rowwise mean and variance from mtx file using Rcpp

rowmeanvar_mtx.cpp

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


// [[Rcpp::export]]
List rowmeanvar_mtx(const int & n_row, const int & n_col,
               const std::string & readtxt) {
  int x;
  int y;
  double v;
  std::ifstream file(readtxt);
  std::string str;    
  int index = 0;
  int n = 0;
  arma::vec vout = arma::zeros<arma::vec>(n_row);
  arma::vec v2out = arma::zeros<arma::vec>(n_row);
  while (std::getline(file, str)){
    if(index > 2){
      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]);
      vout(x-1) += v;
      v2out(x-1) += pow(v,2);
    }
    index++;
  }
  vout /= n_col;
  v2out /= n_col-1;
  v2out -= pow(vout,2);
  return List::create(Named("mean")=vout, Named("var")=v2out);
}