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R (programming language)/Tutorials/Connecting Fortran and R

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This is to briefly describe the process of connecting the programming languages: Fortran and R. At least basic knowledge of both programming languages is assumed here.


Basic requirements

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  • R - R should be installed on your computer. In this tutorial R version 2.10.1 (2009-12-14) was used on a Windows XP machine. For information about setup visit the installation how-to.
  • The Fortran program - You have to have a program source code, that you want to use in R. Please note, that R is able to call only subroutines. In case of functions a so called "wrapper subroutine" should be used. You find more about this here.

Sample program

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Copy the program below to a text file, and name it: multiply.f 

c sample subroutine to include for R
       subroutine multiply(ax,bx,cx)
       implicit none

       integer ax,bx,cx

       cx=ax*bx

       end

Making a dll

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In order to connect these two programming languages you need to compile the Fortran code. You will need a Fortran compiler for this one. The following example assumes a gfortran compiler.

To make a dll (Dynamic-link library) from the source code, open the command prompt, navigate to the directory with the source code and type: 

gfortran -shared -o multiply.dll multiply.f

Where multiply.f is the name of the file which contains the source code.

In Lahey Fortran compiler you should use the -dll switch instead of -shared.

Inside R

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  • Open R. The simplest case is to move the newly created dll file to the working directory of R. (Note: type getwd() to find out your working directory.)
  • You have to load the dll file to R. For this use dyn.load() like this:
dyn.load("d:/work/R_work_files/general/multiply.dll")

Where d:/work/R_work_files/general/multiply is the path to the dll file.

  • To test if it was loaded correctly, type:
is.loaded("multiply")

If you get "[1] TRUE", you are ok. If not, find out the cause of it (some hints are here).

  • The last thing you have to do is call the compiled Fortran subroutine from R using ".Fortran()" like this:
a=5
b=2
.Fortran("multiply",as.integer(a),as.integer(b),c=integer(1))

and you will get: 

[[1]]
[1] 5

[[2]]
[1] 2

$c
[1] 10


Where $c equals 10, which is exactly 5 times 2  .

Move to R function

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The process below is optional, but it makes the usage of the "final product" much more simple and user friendly.

Source code

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  • Open a new script, paste the following code there and save it as fortranConnection.r
# script to test the link between Fortran and R
star = function(a,b){
     # res holds the results for the Fortran call
     res =.Fortran("multiply",as.integer(a),as.integer(b),c=integer(1))
     return(res$c)
}

Note: As an outcome you are interested just in the value of c, so the return value will be res$c and not the whole res.

  • In R open Files> Source R code..., find fortranConnection.r and source it in.
  • Use your brand new R function as:
star(5,2)

Which leads to not-so-surprising result: 

[1] 10

Special cases

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  • Some compilers leave a trailing underscore after the subroutine name inside the dll file. If the is.loaded test above gives a FALSE for you, try this as one of the possible solutions:
is.loaded("multiply_")
  • Also other compiler specific issues might come in, such as for Lahey the following line could be inserted into the Fortran source code:
DLL_EXPORT mySubroutine

Where mySubroutine is the subroutine name to be exported to a dll file. More information here.

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For further reading about the topic:

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