[R] Calling R from C

[Ingmar Visser]

Dear R-helpers,

I'm trying to transfer a loop from R code to C to improve performance using
the .Call interface. However, I ran into problems before even getting
started, and I hope (fear) I'm missing something completely obvious here.

I use the following C function (passing an integer to the C function and
printing it): 

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

SEXP doloop(SEXP nt) {
    int x=INTEGER(nt)[0];
    Rprintf("x: %d \n", x);
    SEXP ans;
    PROTECT(ans=allocVector(REALSXP,1));
    REAL(ans)[0]=0;
    Rprintf("init ans: %f \n",REAL(ans)[0]);
    UNPROTECT(1);
    return(ans);
}

Which compiles without problems into a shared library

a1648:~ ivisser$ R CMD SHLIB  -l
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc/
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc.c
gcc-4.0 -arch ppc -I/Library/Frameworks/R.framework/Resources/include
-I/Library/Frameworks/R.framework/Resources/include/ppc
-I/usr/local/include   -fPIC -fno-common  -g -O2 -std=gnu99 -c
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc.c -o
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc.o
gcc-4.0 -arch ppc -flat_namespace -bundle -undefined suppress
-L/usr/local/lib -o
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc.so
/Users/ivisser/Documents/projectsCurrent/dmm4Project/rfromc.o
-L/Library/Frameworks/R.framework/Resources/lib/ppc -lR

And then call:

>dyn.load("rfromc.so")
> .Call("doloop",nt)

With the following result (basically x is an unpredictable integer):

x: 1072693248 
init ans: 0.000000 
[1] 0

When I replace Rprintf("x: %d \n", x); with Rprintf("x: %f \n", x); (note
the %f) I get:

> .Call("doloop",nt)
x: 1.000000 
init ans: 0.000000 
[1] 0

Apparently x has the right value but is somehow represented as a double
instead of as integer

I'm using:
Version 2.3.0 (2006-04-24)
powerpc-apple-darwin8.6.0

attached base packages:
[1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets"
"base"     

Best, Ingmar