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

[xiagao1982]

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
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