R crashes when I try to access to an element in a C++ matrix using Rcpp

Question

I have encountered a weird problem : when I try to access to an element using brackets( A(1,2) ) R session crashes. When I use A[1,2] it returns the first column and not the second one, and I get the result when I try A[1,3], A[1,4], ...

Here is my code :

#include <Rcpp.h>
#include <String.h> 

using namespace Rcpp;


// [[Rcpp::export]]
Rcpp::NumericMatrix Function_bag_ngrams(Rcpp::NumericMatrix idv_x_sacs, Rcpp::StringMatrix idv, Rcpp::StringMatrix liste_ngrammes_gardes, Rcpp::StringVector colNamesIdBags )
{

  int nbr_idv = idv_x_sacs.nrow();
  int nbr_sac = idv_x_sacs.ncol();

  int nbr_composants_sacs = liste_ngrammes_gardes.nrow();

  std::string str_aux = "";
  std::string idSac = "";
  int indexIdSac = 0;

  for(int i=0; i<nbr_idv; i++)
  {
    str_aux = (String) idv(i,1);

    for(int j=0; j<nbr_composants_sacs; j++)
    {
      if(str_aux.find(liste_ngrammes_gardes(j,2)) != std::string::npos)
      {
        idSac = (String) liste_ngrammes_gardes(j,1);
        indexIdSac = 0;

        for(int k=0;k<nbr_sac;k++)
        {
          if((String) colNamesIdBags(k)==idSac)
          {
            indexIdSac = k;
            break;
          }
        }

        idv_x_sacs(i,indexIdSac) = 1;
      }
    }

  }


  return idv_x_sacs;
}

I know that the problem comes from the brackets because I tried a simple code trying to access an element and it crashed.

Here is a simple code that I tried :

#include <Rcpp.h>
#include <String.h> 

using namespace Rcpp;


// [[Rcpp::export]]

std::string Function(Rcpp::StringMatrix M)
{

    std:string tmp = M(1,1)

  return tmp;
}

And here is how I use it in R :

sourceCpp("Function.cpp")
Function(matrix(rep("test",4),nrow=2,ncol=2))

Thank's.

Answer 1

You are creating a numeric matrix with

matrix(rep(0,4),nrow=2,ncol=2)

which you are sending into a function expecting a StringMatrix

Function(Rcpp::StringMatrix M)

That cannot possibly work.