CODEWITHSUNDEEP
c++rrcpp
Jonathan1234
Jonathan1234

Reputation: 499

Rcpp, Value Assignment Error, Meaning of SEXPREC*

I wrote the following code in Rcpp

//#include <Rcpp.h>
#include <RcppArmadilloExtensions/sample.h>
#include <random>
#include <iostream>
using namespace Rcpp;


// [[Rcpp::export]]

arma::vec SimulBetaBin(int K, arma::vec N){
  
  arma::vec D;

  
  Environment pkg = Environment::namespace_env("extraDistr");
  Function f = pkg["rbbinom"];

  for(int i=0; i<K; ++i){
    D[i] = f(1, N[i],  1,  1);
  }
  
  
  return  D;
}

The purpose of this function is to simulate a Beta Binomial Distribution. However, when I compile the code in R, I get the following error

error: cannot convert 'SEXP' {aka 'SEXPREC*'} to 'double' in assignment
     D[i] = f( N[i],  1,  1);
                           ^

I tried to understand what a SEXPREC* is but I got even more confused

What R users think of as variables or objects are symbols which are bound to a value. The value can be thought of as either a SEXP (a pointer), or the structure it points to, a SEXPREC, what do they mean by that ??

Because I think I have to understand that first in order to solve the error.

In case

Upvotes: 1

Views: 254

Answers (1)

Dirk is no longer here
Dirk is no longer here

Reputation: 368499

You get a SEXP back from calling a Rcpp::Function() object, so you need to cast it. A modified version (also simplifying headers down to what you actually used and need) of your function follows, this one compiles for me.

#include <RcppArmadillo.h>
    
// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
arma::vec SimulBetaBin(int K, arma::vec N) {
  arma::vec D;
  Rcpp::Environment pkg = Rcpp::Environment::namespace_env("extraDistr");
  Rcpp::Function f = pkg["rbbinom"];

  for (int i=0; i<K; ++i) {
    SEXP val = f(1, N[i],  1,  1);
    D[i] = Rcpp::as<double>(val);
  }

  return  D;
}

Edit: Removed another unused header.

Upvotes: 4

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