! split_ex.f90
! An example from Rutgers.
include "mpiSim.f90"
program another
include "useSim.f90"

! SIMPLE MPI PROGRAM TO SPLIT UP DATA AMONG PROCESSES AND
! DO A CALCULATION. RESULTS ARE SENT BACK TO PROCESS 0
! AND SOME RESULTS ARE PRINTED. ASSUMPTION IS MADE THAT
! THE ARRAYSIZE IS EVENLY DISIBLE BY NUMBER OF PROCESSES
! include 'mpif.h'
integer status(MPI_STATUS_SIZE)
integer ARRAYSIZE
parameter (ARRAYSIZE=60000)
integer nprocs, taskid, dest, index, i, chunksize, ierror
integer sendindex, source, sourceindex
real data(ARRAYSIZE), reslt(ARRAYSIZE)
! START MPI
call mpi_init(ierror)
call mpi_comm_rank(MPI_COMM_WORLD,taskid,ierror)
call mpi_comm_size(MPI_COMM_WORLD,nprocs,ierror)
write(*,*)'taskid=',taskid, ' nprocs= ', nprocs
chunksize=ARRAYSIZE/nprocs
index=taskid*chunksize + 1
! INITIALIZE DATA
if (taskid .eq. 0) then
  do 20 i=1, ARRAYSIZE
   data(i)= i
  20 continue
  print*, '*******Start MPI **********'
! SEND EACH PROCESS A CHUNK OF DATA
  do 30 dest=1,nprocs-1
    write(*,*) 'Sending to processes ', dest
! Note, the following +1 is wrong and will cause a subscript range
! error (with compilers that trap this error.)
    sendindex=dest*chunksize + 1
! F90 array notation is used instead F77 array segment, data(sendindex).
    call mpi_send(data(sendindex:sendindex+chunksize-1),   &
         chunksize, MPI_REAL,dest,0,MPI_COMM_WORLD,ierror)
  30 continue
! EACH PROCESS EXCEPT O RECEIVES A CHUNK OF DATA
else
! F90 array notation is used instead F77 array segment, data(sendindex).
  call mpi_recv(data(index:index+chunksize-1), chunksize,  &
  MPI_REAL, 0,0, MPI_COMM_WORLD, status,ierror)
  print*, taskid , ' received data ' , "index = ", index
endif
! COMPUTE A VALUE (RESULT) FOR A CHUNK OF DATA
do 40 i=index, index + chunksize - 1
  reslt(i)= data(i) +1.
40 continue
! SEND COMPUTED VALUES BACK TO 0
if (taskid .gt. 0) then
! F90 array notation is used instead F77 array segment, data(sendindex).
  call mpi_send(reslt(index:index+chunksize-1), chunksize,  &
  MPI_REAL, 0, 0, MPI_COMM_WORLD, ierrror)
  print *, taskid, ' sent values to 0'
else
! PROCESS 0 RECEIVE THE RESULTS FROM OTHER PROCESSES AND
! PRINTS SOME VALUES.
    do 50 source= 1, nprocs-1
     sourceindex= source*chunksize +1
! F90 array notation is used instead F77 array segment, data(sendindex).
     call mpi_recv(reslt(sourceindex:sourceindex+chunksize-1),  &
       chunksize, MPI_REAL, source,0, MPI_COMM_WORLD, status, &
       ierror)
     print *,'--------------------'
     print *,' Sample results from ', source
     print *, 'reslt(', sourceindex, ') = ', reslt(sourceindex)
 ! $$ The following sourceindex values of reslt have not been defined.
     print *, 'reslt(', sourceindex+100, ') = ', &
               reslt(sourceindex+100)
     print *, 'reslt(', sourceindex+1000, ') = ', &
               reslt(sourceindex+1000)
     print *, ' '
  50 continue
endif
! ENDING MPI
print *, 'process calling finalize ', taskid
call mpi_finalize(ierror)
end program another