C vs Rcpp for monte carlo pi approximation

gistfile1.txt

Rcpp_code <- "
#include <Rcpp.h>

// [[Rcpp::export]]
double mcsim_rcpp(const int n){
  int i, r = 0;
  double u, v;
  Rcpp::RNGScope scope;
  
  for (i=0; i<n; i++){
    u = R::runif(0, 1);
    v = R::runif(0, 1);
    
    if (u*u + v*v <= 1)
      r++;
  }
  
  return (double) 4.*r/n;
}
"


C_code <- "
#include <R.h>
#include <Rinternals.h>

// [[Rcpp::export]]
SEXP mcsim_c(SEXP n_){
  SEXP ret;
  int i, r = 0;
  const int n = INTEGER(n_)[0];
  double u, v;
  
  GetRNGstate();
  
  for (i=0; i<n; i++){
    u = unif_rand();
    v = unif_rand();
    
    if (u*u + v*v <= 1)
      r++;
  }
  
  PutRNGstate();
  
  PROTECT(ret = allocVector(REALSXP, 1));
  REAL(ret)[0] = (double) 4.*r/n;
  UNPROTECT(1);
  return ret;
}
"



library(Rcpp)

sourceCpp(code=Rcpp_code)
sourceCpp(code=C_code)

mcsim_c(100L)
mcsim_rcpp(100L)

mcsim_rcpp(100)
mcsim_c(100) ### uh-oh