#include <iostream>   // std ::cout
#include <fstream>    // std::ifstream
#include <vector>
#include <string>
#include "TFile.h"
#include <TCanvas.h>
#include "TLegend.h"
#include "TGraph.h"
#include "TSystemDirectory.h"
#include "TList.h"

using namespace std;

void list_files(vector<string> &filenames, const char *dirname="C:/root/folder/",
                const char *ext=".txt")
{
   TSystemDirectory dir(dirname, dirname);
   TList *files = dir.GetListOfFiles();
   if (files) {
      TSystemFile *file;
      TString fname;
      TIter next(files);
      while ((file=(TSystemFile*)next())) {
         fname = file->GetName();
         if (!file->IsDirectory() && fname.EndsWith(ext)) {
            string filename(fname.ReplaceAll(ext, "").Data()); 
            cout << filename << endl;
            filenames.push_back(filename);
         }
      }
   }
}

void Example()
{
   vector<string> filenames;
   string filename,infile,fileout,extin,extout,input_dir;

   extin=".txt";
   extout=".root";
   cout << "Please enter the directory containing the " <<
        extin << " files you wish to plot "<< endl;
   cin >> input_dir;
   cout<<"Reading files from "<<input_dir<<endl;    //Just a check
   list_files(filenames, input_dir.c_str(), extin.c_str());
   for (auto name : filenames) {
      infile=input_dir+name+extin;
      fileout=name+extout;
      cout<<"Processing Data File: "<<infile<<endl;    //Just a check

      new TCanvas(infile.c_str(), "My Plot");
      TGraph *gr = new TGraph(infile.c_str(), "%lg %lg"); 
      gr->Draw("AL");                               

      TFile *f0 = TFile::Open(fileout.c_str(), "RECREATE");
      gr->Write();  
      f0->Close(); 
      delete f0;
   }
}