
static char help[] = "Tests vector scatter-gather operations.  Input arguments are\n\
  -n <length> : vector length\n\n";

#include <petscvec.h>

#undef __FUNCT__
#define __FUNCT__ "main"
int main(int argc,char **argv)
{
  PetscErrorCode ierr;
  PetscInt       n   = 5,idx1[2] = {0,3},idx2[2] = {1,4};
  PetscScalar    one = 1.0,two = 2.0;
  Vec            x,y;
  IS             is1,is2;
  VecScatter     ctx = 0;

  ierr = PetscInitialize(&argc,&argv,(char*)0,help);CHKERRQ(ierr);
  ierr = PetscOptionsGetInt(NULL,NULL,"-n",&n,NULL);CHKERRQ(ierr);

  /* create two vector */
  ierr = VecCreateSeq(PETSC_COMM_SELF,n,&x);CHKERRQ(ierr);
  ierr = VecDuplicate(x,&y);CHKERRQ(ierr);

  /* create two index sets */
  ierr = ISCreateGeneral(PETSC_COMM_SELF,2,idx1,PETSC_COPY_VALUES,&is1);CHKERRQ(ierr);
  ierr = ISCreateGeneral(PETSC_COMM_SELF,2,idx2,PETSC_COPY_VALUES,&is2);CHKERRQ(ierr);

  ierr = VecSet(x,one);CHKERRQ(ierr);
  ierr = VecSet(y,two);CHKERRQ(ierr);
  ierr = VecScatterCreate(x,is1,y,is2,&ctx);CHKERRQ(ierr);
  ierr = VecScatterBegin(ctx,x,y,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
  ierr = VecScatterEnd(ctx,x,y,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);

  ierr = VecView(y,PETSC_VIEWER_STDOUT_SELF);CHKERRQ(ierr);

  ierr = VecScatterBegin(ctx,y,x,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
  ierr = VecScatterEnd(ctx,y,x,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
  ierr = VecScatterDestroy(&ctx);CHKERRQ(ierr);

  ierr = PetscPrintf(PETSC_COMM_SELF,"-------\n");CHKERRQ(ierr);
  ierr = VecView(x,PETSC_VIEWER_STDOUT_SELF);CHKERRQ(ierr);

  ierr = ISDestroy(&is1);CHKERRQ(ierr);
  ierr = ISDestroy(&is2);CHKERRQ(ierr);

  ierr = VecDestroy(&x);CHKERRQ(ierr);
  ierr = VecDestroy(&y);CHKERRQ(ierr);

  ierr = PetscFinalize();
  return 0;
}

