Actual source code: ex8.c

      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>

     17: int main(int argc,char **argv)
     18: {
     20:   PetscMPIInt    rank;
     21:   PetscInt       i,ng,*gindices,rstart,rend,M;
     22:   PetscScalar    one = 1.0;
     23:   Vec            x;

     25:   PetscInitialize(&argc,&argv,(char*)0,help);if (ierr) return ierr;
     26:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

     28:   /*
     29:      Create a parallel vector.
     30:       - In this case, we specify the size of each processor's local
     31:         portion, and PETSc computes the global size.  Alternatively,
     32:         PETSc could determine the vector's distribution if we specify
     33:         just the global size.
     34:   */
     35:   VecCreate(PETSC_COMM_WORLD,&x);
     36:   VecSetSizes(x,rank+1,PETSC_DECIDE);
     37:   VecSetFromOptions(x);
     38:   VecSet(x,one);

     40:   /*
     41:      Set the local to global ordering for the vector. Each processor
     42:      generates a list of the global indices for each local index. Note that
     43:      the local indices are just whatever is convenient for a particular application.
     44:      In this case we treat the vector as lying on a one dimensional grid and
     45:      have one ghost point on each end of the blocks owned by each processor.
     46:   */

     48:   VecGetSize(x,&M);
     49:   VecGetOwnershipRange(x,&rstart,&rend);
     50:   ng   = rend - rstart + 2;
     51:   PetscMalloc1(ng,&gindices);
     52:   gindices[0] = rstart - 1;
     53:   for (i=0; i<ng-1; i++) gindices[i+1] = gindices[i] + 1;
     54:   /* map the first and last point as periodic */
     55:   if (gindices[0]    == -1) gindices[0]    = M - 1;
     56:   if (gindices[ng-1] == M)  gindices[ng-1] = 0;
     57:   {
     58:     ISLocalToGlobalMapping ltog;
     59:     ISLocalToGlobalMappingCreate(PETSC_COMM_SELF,1,ng,gindices,PETSC_COPY_VALUES,&ltog);
     60:     VecSetLocalToGlobalMapping(x,ltog);
     61:     ISLocalToGlobalMappingDestroy(&ltog);
     62:   }
     63:   PetscFree(gindices);

     65:   /*
     66:      Set the vector elements.
     67:       - In this case set the values using the local ordering
     68:       - Each processor can contribute any vector entries,
     69:         regardless of which processor "owns" them; any nonlocal
     70:         contributions will be transferred to the appropriate processor
     71:         during the assembly process.
     72:       - In this example, the flag ADD_VALUES indicates that all
     73:         contributions will be added together.
     74:   */
     75:   for (i=0; i<ng; i++) {
     76:     VecSetValuesLocal(x,1,&i,&one,ADD_VALUES);
     77:   }

     79:   /*
     80:      Assemble vector, using the 2-step process:
     81:        VecAssemblyBegin(), VecAssemblyEnd()
     82:      Computations can be done while messages are in transition
     83:      by placing code between these two statements.
     84:   */
     85:   VecAssemblyBegin(x);
     86:   VecAssemblyEnd(x);

     88:   /*
     89:       View the vector; then destroy it.
     90:   */
     91:   VecView(x,PETSC_VIEWER_STDOUT_WORLD);
     92:   VecDestroy(&x);

     94:   PetscFinalize();
     95:   return ierr;
     96: }

     98: /*TEST

    100:      test:
    101:        nsize: 4

    103: TEST*/