=%d\n", rank);CHKERRQ(ierr);
for (i = x; i < x + xm; ++i) {
    ierr = PetscSynchronizedPrintf(PETSC_COMM_SELF, "%4.f ", val[i]);CHKERRQ(ierr);
}
ierr = DMDAVecRestoreArray(da, global, &val);CHKERRQ(ierr);

and the results are:

$ ./test0
Vec Object: 1 MPI processes
  type: seq
Vec Object: Vec_0x7ffff3cbc500_0 1 MPI processes
  type: mpi
Process [0]
0.
1.
2.
3.
4.
Rank=0
   0    1    2    3    4

$ mpiexec -n 2 ./test0
Vec Object: 2 MPI processes
  type: mpi
Vec Object: Vec_0x7fffcf948a90_0 2 MPI processes
  type: mpi
Process [0]
0.
1.
2.
Process [1]
3.
4.
Rank=0
   0    0    0 Rank=1
   0    0


the seq version is correct, but the mpi version is wrong. Every element in val is 0.

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