Actual source code: ex8.c
petsc-3.4.2 2013-07-02

      2: static char help[] = "Demonstrates using a local ordering to set values into a parallel vector.\n\n";

      4: /*T
      5:    Concepts: vectors^assembling vectors with local ordering;
      6:    Processors: n
      7: T*/

      9: /*
     10:   Include "petscvec.h" so that we can use vectors.  Note that this file
     11:   automatically includes:
     12:      petscsys.h       - base PETSc routines   petscis.h     - index sets
     13:      petscviewer.h - viewers
     14: */
     15: #include <petscvec.h>

     19: int main(int argc,char **argv)
     20: {
     22:   PetscMPIInt    rank;
     23:   PetscInt       i,N,ng,*gindices,rstart,rend,M;
     24:   PetscScalar    one = 1.0;
     25:   Vec            x;

     27:   PetscInitialize(&argc,&argv,(char*)0,help);
     28:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

     30:   /*
     31:      Create a parallel vector.
     32:       - In this case, we specify the size of each processor's local
     33:         portion, and PETSc computes the global size.  Alternatively,
     34:         PETSc could determine the vector's distribution if we specify
     35:         just the global size.
     36:   */
     37:   VecCreate(PETSC_COMM_WORLD,&x);
     38:   VecSetSizes(x,rank+1,PETSC_DECIDE);
     39:   VecSetFromOptions(x);
     40:   VecGetSize(x,&N);
     41:   VecSet(x,one);

     43:   /*
     44:      Set the local to global ordering for the vector. Each processor
     45:      generates a list of the global indices for each local index. Note that
     46:      the local indices are just whatever is convenient for a particular application.
     47:      In this case we treat the vector as lying on a one dimensional grid and
     48:      have one ghost point on each end of the blocks owned by each processor.
     49:   */

     51:   VecGetSize(x,&M);
     52:   VecGetOwnershipRange(x,&rstart,&rend);
     53:   ng   = rend - rstart + 2;
     54:   PetscMalloc(ng*sizeof(PetscInt),&gindices);
     55:   gindices[0] = rstart - 1;
     56:   for (i=0; i<ng-1; i++) gindices[i+1] = gindices[i] + 1;
     57:   /* map the first and last point as periodic */
     58:   if (gindices[0]    == -1) gindices[0]    = M - 1;
     59:   if (gindices[ng-1] == M)  gindices[ng-1] = 0;
     60:   {
     61:     ISLocalToGlobalMapping ltog;
     62:     ISLocalToGlobalMappingCreate(PETSC_COMM_SELF,ng,gindices,PETSC_COPY_VALUES,&ltog);
     63:     VecSetLocalToGlobalMapping(x,ltog);
     64:     ISLocalToGlobalMappingDestroy(&ltog);
     65:   }
     66:   PetscFree(gindices);

     68:   /*
     69:      Set the vector elements.
     70:       - In this case set the values using the local ordering
     71:       - Each processor can contribute any vector entries,
     72:         regardless of which processor "owns" them; any nonlocal
     73:         contributions will be transferred to the appropriate processor
     74:         during the assembly process.
     75:       - In this example, the flag ADD_VALUES indicates that all
     76:         contributions will be added together.
     77:   */
     78:   for (i=0; i<ng; i++) {
     79:     VecSetValuesLocal(x,1,&i,&one,ADD_VALUES);
     80:   }

     82:   /*
     83:      Assemble vector, using the 2-step process:
     84:        VecAssemblyBegin(), VecAssemblyEnd()
     85:      Computations can be done while messages are in transition
     86:      by placing code between these two statements.
     87:   */
     88:   VecAssemblyBegin(x);
     89:   VecAssemblyEnd(x);

     91:   /*
     92:       View the vector; then destroy it.
     93:   */
     94:   VecView(x,PETSC_VIEWER_STDOUT_WORLD);
     95:   VecDestroy(&x);

     97:   PetscFinalize();
     98:   return 0;
     99: }