NAME

       MPI_Graph_create   -  Makes  a  new  communicator  to  which topology information has been
       attached.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Graph_create(MPI_Comm comm_old, int nnodes, const int index[],
            const int edges[], int reorder, MPI_Comm *comm_graph)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_GRAPH_CREATE(COMM_OLD, NNODES, INDEX, EDGES, REORDER,
                 COMM_GRAPH, IERROR)
            INTEGER   COMM_OLD, NNODES, INDEX(*), EDGES(*)
            INTEGER   COMM_GRAPH, IERROR
            LOGICAL   REORDER

C++ Syntax

       #include <mpi.h>
       Graphcomm Intracomm::Create_graph(int nnodes, const int index[],
            const int edges[], bool reorder) const

INPUT PARAMETERS

       comm_old  Input communicator without topology (handle).

       nnodes    Number of nodes in graph (integer).

       index     Array of integers describing node degrees (see below).

       edges     Array of integers describing graph edges (see below).

       reorder   Ranking may be reordered (true) or not (false) (logical).

OUTPUT PARAMETERS

       comm_graph
                 Communicator with graph topology added (handle).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       MPI_Graph_create returns a handle to a  new  communicator  to  which  the  graph  topology
       information is attached. If reorder = false then the rank of each process in the new group
       is identical to its rank in the  old  group.  Otherwise,  the  function  may  reorder  the
       processes.  If  the  size,  nnodes,  of the graph is smaller than the size of the group of
       comm_old, then some processes are returned MPI_COMM_NULL, in  analogy  to  MPI_Cart_create
       and  MPI_Comm_split. The call is erroneous if it specifies a graph that is larger than the
       group size of the input communicator.

       The three parameters nnodes, index, and edges define the graph structure.  nnodes  is  the
       number  of nodes of the graph. The nodes are numbered from 0 to nnodes-1. The ith entry of
       array index stores the total number of neighbors of the first i graph nodes. The lists  of
       neighbors of nodes 0, 1, ..., nnodes-1 are stored in consecutive locations in array edges.
       The array edges is a flattened representation of the  edge  lists.  The  total  number  of
       entries in index is nnodes and the total number of entries in edges is equal to the number
       of graph edges.

       The definitions of the arguments  nnodes,  index,  and  edges  are  illustrated  with  the
       following simple example.

       Example: Assume there are four processes 0, 1, 2, 3 with the following adjacency matrix:

           Process    Neighbors
              0          1, 3
              1          0
              2          3
              3          0, 2

       Then, the input arguments are:
           nnodes = 4
           index  = 2, 3, 4, 6
           edges  = 1, 3, 0, 3, 0, 2

       Thus,  in  C, index[0] is the degree of node zero, and index[i] - index[i-1] is the degree
       of node i, i=1, . . . , nnodes-1;  the list  of  neighbors  of  node  zero  is  stored  in
       edges[j],  for  0  <=  j  <= index[0] - 1 and the list of neighbors of node i, i > 0 ,  is
       stored in edges[j], index[i-1] <= j <= index[i] - 1.

       In Fortran, index(1) is the degree of node zero, and index(i+1) - index(i) is  the  degree
       of  node  i,  i=1,  .  .  .  ,  nnodes-1;  the list of neighbors of node zero is stored in
       edges(j), for 1 <= j <= index(1) and the list of neighbors of node  i, i > 0, is stored in
       edges(j), index(i) + 1 <= j <= index(i + 1).

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran routines in the last argument. C++ functions do not return errors. If the  default
       error  handler  is  set  to  MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
       mechanism will be used to throw an MPI::Exception object.

       Before the error value is returned, the current MPI error handler is called.  By  default,
       this  error  handler aborts the MPI job, except for I/O function errors. The error handler
       may   be   changed   with   MPI_Comm_set_errhandler;   the   predefined   error    handler
       MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
       guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Graph_get
       MPI_Graphdims_get