!4. Modify example4.f90 to update RHS guard cells. 
!-----------------------------------------------------------

      program main

!-- On a 1D grid of 4 blocks. Each block updates its guard
!   cells (v0 and v3) with data from its left and
!   right neighbors.

      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
      integer sendtag,recvtag

      parameter (ndims=1)    !dimension of the Cartesian grid
      integer ndims,dims(ndims)
      logical periods(ndims),reorder
      integer comm_cart
      integer coords(ndims),neig_coords(ndims)

      integer direction,disp,source,dest
      integer MPI_PROC_NULL
      real vel(0:3),v0,v3    !MODIFIED!

!--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)=4
      periods=.true.
      reorder=.false.
      call MPI_CART_CREATE(MPI_COMM_WORLD,ndims,dims,periods, &
                        &  reorder,comm_cart,ierror)
     
!--Find rank of left and right (along x) blocks
      direction=0                                  !(along x)
      disp=-1         !MODIFIED!                          !immediate neighbors

      call MPI_CART_SHIFT(comm_cart,direction,disp,source, &
                        & dest,ierror)

      write(*,*)'my_rank=',my_rank,' source=',source,' dest=',dest

!--Am I at a physical boundary?
      if (dest==MPI_PROC_NULL) &
         write(*,*)'dest=MPI_PROC_NULL at proc ',my_rank
      if (source==MPI_PROC_NULL) &
         write(*,*)'source=MPI_PROC_NULL at proc ',my_rank


!--Find the coordinate parameters of this block
      call MPI_CART_COORDS(comm_cart,my_rank,ndims,coords,ierror)

!--Assign some values to vel(0:2)
      do i=0,2
       vel(i)=float(i+100*coords(1))
      end do

!--Send vel(1) to the block to my left  and receive v3 from my right -->Modified
       sendtag=1
      recvtag=1
      call MPI_SENDRECV(vel(1),1,MPI_REAL,dest,sendtag, &          !MODIFIED!
                    v3,1,MPI_REAL,source,recvtag, &          !MODIFIED!
                    comm_cart,status,ierror)

!--Print out the updated v3
      write(*,*)'my coords are: ',coords,'   v3= ',v3     !MODIFIED!
!--Finilize MPI
      call MPI_FINALIZE(irc)

      stop
      end