.. _mpi_graph_neighbors:


MPI_Graph_neighbors
===================

.. include_body

:ref:`MPI_Graph_neighbors` |mdash| Returns the neighbors of a node associated
with a graph topology.

.. The following file was automatically generated
.. include:: ./bindings/mpi_graph_neighbors.rst

INPUT PARAMETERS
----------------
* ``comm``: Communicator with graph topology (handle).
* ``rank``: Rank of process in group of comm (integer).
* ``maxneighbors``: Size of array neighbors (integer).

OUTPUT PARAMETERS
-----------------
* ``neighbors``: Ranks of processes that are neighbors to specified process (array of integers).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

**Example:** Suppose that comm is a communicator with a shuffle-exchange
topology. The group has 2n members. Each process is labeled by a(1),
..., a(n) with a(i) E{0,1}, and has three neighbors: exchange (a(1),
..., a(n) = a(1), ..., a(n-1), a(n) (a = 1 - a), shuffle (a(1), ...,
a(n)) = a(2), ..., a(n), a(1), and unshuffle (a(1), ..., a(n)) = a(n),
a(1), ..., a(n-1). The graph adjacency list is illustrated below for
n=3.

::

           		exchange		shuffle		unshuffle
       node		neighbors(1)	neighbors(2)	neighbors(3)
       0(000)	    1		    0		    0
       1(001)	    0		    2		    4
       2(010)	    3		    4		    1
       3(011)	    2		    6		    5
       4(100)	    5		    1		    2
       5(101)	    4		    3		    6
       6(110)	    7		    5		    3
       7(111)	    6		    7		    7

Suppose that the communicator comm has this topology associated with it.
The following code fragment cycles through the three types of neighbors
and performs an appropriate permutation for each.

.. code-block:: fortran

   !  assume: each process has stored a real number A.
   !  extract neighborhood information
   CALL MPI_COMM_RANK(comm, myrank, ierr)
   CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr)
   !  perform exchange permutation
   CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0, &
                             neighbors(1), 0, comm, status, ierr)
   !  perform shuffle permutation
   CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0, &
                             neighbors(3), 0, comm, status, ierr)
   !  perform unshuffle permutation
   CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0, &
                             neighbors(2), 0, comm, status, ierr)


ERRORS
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Graph_neighbors_count`