program main

!-- Create a Cartesian 2D topology and print out the
!   coordinate parameters of each process.

!--Include the mpi header file
      use mpi
      implicit none

      integer i,j,k,ierr,rank,my_rank,numprocs,itag,ierror
      integer status(MPI_STATUS_SIZE)
      integer irc

      parameter (ndims=2)    !dimension of the cartesian grid
      integer ndims,dims(ndims)
      integer COMM_CART
      logical periods(ndims),reorder
      integer coords(ndims)

      integer count,tag, datatype,comm,source,nxb,nyb
      integer, parameter :: nx=4, ny=4
      integer  x_corner, y_corner
      real :: x(2,2), y(2,2),x0(4),y0(4)
      real, parameter :: dx=0.2, dy=0.2
! The cell size is dx=dy=0.2 

       nxb=nx/ndims
       nyb=ny/ndims

!--Initialize MPI
      call MPI_INIT( ierr )

!--Who am I? --- get my rank=my_rank
      call MPI_COMM_RANK( MPI_COMM_WORLD, my_rank, ierr )

!--How many processes in the global group?
      call MPI_COMM_SIZE( MPI_COMM_WORLD, numprocs, ierr )

!--Create a cartesian topology
       dims(1:ndims)=2    ! 2x2 grid of sub-domains
      periods(1)=.true.  ! periodic bdry. cond. along x
      periods(2)=.false. ! non-periodic bdry. cond. along y

      call MPI_CART_CREATE(MPI_COMM_WORLD,ndims,dims,periods, &
                           reorder,COMM_CART,ierror)

!           ----------------------------------------------------------------
!          | MPI_CART_CREATE will provide a 2D cartesian array of processes |
!   ======>| of dimensions 2x2.                                             |
!          | Each process rank is associated with a given sub-domain.       |
!           ---------------------------------------------------------------- 

! --Find and print the coordinate parameters of each process in the 
!   Cartesian topology.

! --the common calling arguments      
   count   = 2
   datatype=MPI_REAL      
   tag     = 1      
   comm    = MPI_COMM_WORLD

   if (my_rank == 0) then
      !build x,y matrices
      x0(:)=(/0.1, 0.3, 0.5, 0.7/)
      y0(:)=(/0.1, 0.3, 0.5, 0.7/)
       do i=1,numprocs
         rank=i-1
         call MPI_CART_COORDS(COMM_CART,rank,2,coords,ierror)
         do j=1,nx
           x(j,:)=x0(coords(1)*nxb+j)
           y(:,j)=y0(coords(2)*nyb+j)
         end do
         !send the subportions
         call MPI_SEND(x(1),count,datatype, rank,tag,comm,ierr)
         call MPI_SEND(y(1),count,datatype, rank,tag,comm,ierr)
       end do
   endif

      !get the x,y subarrays
      source=0
      call MPI_CART_COORDS(COMM_CART,my_rank,2,coords,ierror)
      call MPI_RECV(x(1),count,datatype, source,tag,comm,status,  ierr)
      call MPI_RECV(y(1),count,datatype, source,tag,comm,status,  ierr)

   write(*,"('Rank[',i2,'] Coords=[',2i2,'], x=[',4f4.1,'] y=[',4f4.1,']\n')" ) &
              my_rank, coords, x,y

!--Finilize MPI
      call MPI_FINALIZE(irc)

      stop
      end