using a Fortran module in R?

Question

I am able to use a fortran subroutine in R as long as I don't have it inside a module.For example:

subroutine dboard(darts, dartsscore)
  implicit none
  integer, intent(in)           :: darts
  double precision, intent(out) :: dartsscore
  double precision              :: x_coord, y_coord
  integer                       :: score, n

score = 0
do n = 1, darts
  call random_number(x_coord)
  call random_number(y_coord)

  if ((x_coord**2 + y_coord**2) <= 1.0d0) then
  score = score + 1
  end if
end do

dartsscore = 4.0d0*score/darts

end subroutine dboard

subroutine pi(avepi, DARTS, ROUNDS)
  implicit none
  double precision, intent(out)   ::  avepi
  integer, intent(in)             ::  DARTS, ROUNDS
  integer                         ::  MASTER, rank, i, n
  integer, allocatable            ::  seed(:)
  double precision                ::  pi_est, homepi, pirecv, pisum

interface 
   subroutine dboard(darts, dartsscore)
      implicit none
      integer, intent(in)           :: darts
      double precision, intent(out) :: dartsscore
   end subroutine dboard
end interface

! we set it to zero in the sequential run
rank = 0
! initialize the random number generator
! we make sure the seed is different for each task
call random_seed()
call random_seed(size = n)
allocate(seed(n))
seed = 12 + rank*11
call random_seed(put=seed(1:n))
deallocate(seed)

avepi = 0
do i = 0, ROUNDS-1
  call dboard(darts, pi_est)
  ! calculate the average value of pi over all iterations
  avepi = ((avepi*i) + pi_est)/(i + 1)
end do
end subroutine pi    

Can be compiled for R with:

R CMD SHLIB ./Fortran/Fpi.f90

and I can run it in R with:

mypi <- function(DARTS, ROUNDS) {
  dyn.load("./Fortran/Fpi.so")
  retvals <- .Fortran("pi", avepi = as.numeric(1), DARTS =  as.integer(DARTS), ROUNDS =  as.integer(ROUNDS))
  return(retvals$avepi)
}

mypi(DARTS = 50000, ROUNDS = 10)

I would like to write my fortran subroutines inside a module. I thought this was not possible, but @roygvib and @francescalus mentioned it is in one of my previous questions

How do you do the "attaching bind("c",name=...)" thing that @roygvib mentioned in that post?

Thanks!

---

I made the changes suggested in the comments:

Module Fortranpi
IMPLICIT NONE
contains
subroutine dboard(darts, dartsscore)
  implicit none
  integer, intent(in)           :: darts
  double precision, intent(out) :: dartsscore
  double precision              :: x_coord, y_coord
  integer                       :: score, n

score = 0
do n = 1, darts
  call random_number(x_coord)
  call random_number(y_coord)

  if ((x_coord**2 + y_coord**2) <= 1.0d0) then
  score = score + 1
  end if
end do

dartsscore = 4.0d0*score/darts

end subroutine dboard

subroutine pi(avepi, DARTS, ROUNDS) bind(C, name="pi_")
  implicit none
  double precision, intent(out)   ::  avepi
  integer, intent(in)             ::  DARTS, ROUNDS
  integer                         ::  MASTER, rank, i, n
  integer, allocatable            ::  seed(:)
  double precision                ::  pi_est, homepi, pirecv, pisum

interface
   subroutine dboard(darts, dartsscore)
      implicit none
      integer, intent(in)           :: darts
      double precision, intent(out) :: dartsscore
   end subroutine dboard
end interface

! we set it to zero in the sequential run
rank = 0
! initialize the random number generator
! we make sure the seed is different for each task
call random_seed()
call random_seed(size = n)
allocate(seed(n))
seed = 12 + rank*11
call random_seed(put=seed(1:n))
deallocate(seed)

avepi = 0
do i = 0, ROUNDS-1
  call dboard(darts, pi_est)
  ! calculate the average value of pi over all iterations
  avepi = ((avepi*i) + pi_est)/(i + 1)
end do
end subroutine pi

end module Fortranpi

When I try to build the package with Rstudio I get these errors:

==> R CMD INSTALL --no-multiarch --with-keep.source MyPi

* installing to library ‘/home/ignacio/R/x86_64-pc-linux-gnu-library/3.2’
make: Nothing to be done for 'all'.
* installing *source* package ‘MyPi’ ...
** libs
installing to /home/ignacio/R/x86_64-pc-linux-gnu-library/3.2/MyPi/libs
** R
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
Error in library.dynam(lib, package, package.lib) : 
  shared object ‘Fortranpi.so’ not found
Error: loading failed
Execution halted
ERROR: loading failed
* removing ‘/home/ignacio/R/x86_64-pc-linux-gnu-library/3.2/MyPi’
* restoring previous ‘/home/ignacio/R/x86_64-pc-linux-gnu-library/3.2/MyPi’

Exited with status 1.

I believe the problem could be the NAMESPACE

useDynLib(Fpi)
exportPattern("^[[:alpha:]]+")

Answer 1

Thanks to @roygvib and @francescalus this is my working module:

Module Fortranpi
IMPLICIT NONE
contains
subroutine dboard(darts, dartsscore)
  integer, intent(in)                    :: darts
  double precision, intent(out)          :: dartsscore
  double precision                       :: x_coord, y_coord
  integer                                :: score, n

score = 0
do n = 1, darts
  call random_number(x_coord)
  call random_number(y_coord)

  if ((x_coord**2 + y_coord**2) <= 1.0d0) then
  score = score + 1
  end if
end do

dartsscore = 4.0d0*score/darts

end subroutine dboard

subroutine pi(avepi, DARTS, ROUNDS) bind(C, name="pi_")
  use, intrinsic                         :: iso_c_binding, only : c_double, c_int
  real(c_double), intent(out)            ::  avepi
  integer(c_int), intent(in)             ::  DARTS, ROUNDS
  integer                                ::  MASTER, rank, i, n
  integer, allocatable                   ::  seed(:)
  double precision                       ::  pi_est, homepi, pirecv, pisum

! we set it to zero in the sequential run
rank = 0
! initialize the random number generator
! we make sure the seed is different for each task
call random_seed()
call random_seed(size = n)
allocate(seed(n))
seed = 12 + rank*11
call random_seed(put=seed(1:n))
deallocate(seed)

avepi = 0
do i = 0, ROUNDS-1
  call dboard(darts, pi_est)
  ! calculate the average value of pi over all iterations
  avepi = ((avepi*i) + pi_est)/(i + 1)
end do
end subroutine pi

end module Fortranpi