R help - Dec 2001 - passing lists through .C

I have a list,
> rb
$t
[1] "tree"
 
$x
[1] 0
 
$cut
[1] 0.8
 
$l
 [1] 0 0
 
and pass it through .C("fn",rb) to

void fn(ev)
int **ev;
{ double cut;
  cut = *(double *)ev[2][0];
  printf("%8.5f\n",cut);
}

in S-4, it produces 0.8, as I want.
But R (version 1.3.1, linux) produces a segmentation fault.
Is it possible to access list elements in R? The manual seems
to suggest writing .Call interfaces, which I want to avoid.

Thanks,
Catherine.
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On Tue, 11 Dec 2001, Catherine Loader wrote:
> I have a list,
>
> > rb
> $t
> [1] "tree"
>
> $x
> [1] 0
>
> $cut
> [1] 0.8
>
> $l
>  [1] 0 0
>
> and pass it through .C("fn",rb) to
>
> void fn(ev)
> int **ev;
> { double cut;
>   cut = *(double *)ev[2][0];
>   printf("%8.5f\n",cut);
> }
>
> in S-4, it produces 0.8, as I want.
> But R (version 1.3.1, linux) produces a segmentation fault.
> Is it possible to access list elements in R? The manual seems
> to suggest writing .Call interfaces, which I want to avoid.
Yes, but it would be no easier than using .Call.  ?Foreign says

Lists are passed as C arrays of `SEXP' and can be declared as `void *'
or
`SEXP *'.

so you do need to mess with SEXPs.  Something like (minimally tested)

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

void fn(SEXP *ev)
{
   SEXP cut = ev[2];
   printf("%8.5f\n", REAL(cut)[0]);
}

Note that using printf is not portable, but presumably this was just a
test.

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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>>>>> "CL" == Catherine Loader <catherine at
research.bell-labs.com> writes:
    CL> I have a list,
    >> rb
    CL> $t [1] "tree"
    CL> $x [1] 0
    CL> $cut [1] 0.8
    CL> $l [1] 0 0

    CL> and pass it through .C("fn",rb) to

    CL> void fn(ev)
    CL> int **ev;
    CL> { double cut;
    CL>  cut = *(double *)ev[2][0];
    CL>  printf("%8.5f\n",cut);
    CL> }

    CL> in S-4, it produces 0.8, as I want.

which is astonishing, and AFAIK not at all to be relied on.

    CL> But R (version 1.3.1, linux) produces a segmentation fault.
    CL> Is it possible to access list elements in R? 
yes, via .Call()

    CL> The manual seems to suggest writing .Call
    CL> interfaces, which I want to avoid.

no way (to avoid this)!
It's also the (only?) way publicized for S4 (based versions of
S-PLUS).

Regards,
Martin
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