NAME

       MPI_Cart_shift   -   Returns  the  shifted  source  and  destination  ranks, given a shift
       direction and amount.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
            int *rank_source, int *rank_dest)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_CART_SHIFT(COMM, DIRECTION, DISP, RANK_SOURCE,
                 RANK_DEST, IERROR)
            INTEGER   COMM, DIRECTION, DISP, RANK_SOURCE
            INTEGER   RANK_DEST, IERROR

C++ Syntax

       #include <mpi.h>
       void Cartcomm::Shift(int direction, int disp, int& rank_source,
            int& rank_dest) const

INPUT PARAMETERS

       comm      Communicator with Cartesian structure (handle).

       direction Coordinate dimension of shift (integer).

       disp      Displacement ( > 0: upward shift, < 0: downward shift) (integer).

OUTPUT PARAMETERS

       rank_source
                 Rank of source process (integer).

       rank_dest Rank of destination process (integer).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       If the process topology is a Cartesian structure, an MPI_Sendrecv operation is  likely  to
       be  used  along  a coordinate direction to perform a shift of data. As input, MPI_Sendrecv
       takes the rank of a source process for the receive, and the rank of a destination  process
       for  the  send. If the function MPI_Cart_shift is called for a Cartesian process group, it
       provides the calling process with the above identifiers,  which  then  can  be  passed  to
       MPI_Sendrecv.  The  user  specifies  the  coordinate  direction  and  the size of the step
       (positive or negative). The function is local.

       The direction argument indicates the dimension of the shift, i.e.,  the  coordinate  whose
       value  is  modified  by  the  shift. The coordinates are numbered from 0 to ndims-1, where
       ndims is the number of dimensions.

       Note:  The direction argument is in the range [0,  n-1]  for  an  n-dimensional  Cartesian
       mesh.

       Depending on the periodicity of the Cartesian group in the specified coordinate direction,
       MPI_Cart_shift provides the identifiers for a circular or an end-off shift. In the case of
       an  end-off  shift,  the  value MPI_PROC_NULL may be returned in rank_source or rank_dest,
       indicating that the source or the destination for the shift is out of range.

       Example: The communicator, comm, has  a  two-dimensional,  periodic,  Cartesian   topology
       associated with it. A two-dimensional array of REALs is stored one element per process, in
       variable A. One wishes to skew this array, by shifting column i (vertically,  i.e.,  along
       the column) by i steps.

         ....
         C find process rank
               CALL MPI_COMM_RANK(comm, rank, ierr))
         C find Cartesian coordinates
               CALL MPI_CART_COORDS(comm, rank, maxdims, coords,
                                    ierr)
         C compute shift source and destination
               CALL MPI_CART_SHIFT(comm, 0, coords(2), source,
                                   dest, ierr)
         C skew array
               CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, dest, 0,
                                         source, 0, comm, status,
                                         ierr)

NOTE

       In  Fortran,  the  dimension indicated by DIRECTION = i has DIMS(i+1) nodes, where DIMS is
       the array that was used to create the grid. In C, the dimension indicated by direction = i
       is the dimension specified by dims[i].

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.