initial values of matrix and vector

[Matthew Knepley]

On Tue, Feb 24, 2009 at 3:47 PM, Yixun Liu <enjoywm at cs.wm.edu> wrote
You mean I have no need to call

> VecSet(v, 0.0); and
> MatZeroEntries(mat);
>
> after I set the size for the vector or matrix?
>

Yes, do not call them.

  Matt


> Barry Smith wrote:
> >
> > 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
> >
>
>


-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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