Writing DLL's for R in Delphi

Delphi is not the most natural choice to make DLL's for R in, because R is based on C. When your DLL's will contain a lot of pointer gymnastics it is a little clumbsy because Object Pascal does not allow much pointer arithmitic.

Anyway, it can be done. Here is the source code for the Max function. Notice

1. The function is "void" - the result is returned in the third argument
2. Arguments are pointers.

library MyMaxLib; // File MyMax.pas

// This is a DLL with an R-style interface ...
// it exports the MyMax function.

uses
  SysUtils,
  Classes;

type
  pD = ^Double;

procedure MyMax(X,Y,Z: pD); stdcall;
begin
   if (X^ < Y^) then Z^:=Y^ else Z^:= X^;
end;

exports 
  MyMax index 1;
begin
end.

To compile this into a DLL, execute "dcc32 MyMax.pas" from the command prompt (assuming that the source code is residing in MyMax.pas)

To call this from R, you need a wrapper. Here's one, called test.r

# Set-up test of the MyMax DLL
setwd("c:/uht/Delphi/R-DLL")  # ... replace with the proper path

dyn.load("MyMax.dll")
MyMax <- function(a,b) 
{
  c <- as.integer(0); 
  ans <- .C("MyMax",
	as.double(a),
	as.double(b),
	as.double(c)
    )
  ans[[3]]
}

MyMax(12,23.1)

Notice the explicit type-casting ("as.double ..."). This is to make sure that the DLL gets what it expects, a pointer to a double.

Notice: This DLL is not "vectorised" as most R functions are and should be. What I mean is, if you call this with

> MyMax(c(12,34.1),17.1)

then you get just 17.1 - the largest of the first elements. But hey.

How to write DLL's for R in C (or C++)

See the pages for the C++ environments, MicroSoft Visual C++ or Borland C++ Builder.