# 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#

• sf - scatter context generated by VecScatterCreate()

• x - the vector from which we scatter

• y - the vector to which we scatter

• addv - either 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.

• mode - the scattering mode, usually 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.

Low-level Vector Communication, VecScatter, VecScatterCreate(), VecScatterEnd()

intermediate

## Location#

src/vec/is/sf/interface/vscat.c

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