Actual source code: ex8.c

  1: /*$Id: ex8.c,v 1.24 2001/09/11 16:32:18 bsmith Exp $*/

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

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

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

 18: int main(int argc,char **argv)
 19: {
 20:   int     i,N,ierr,rank,ng,*gindices,rstart,rend,M;
 21:   PetscScalar  one = 1.0;
 22:   Vec     x;

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

 27:   /*
 28:      Create a parallel vector.
 29:       - In this case, we specify the size of each processor's local
 30:         portion, and PETSc computes the global size.  Alternatively,
 31:         PETSc could determine the vector's distribution if we specify
 32:         just the global size.
 33:   */
 34:   VecCreate(PETSC_COMM_WORLD,&x);
 35:   VecSetSizes(x,rank+1,PETSC_DECIDE);
 36:   VecSetFromOptions(x);
 37:   VecGetSize(x,&N);
 38:   VecSet(&one,x);

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

 67:   /*
 68:      Set the vector elements.
 69:       - In this case set the values using the local ordering
 70:       - Each processor can contribute any vector entries,
 71:         regardless of which processor "owns" them; any nonlocal
 72:         contributions will be transferred to the appropriate processor
 73:         during the assembly process.
 74:       - In this example, the flag ADD_VALUES indicates that all
 75:         contributions will be added together.
 76:   */
 77:   for (i=0; i<ng; i++) {
 78:     VecSetValuesLocal(x,1,&i,&one,ADD_VALUES);
 79:   }

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

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

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