filtering rows in mtx file using Rcpp

rowfilter_mtx.cpp

#include "RcppArmadillo.h"
// [[Rcpp::depends(RcppArmadillo)]]
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace Rcpp;


// [[Rcpp::export]]
void rowfilter_mtx(const std::string & readtxt,
                const std::string & writetxt,
                const arma::vec & rowind){
  int x;
  //int y;
  //double v;
  std::ifstream file(readtxt);
  std::string str;    
  int index = 0;
  //int n = 0;
  std::ofstream myfile;
  myfile.open(writetxt);
  while (std::getline(file, str)){
    if(index <= 2){
      myfile << str + "\n";
    }else{
      std::stringstream ss(str);
      std::vector<std::string> svec;
      while( ss.good() ){
        std::string substr;
        getline(ss, substr, ' ');
        svec.push_back(substr);
      }
      x = stoi(svec[0]);
      //y = stoi(svec[1]);
      //v = stod(svec[2]);
      if(arma::any(rowind==x)){
        myfile << svec[0] + " " + svec[1] + " " + svec[2] + "\n";
      }
    }
    index++;
  }
  myfile.close(); 
}