c In this subroutine below, the first call to MPI_COMM_SPLIT divides 
c MPI_COMM_WORLD into three communicators corresponding to the three rows 
c in the 3x4 grid. 
c One communicator includes all processes for which color = 1 (row 1), 
c another includes all processes for which color = 2 (row 2), 
c and a third includes all processes for which color = 3 (row 3). 
c Within the new communicators, processes are ranked in order of 
c increasing key value (in this case, according to
c increasing rank in the original communicator MPI_COMM_WORLD).      

      subroutine set_group(row_comm,col_comm)

      include 'mpif.h'
      parameter (NROW=3, NCOL=4)
      integer row_comm, coll_comm, color, key

c  -------------------------------------------------------------
c   Establish the row and column to which this processor belongs 
c  -------------------------------------------------------------
c
      call MPI_COMM_RANK(MPI_COMM_WORLD,irank,ierr)
      irow = MOD(irank,NROW) + 1
      icol = INT(irank/NROW) + 1
c
c  -------------------------
c   Build row communicators
c  -------------------------
c
      color = irow
      key = irank
      call MPI_COMM_SPLIT (MPI_COMM_WORLD, color, key, row_comm, ierr)
c
c  ---------------------------
c   Build column communicators
c  ---------------------------
c
      color = icol
      call MPI_COMM_SPLIT (MPI_COMM_WORLD, color, key, col_comm, ierr)
      return
      end