program myjacobi
c%%%%%%%%%%%%%%%%%%%
c%%% 
c%%%%%%%%%%%%%%%%%%%%
c%%% calling sequence  .... 
c%%% 
c%%%%%%%%%%%%%%%%%%%%
c%% myn = size of problem
c%% tol = tolerance for size of residual
c%% maxiter = max number of iterations acceptable
c%%%%%%%%%%%%%%%%%%%%
      implicit none
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
c%%%%% MPI declarations
      include 'mpif.h'
      integer nproc, rank, p, ierr, rc
      integer tag,status(MPI_STATUS_SIZE)
c%%%%% problem declarations
      integer maxn
      integer myn
      parameter (maxn=100)
c
      integer i
      double precision uleft, uright,u(maxn)
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
c%%%% start MPI 
c23456
      call MPI_INIT(ierr)
      if (ierr .ne. MPI_SUCCESS) then
         print *,'Error starting MPI program. Terminating.'
         call MPI_ABORT(MPI_COMM_WORLD, rc, ierr)
      end if
c%%%% what is my number (rank+1) and total number of processors 
      call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)
      call MPI_COMM_SIZE(MPI_COMM_WORLD, nproc, ierr)
c
      p = rank + 1
      if (rank.eq.0) then
          write(6,*) 'Number of processors=',nproc 
      endif
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      myn = 4

c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      uleft = 0D0;
      uright = 0D0;

c%% %%% every processor records its number
      do i = 1, myn
         u(i) = p;
      enddo
c
      write(6,*) 'Proc ',p,' data before ',uleft,(u(i),i=1,myn),uright

c%%%%%%%%%%%%%%%%%%%%%%%%%%%% example of exchange data

c exchaNnge data 
c     CORRECT THE CODE 
       tag = 0

         if (p.lt.nproc) then
            call MPI_Send(u(myn), 1, MPI_DOUBLE_PRECISION, rank + 1, 
     &        tag,MPI_COMM_WORLD,ierr );
         endif
         if (p.gt.1) then
            call MPI_Send(u(1), 1, MPI_DOUBLE_PRECISION, rank - 1,  
     &        tag,MPI_COMM_WORLD,ierr );
         endif
         if (p.lt.nproc) then
            call MPI_RECV (uright, 1, MPI_DOUBLE_PRECISION, rank,  
     &        tag,MPI_COMM_WORLD,status,ierr );
         endif
         if (p.ge.1) then
            call MPI_RECV (uleft, 1, MPI_DOUBLE_PRECISION, rank - 1, 
     &        tag,MPI_COMM_WORLD,status,ierr );
         endif

      write(6,*) 'Proc ',p,' data after ',uleft,(u(i),i=1,myn),uright

c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      call MPI_FINALIZE(ierr)
      end