initial values of matrix and vector

[Jed Brown]

On Tue 2009-02-24 06:53, Matthew Knepley wrote:
> On Mon, Feb 23, 2009 at 8:26 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> 
>       Vectors have all entries equal to zero. Dense matrices have all entries
>     equal to zero. Sparse matrices have no entries (logically this is the same
>     as entries equal to zero).
> 
> 
> We do not automatically zero the entires upon allocation. You have to call
> VecZeroEntries() or VecSet()
> to initialize the vector.

That's what I would have expected, but Barry is correct, at least for
the usual types.  See, for instance

PetscErrorCode PETSCVEC_DLLEXPORT VecCreate_Seq(Vec V)
{
  Vec_Seq        *s;
  PetscScalar    *array;
  PetscErrorCode ierr;
  PetscInt       n = PetscMax(V->map->n,V->map->N);
  PetscMPIInt    size;

  PetscFunctionBegin;
  ierr = MPI_Comm_size(((PetscObject)V)->comm,&size);CHKERRQ(ierr);
  if (size > 1) {
    SETERRQ(PETSC_ERR_ARG_WRONG,"Cannot create VECSEQ on more than one process");
  }
  ierr = PetscMalloc(n*sizeof(PetscScalar),&array);CHKERRQ(ierr);
  ierr = PetscLogObjectMemory(V, n*sizeof(PetscScalar));CHKERRQ(ierr);
  ierr = PetscMemzero(array,n*sizeof(PetscScalar));CHKERRQ(ierr);
  ierr = VecCreate_Seq_Private(V,array);CHKERRQ(ierr);
  s    = (Vec_Seq*)V->data;
  s->array_allocated = array;
  PetscFunctionReturn(0);
}



In well-structured code, the cost of zeroing the vector is tiny and the
effort to track down uninitialized bugs is significant enough that this
seems like a sane default.

Jed
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