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