R help - Jan 2005 - converting R objects to C types in .Call

Dear People,

I'm trying to write an R wrapper for a C++ library, using .Call. I've 
never used .Call before. I'm currently having some difficulties converting 
a R character string to a C one.

Here is a little test program.

#include <R.h>
#include <Rinternals.h>
#include <stdio.h>

SEXP testfn(SEXP chstr)
{
   char * charptr = CHAR(chstr);
   printf("%s", charptr);
}

This compiles without problems, but when I try to run this as
> .Call("testfn", "foo")
I get a segmentation fault.

I am sure I am making an obvious mistake, but can someone help me to sort 
it out? Thanks in advance. Please cc me, I'm not subscribed.

                                                              Faheem.

Faheem Mitha wrote:
> SEXP testfn(SEXP chstr)
> {
>   char * charptr = CHAR(chstr);
>   printf("%s", charptr);
> }
> 
> I am sure I am making an obvious mistake, but can someone help me to 
> sort it out? Thanks in advance. Please cc me, I'm not subscribed.
> 
Firstly, R is expecting an SEXP as a return value! And secondly, your 
SEXP chstr is still a vector - so you need to get an element from it. If 
there's only one element, then that'll be the zeroth element. Here's
my
code:

#include <R.h>
#include <Rinternals.h>
#include <stdio.h>

SEXP testfn(SEXP chstr)
{
   char * charptr = CHAR(VECTOR_ELT(chstr,0));
   printf("%s", charptr);
   return(chstr);
}

  - I'm just returning the same object back in the return() statement, 
and using VECTOR_ELT(chstr,0) to get to the 0'th element.

  SO in R:

  > foo = .Call("testfn","fnord")

should print 'fnord' and return foo as "fnord".

  > foo = .Call("testfn",c("bar","baz"))

will print 'bar' and return foo as c("bar","baz")

Baz

On Mon, 24 Jan 2005, Faheem Mitha wrote:
> I'm trying to write an R wrapper for a C++ library, using .Call.
I've never
> used .Call before. I'm currently having some difficulties converting a
R
> character string to a C one.
> Here is a little test program.
>
> #include <R.h>
> #include <Rinternals.h>
> #include <stdio.h>
>
> SEXP testfn(SEXP chstr)
> {
>  char * charptr = CHAR(chstr);
CHAR(STRING_ELT(chstr, 0));

First you select the first element of the character vector, then select 
its contents as a C-level character array.
>  printf("%s", charptr);
> }
>
> This compiles without problems, but when I try to run this as
>
>> .Call("testfn", "foo")
>
> I get a segmentation fault.
>
> I am sure I am making an obvious mistake, but can someone help me to sort
it
> out? Thanks in advance. Please cc me, I'm not subscribed.
Please read `Writing R Extensions' for more examples.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Is there a specific reason why, instead of

    CHAR(STRING_ELT(chstr, 0));

The S-Plus compatible

   CHARACTER_POINTER(chstr)[0]

does not work in R?
--
Edzer

On Tue, 25 Jan 2005, Edzer J. Pebesma wrote:
> Is there a specific reason why, instead of
>
>   CHAR(STRING_ELT(chstr, 0));
>
> The S-Plus compatible
>
>  CHARACTER_POINTER(chstr)[0]
>
> does not work in R?
Yes.

This is really a question for R-devel, but briefly, R and S-PLUS store 
character vectors in very different ways.  This worked prior to 1.2.0, but 
was changed as part of the price of the new (then) garbage collector.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595