!3a. Copy and modify example2.f90 to find the nearest neighbor along y+ 
!    of each process.
!-----------------------------------------------------------------------

      program main

!------------------------------------------------------------------
!    WHO IS MY NEIGHBOR IN THE POSITIVE Y DIRECTION?              
!    Strategy 1:
!      I.Find out what my process's coordinate parameters are [coords(ndims)]
!        (Use MPI_CART_GET for this)
!     II.Use MPI_CART_RANK to query for my neighbor's rank.
!------------------------------------------------------------------

!-- All processes send a message to process 0

      implicit none

!--Include the mpi header file
      include 'mpif.h'
      integer i,ierr,my_rank,numprocs,itag
      integer message,mess_received,iroot_pe,itag,icount,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 maxdims
      integer nndims,ddims(ndims)
      integer r_coords(ndims)
      integer r_rank
      logical l_periods(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 )
!     print *, "Process ", my_rank, " of ", numprocs, " is alive"

!--Create a Cartesian topology
       dims(1:ndims)=int(sqrt(float(numprocs)))    ! 2x2 grid of sub-domains
      periods(1)=.true.  ! periodic bdry. cond. along x
      periods(2)=.true. ! 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.       |
!           ---------------------------------------------------------------- 

! --Given my rank (my_rank), find what my coordinate parameters on the block 
!   grid are.
       call MPI_CART_COORDS(COMM_CART,my_rank,ndims,coords,ierror)
       print *,"my_rank=",my_rank," my coordinates parameters, coords, are ",&
              & coords

!--Who is my TOP neighbor?
!  Well... Since my corrdinate parameters are [coords(1),coords(2)]
!          the coordinate parameters of my TOP neighbor are 
!          [coords(1),coords(2)+1]
!          unless I am at the "right" bdry and bdr. conditions are NON-periodic
!  What I need is the rank of the process containing this block.
!  =====> Use MPI_CART_RANK --- [ note period(1)=.true. !]
       r_coords(1)=coords(1)
       r_coords(2)=coords(2)+1
       call MPI_CART_RANK(COMM_CART,r_coords,r_rank,ierror)

       print*,"my_rank=",my_rank," my TOP neighbor is: ",r_rank


!--Finilize MPI
      call MPI_FINALIZE(irc)

      stop
      end