skeleton.f90

program cartesian_grid
  use mpi
  implicit none

  integer :: ntask,  & ! number of MPI tasks
       my_id,        & ! MPI rank of the task
       rc,           & ! return code
       comm2d,       & ! Cartesian communicator
       neighbors(4), & ! neighbors in the 2D grid
       coord(0:1),   & ! coordinates in the grid
       dims(0:1)       ! dimensions of the grid
  logical, dimension(2) :: period = (/ .true., .true. /)
  integer :: irank

  call mpi_init(rc)
  call mpi_comm_size(MPI_COMM_WORLD, ntask, rc)
  call mpi_comm_rank(MPI_COMM_WORLD, my_id, rc)

  ! Determine the process grid (dims(0) x dims(1) = ntask)
  if (ntask < 16) then
     dims(0) = 2
  else if (ntask >= 16 .and. ntask < 64) then
     dims(0) = 4
  else if (ntask >= 64 .and. ntask < 256) then
     dims(0) = 8
  else
     dims(0) = 16
  end if

  dims(1) = ntask / dims(0)
  if (dims(0) * dims(1) /= ntask) then
     write(*,'(A,I3,A1,I3,A2,I4)') 'sorry, no go', dims(0), 'x', &
          dims(1),'/=', ntask
     call mpi_abort(mpi_comm_world, 1, rc)
  end if

  ! Create the 2D Cartesian communicator
  ! TO DO
  ! Find out & store the neighboring ranks
  ! TO DO
  ! Find out & store also the Cartesian coordinates of a rank
  ! TO DO

  do irank = 0, ntask-1
     if (my_id == irank) print '(I3,A,2I2,A,4I3)', &
          my_id, '=', coord, ' neighbors=', neighbors(:)
     call mpi_barrier(mpi_comm_world, rc)
  end do

  call mpi_finalize(rc)

end program cartesian_grid