initial values of matrix and vector

[Barry Smith]

   Jed is right. Our intention is to have the vector initialized to  
zero at creation time IF we allocate the array space.
Tracking down a not properly initialized vector is the type of bug  
that takes forever to find; plus I wanted consistency
between vectors and matrix. Users SHOULD NOT call a MatZeroEntries()  
initially on matrices (since it screws up
preallocation) so we should not have them do it on vectors either for  
consistency.

    Barry

On Feb 24, 2009, at 10:57 AM, Jed Brown wrote:

> 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