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.