# VecScatterBegin#

Begins a generalized scatter from one vector to another. Complete the scattering phase with `VecScatterEnd()`

.

## Synopsis#

```
#include "petscsf.h"
PetscErrorCode VecScatterBegin(VecScatter sf, Vec x, Vec y, InsertMode addv, ScatterMode mode)
```

Neighbor-wise Collective

## Input Parameters#

scatter context generated by**sf -**`VecScatterCreate()`

the vector from which we scatter**x -**the vector to which we scatter**y -**either**addv -**`ADD_VALUES`

,`MAX_VALUES`

,`MIN_VALUES`

or`INSERT_VALUES`

, with`INSERT_VALUES`

mode any location not scattered to retains its old value; i.e. the vector is NOT first zeroed.the scattering mode, usually**mode -**`SCATTER_FORWARD`

. The available modes are:`SCATTER_FORWARD`

or`SCATTER_REVERSE`

## Notes#

The vectors `x`

and `y`

need not be the same vectors used in the call
to `VecScatterCreate()`

, but `x`

must have the same parallel data layout
as that passed in as the `x`

to `VecScatterCreate()`

, similarly for the `y`

.
Most likely they have been obtained from `VecDuplicate()`

.

You cannot change the values in the input vector between the calls to `VecScatterBegin()`

and `VecScatterEnd()`

.

If you use `SCATTER_REVERSE`

the two arguments `x`

and `y`

should be reversed, from
the `SCATTER_FORWARD`

.

y[iy[i]] = x[ix[i]], for i=0,…,ni-1

This scatter is far more general than the conventional
scatter, since it can be a gather or a scatter or a combination,
depending on the indices ix and iy. If x is a parallel vector and y
is sequential, `VecScatterBegin()`

can serve to gather values to a
single processor. Similarly, if `y`

is parallel and `x`

sequential, the
routine can scatter from one processor to many processors.

## See Also#

Low-level Vector Communication, `VecScatter`

, `VecScatterCreate()`

, `VecScatterEnd()`

## Level#

intermediate

## Location#

## Examples#

src/ts/tutorials/ex29.c

src/vec/vec/tutorials/ex44.c

src/dm/tutorials/ex6.c

src/vec/vec/utils/tagger/tutorials/ex1.c

src/ts/tutorials/ex30.c

src/ksp/ksp/tutorials/ex43.c

src/dm/tutorials/ex14.c

src/dm/tutorials/ex25.c

src/dm/tutorials/ex22.c

src/ksp/ksp/tutorials/ex73.c

Index of all PetscSF routines

Table of Contents for all manual pages

Index of all manual pages