[R] Calling R from C

[Roger Bivand]

On Tue, 9 May 2006, Ingmar Visser wrote:

> 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)

Try:

.Call("doloop", as.integer(nt))

I think nt probably is numeric but not integer; it is safer to impose the 
required storage mode than leave it to chance:

> nt <- 1 
> storage.mode(nt)
[1] "double"
>  .Call("doloop",nt)
x: 0 
init ans: 0.000000 
[1] 0
> .Call("doloop", as.integer(nt))
x: 1 
init ans: 0.000000 
[1] 0
> sessionInfo()
Version 2.3.0 (2006-04-24) 
i686-pc-linux-gnu 

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



> 
> 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
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no