[Rcpp-devel] About calling C/C++ functions in R

[Romain Francois]

Hi,

I totally second Dirk in the advice about reading the appropriate 
documents. I would add the book from John Chambers "Programming with 
Data" as it has very clear chapters about R and C(++).

Once you digested these documents, you can read the Rcpp-modules 
vignette from Rcpp, which might help you.

Also, I just commited some code in Rcpp (which will be available in the 
next version of Rcpp : 0.8.3) that adds the Rcpp::InternalFunction 
class, that you can use like this:

// grab the global environment
Environment g = Environment::global_env() ;

// assign the "hello" R variable to the internal function
g["hello"] = InternalFunction( &hello ) ;



Here is a complete example you can run from the R prompt:

require( Rcpp)
require( inline)

inc <- '

	const char* hello( std::string who ){
     		std::string result( "hello " ) ;
     		result += who ;
     		return result.c_str() ;
	}
	
'
code <- '
	Environment g = Environment::global_env() ;
	g["hello"] = InternalFunction( &hello ) ;
	return R_NilValue ;
'
fx <- cxxfunction( signature(), code, inc, plugin = "Rcpp" )
f <- fx() # force loading the dynamic library

hello( "world" )

I have not tried it yet from RInside.

Romain

Le 16/06/10 02:13, xiagao1982 a écrit :
> Dear friends,
> I am a newcomer of Rcpp and RInside. I installed them in my system and
> successfully build the following example:
> #include <RInside.h> // for the embedded R via RInside
> Rcpp::NumericMatrix createMatrix(const int n) {
> Rcpp::NumericMatrix M(n,n);
> for (int i=0; i<n; i++) {
> for (int j=0; j<n; j++) {
> M(i,j) = i*10+j;
> }
> }
> return(M);
> }
 >
> int main(int argc, char *argv[]) {
> const int mdim = 4; // let the matrices be 4 by 4
> SEXP ans;
> RInside R(argc, argv); // create an embedded R instance
> Rcpp::NumericMatrix M = createMatrix(mdim); // create and fill a sample
> data Matrix
> R["M"] = M; // assign C++ matrix M to R's 'M' var
> std::string evalstr = "\
> cat('Running ls()\n'); print(ls()); \
> cat('Showing M\n'); print(M); \
> cat('Showing colSums()\n'); Z <- colSums(M); print(Z); \
> Z"; // returns Z
> ans = R.parseEval(evalstr); // eval the init string -- Z is now in ans
> Rcpp::NumericVector v(ans); // convert SEXP ans to a vector of doubles
> for (int i=0; i< v.size(); i++) { // show the result
> std::cout << "In C++ element " << i << " is " << v[i] << std::endl;
> }
> exit(0);
> }
> Now I add a function in the C++ code:
> const char* hello( std::string who ){
> std::string result( "hello " ) ;
> result += who ;
> return result.c_str() ;
> }
> And I try to call this function in R:
> std::string txt = "ret = hello('Friends'); print(ret);";
> R.parseEvalQ(txt); // eval string quietly, no result
> It fails to do that.
> Could you please tell me how to achieve that? Thanks very much!
> Gao Xia
> ------------------------------------------------------------------------
> xiagao1982
> 2010-06-16


-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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