[R] mutable objects

[Duncan Temple Lang]

Vadim Ogranovich wrote:
> Hi,
> 
> I need "mutable" objects that I can safely modify in my C code. All call
> interfaces, e.g. .C, .Call, etc., require you to not modify the "in" args.

I took a quick look at the code for .Call() and it appears not to copy
its arguments. As a result, however, you can use this to modify objects in
place and they effects will persist. This is not "recommended" but
does work for many purposes.

A simple example where one can see  this effect is 
with the following C code.

#include "Rinternals.h"

SEXP
foo(SEXP x, SEXP y)
{
  int n = LENGTH(x), i;

  for(i = 0; i < n; i++) {
          REAL(x)[i] += 1.0;
          REAL(y)[i] *= REAL(y)[i];
  }

  return(x);
}

Calling this from R as 

 x <- as.numeric(1:10)
 y <- as.numeric(1:10)
 z <- .Call("foo", x, y)

Then look at the values of x, y and z.



The .Call() is currently incompatabile with S4 & S-Plus 6, I believe.
We should probably correct this by adding a COPY argument and making
duplication the default behavior.


The OOP package provides both i) a way for creating mutable objects
using environments and ii) a formal mechanism for specifying classes
and inheritance from those classes. It sounds like you (currently)
only need environments. The package is available from 
   http://www.omegahat.org/OOP

 D.



> The description of the OOP package in R-news from Sep 2001 alludes to a
> possiblity of such objects, but the package itself is no longer on Omegahat
> and I am not sure I need the full machinery.
> 
> Thanks, Vadim
> 
> 
> P.S. I wonder what is the relationship between the "methods" package and the
> OOP package described in R-news from Sep 2001.
> 
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-- 
_______________________________________________________________

Duncan Temple Lang                duncan at research.bell-labs.com
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