[R] Fortran Code to R Code

[Dave Roberts]

Bryan,

     The previous responses will point you in the right direction.  I 
have found, however, that it takes a while to get used to the 
requirements, and there are many possible sources of error.

1) You have to convert the main program to a subroutine.  Any arrays 
declared in the main program or any nested subroutine have to created in 
R before the subroutine is called, and passed to the subroutine.  On the 
other hand, this provides the advantage of allocatable array size in 
FORTRAN 77.

2) You have to make sure that you are very careful about storage.  I 
have reluctantly decided that attempting minimize storage by using 
integer*2 and real*4 is not worth it, and I now use just integer and 
double precision to minimize problems with R.

3) Consider casting every variable passed in the .Fortran() call.  Many 
of them would be correct by default possibly, but it is easier to simply 
cast them all, e.g. as.integer() for all integers, as.double() for real 
numbers.  These get the storage right, and strip of any attributes 
besides he actual values.  Fortunately, R and FORTRAN agree about 
storage order in arrays and you don't have to micro-manage that.

4) When you get it wrong, the probability of a seg fault and hard crash 
is very high.  So, every time you modify the R function that calls the 
.Fortran you have written, do a save.image() before you try the code. 
Otherwise you lose all your changes.  If you have the luxury of working 
in linux (and presumably other *nixes), you can use "write(6,*) 
whatever" debug statements in your code and the output goes to your R 
session.  In Windows, you have to use the specific debug routines 
described in the R manuals.

5) Managing the returned values has to handled in the .Fortran() call by 
giving the argument a name.  Then, that name can be used as the 
component name of the returned object.  Just below is an example from 
labdsv that converts any dissimilarity or distance matrix to the nearest 
euclidean distance object.  The PACKAGE= argument is used when the 
routine is pat of a package; it's not necessary for ad hoc functions.

euclidify <- function (x,upper=FALSE,diag=FALSE)
{
     x <- as.dist(x)
     tmp <- .Fortran("euclid",
         x=as.matrix(x),
         as.integer(attr(x,"Size")),
         PACKAGE='labdsv')
     tmp2 <- as.dist(tmp$x)
     attr(tmp2, "Labels") <- dimnames(x)[[1]]
     attr(tmp2, "Diag") <- diag
     attr(tmp2, "Upper") <- upper
     attr(tmp2, "method") <- paste("euclidify", attr(x, "method"))
     attr(tmp2, "call") <- match.call()
     tmp2
}

Dave Roberts


Bryan Klingaman wrote:
> How do you convert Fortran Code to R Code to use and execute in R?  Or how do you take Fortran code and make it run in R?  So what I'm getting at is, I have some code in fortran and I want to be able to run that same code for R.  Please email me back and let me know how to do that, thanks.
>    
>   Bryan
>   Email: BKling9er at yahoo.com
> 
>        
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