How to call a Fortran program from R

Question

I am totally new to Fortran and well versed with R. I was handed down a huge Fortran program with about 30 subroutines and about 15 functions and many other lines of code. I was told that I needed to call the Fortran program from R. I have been searching online for ways to create this bridge between R and Fortran with very little success. I can successfully execute the Fortran exe file from the command line and create the desired outputs. The fortran file is called "FortFish.f"

A question:

From R, Do I call the Fortran program or do I have to call Fortran functions and subroutines separately?

From R, Do I call the entire Fortran program like this?: R CMD SHLIB FortFish.f and then use: dyn.load("FortFish.so")

If I cannot run the entire Fortran program at once, I will post a couple of small fortran functions and subroutines upon request. Does anyone have a running example of using R and Fortran that can share?

My Fortran code is extremely large, otherwise, I would post it here. Thank you.

user_13963867 · Answer

I see three possibilities:

You compile the Fortran program separately, and call it with R function system(). You will have to pass data through files, in a format that this program can read.
You compile a DLL that you load from R with dyn.load(), then you call a Fortran function with .Fortran(). You can easily pass numeric data (scalars, vectors or arrays), but string data is more difficult to handle. And arrays are copied.
This mechanism to call a DLL function is considered too simplistic and now .Call() is prefered, but to use .Call() you would have to write C wrappers.

I will give an example of the second possibility. Consider a subroutine in Fortran that evaluates a polynomial by Horner's algorithm:

subroutine horner(n, a, x, y)
    implicit none
    integer :: n, i
    double precision :: a(n), x, y

    y = a(n)
    do i = n - 1, 1, -1
        y = y * x + a(i)
    end do
end subroutine

Compile from the command line with:

R CMD SHLIB horner.f90

To call it from R:

dyn.load("horner.dll")

horner <- function(a, x) {
  .Fortran("horner", as.integer(length(a)), a, x, y=0)$y
}

horner(c(-2, 0, 1), 1.414)

If you want your Fortran subroutines to print something to RStudio console, you need to do (at least on Windows):

Sys.unsetenv("GFORTRAN_STDOUT_UNIT")
Sys.unsetenv("GFORTRAN_STDERR_UNIT")

This is really a trivial example, and a more complex program will require more work, but you get the idea.

If your Fortran program is standalone (it has a 'program' unit and is supposed to be compiled to an executable called from the command line), and if you are new to Fortran, I would suggest to stick with the first choice, which will be much simpler. That's what the seasonal packaged does: call the executable of Census' X13AS from within R. The executable is in the x13binary package.

How to call a Fortran program from R

1 Answers