2: static char help[] = "Builds a parallel vector with 1 component on the first processor, 2 on the second, etc.\n\
  3:   Then each processor adds one to all elements except the last rank.\n\n";

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

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

 20: int main(int argc,char **argv)
 21: {
 23:   PetscMPIInt    rank;
 24:   PetscInt       i,N;
 25:   PetscScalar    one = 1.0;
 26:   Vec            x;

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

 31:   /*
 32:      Create a parallel vector.
 33:       - In this case, we specify the size of each processor's local
 34:         portion, and PETSc computes the global size.  Alternatively,
 35:         if we pass the global size and use PETSC_DECIDE for the
 36:         local size PETSc will choose a reasonable partition trying
 37:         to put nearly an equal number of elements on each processor.
 38:   */
 39:   VecCreate(PETSC_COMM_WORLD,&x);
 40:   VecSetSizes(x,rank+1,PETSC_DECIDE);
 41:   VecSetFromOptions(x);
 42:   VecGetSize(x,&N);
 43:   VecSet(x,one);

 45:   /*
 46:      Set the vector elements.
 47:       - Always specify global locations of vector entries.
 48:       - Each processor can contribute any vector entries,
 49:         regardless of which processor "owns" them; any nonlocal
 50:         contributions will be transferred to the appropriate processor
 51:         during the assembly process.
 52:       - In this example, the flag ADD_VALUES indicates that all
 53:         contributions will be added together.
 54:   */
 55:   for (i=0; i<N-rank; i++) {
 56:     VecSetValues(x,1,&i,&one,ADD_VALUES);
 57:   }

 59:   /*
 60:      Assemble vector, using the 2-step process:
 61:        VecAssemblyBegin(), VecAssemblyEnd()
 62:      Computations can be done while messages are in transition
 63:      by placing code between these two statements.
 64:   */
 65:   VecAssemblyBegin(x);
 66:   VecAssemblyEnd(x);

 68:   /*
 69:       View the vector; then destroy it.
 70:   */
 71:   VecView(x,PETSC_VIEWER_STDOUT_WORLD);
 72:   VecDestroy(&x);

 74:   PetscFinalize();
 75:   return 0;
 76: }