Int_t getFile=0;

TString filename = "hsimple.root";
TString dir = gROOT->GetTutorialDir();
dir.ReplaceAll("/./","/");
TFile *hfile = nullptr;
if (getFile) {
   // if the argument getFile =1 return the file "hsimple.root"
   // if the file does not exist, it is created
   TString fullPath = dir+"hsimple.root";
   if (!gSystem->AccessPathName(fullPath,kFileExists)) {
      hfile = TFile::Open(fullPath); //in $ROOTSYS/tutorials
      if (hfile) return hfile;
   }
   //otherwise try $PWD/hsimple.root
   if (!gSystem->AccessPathName("hsimple.root",kFileExists)) {
      hfile = TFile::Open("hsimple.root"); //in current dir
      if (hfile) return hfile;
   }
}

if (gSystem->AccessPathName(".",kWritePermission)) {
   printf("you must run the script in a directory with write access\n");
   return nullptr;
}
hfile = (TFile*)gROOT->FindObject(filename); if (hfile) hfile->Close();
hfile = new TFile(filename,"RECREATE","Demo ROOT file with histograms");

TH1F *hpx = new TH1F("hpx","This is the px distribution",100,-4,4);
hpx->SetFillColor(48);
TH2F *hpxpy = new TH2F("hpxpy","py vs px",40,-4,4,40,-4,4);
TProfile *hprof = new TProfile("hprof","Profile of pz versus px",100,-4,4,0,20);
TNtuple *ntuple = new TNtuple("ntuple","Demo ntuple","px:py:pz:random:i");

gBenchmark->Start("hsimple");

TCanvas *c1 = new TCanvas("c1","Dynamic Filling Example",200,10,700,500);
c1->SetFillColor(42);
c1->GetFrame()->SetFillColor(21);
c1->GetFrame()->SetBorderSize(6);
c1->GetFrame()->SetBorderMode(-1);

TRandom3 randomNum;
Float_t px, py, pz;
const Int_t kUPDATE = 1000;
for (Int_t i = 0; i < 25000; i++) {
   randomNum.Rannor(px,py);
   pz = px*px + py*py;
   Float_t rnd = randomNum.Rndm();
   hpx->Fill(px);
   hpxpy->Fill(px,py);
   hprof->Fill(px,pz);
   ntuple->Fill(px,py,pz,rnd,i);
   if (i && (i%kUPDATE) == 0) {
      if (i == kUPDATE) hpx->Draw();
      c1->Modified();
      c1->Update();
      if (gSystem->ProcessEvents())
         break;
   }
}
gBenchmark->Show("hsimple");

hpx->SetFillColor(0);
hfile->Write();
hpx->SetFillColor(48);
c1->Modified();
return hfile;

gROOT->GetListOfCanvases()->Draw()