/*$Id: ex2.c,v 1.44 2001/09/07 20:09:05 bsmith Exp $*/

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

/*T
   Concepts: vectors^assembling vectors;
   Processors: n
T*/

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

#undef __FUNCT__
#define __FUNCT__ "main"
int main(int argc,char **argv)
{
  int          i,N,ierr,rank;
  PetscScalar  one = 1.0;
  Vec          x;

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

  /*
     Create a parallel vector.
      - In this case, we specify the size of each processor's local
        portion, and PETSc computes the global size.  Alternatively,
        if we pass the global size and use PETSC_DECIDE for the 
        local size PETSc will choose a reasonable partition trying 
        to put nearly an equal number of elements on each processor.
  */
  ierr = VecCreate(PETSC_COMM_WORLD,&x);CHKERRQ(ierr);
  ierr = VecSetSizes(x,rank+1,PETSC_DECIDE);CHKERRQ(ierr);
  ierr = VecSetFromOptions(x);CHKERRQ(ierr);
  ierr = VecGetSize(x,&N);CHKERRQ(ierr);
  ierr = VecSet(&one,x);CHKERRQ(ierr);

  /*
     Set the vector elements.
      - Always specify global locations of vector entries.
      - Each processor can contribute any vector entries,
        regardless of which processor "owns" them; any nonlocal
        contributions will be transferred to the appropriate processor
        during the assembly process.
      - In this example, the flag ADD_VALUES indicates that all
        contributions will be added together.
  */
  for (i=0; i<N-rank; i++) {
    ierr = VecSetValues(x,1,&i,&one,ADD_VALUES);CHKERRQ(ierr);  
  }

  /* 
     Assemble vector, using the 2-step process:
       VecAssemblyBegin(), VecAssemblyEnd()
     Computations can be done while messages are in transition
     by placing code between these two statements.
  */
  ierr = VecAssemblyBegin(x);CHKERRQ(ierr);
  ierr = VecAssemblyEnd(x);CHKERRQ(ierr);

  /*
      View the vector; then destroy it.
  */
  ierr = VecView(x,PETSC_VIEWER_STDOUT_WORLD);CHKERRQ(ierr);
  ierr = VecDestroy(x);CHKERRQ(ierr);

  ierr = PetscFinalize();CHKERRQ(ierr);
  return 0;
}