Problem with TChain reset and TEventList

ROOT
1

Hello,
I have 2 files with the same TTree and I’m trying to read them in sequenze applying some cut to the events. The problem is that when I try to read the second file the event loop gives the last entry of the first file. I guess using Reset() on the chain is not enough… I wrote a simple macro to show what I get. Could you check what’s wrong with the following code?
Thanks

Attilio

void test() {

  UInt_t event;

  // write first file
  TFile f1("f1.root","recreate");
  f1.cd();
  TTree *fTree1 = new TTree ("test", "test");
  fTree1->Branch("event", &event, "event/i");
  for(int n=0;n<10;n++) {
    event=n;
    fTree1->Fill();
  }
  f1.Write();
  f1.Close();

  // write second file
  TFile f2("f2.root","recreate");
  f2.cd();
  TTree *fTree2 = new TTree ("test", "test");
  fTree2->Branch("event", &event, "event/i");
  for(int n=10;n<20;n++) {
    event=n;
    fTree2->Fill();
  }
  f2.Write();
  f2.Close();

  // define TChain and cut
  TChain * fChain = new TChain("test","");
  TCut odd("event%2!=0");
  TBranch        *b_event;   //!
  fChain->SetBranchAddress("event", &event, &b_event);

  // read first file OK
  fChain->Add("f1.root");
  fChain->Draw(">>eventlist",odd);
  TEventList *elist1 = (TEventList*)gDirectory->Get("eventlist");
  for (Long64_t i = 0 ; i < elist1->GetN() ; ++i) {
    fChain->GetEntry(elist1->GetEntry(i));
    cout << event << endl;
  }
  fChain->Reset();

  // read second file NOPE
  fChain->Add("f2.root");
  fChain->Draw(">>eventlist",odd);
  TEventList *elist2 = (TEventList*)gDirectory->Get("eventlist");
  for (Long64_t i = 0 ; i < elist2->GetN() ; ++i) {
    fChain->GetEntry(elist2->GetEntry(i));
    cout << event << endl;
  }
  fChain->Reset();

}

Macro output:

   ------------------------------------------------------------------
  | Welcome to ROOT 6.26/06                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for macosxarm64 on Sep 24 2022, 11:23:00                   |
  | From tag , 28 July 2022                                          |
  | With Apple clang version 14.0.0 (clang-1400.0.29.102)            |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

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2

I guess @pcanal can help you.

3

Before I get to the technical answer, why are you using TChain::Reset rather than simply adding both file to the same chain or directly use the TTree from the file?

There is 2 issues with the script mentioned. One is that we have the pattern:

 TChain * fChain = new TChain("test","");
...
  fChain->SetBranchAddress("event", &event, &b_event);
...
  fChain->Reset()
...
     fChain->GetEntry(...);

TChain::Reset (of course) also reset the SetBranchAddress and thus the chain is no longer contect to the value.
Use:

  fChain->Add("f2.root");
  fChain->SetBranchAddress("event", &event, &b_event);

The second issue is that when attempting to retrieve the second TEventList the first one is still list and thus the statement:

  TEventList *elist2 = (TEventList*)gDirectory->Get("eventlist");

actually returns the first list rather than the second one (and thus we have elist1 == elist2.
Instead use

  fChain->Draw(">>eventlist2",odd);
  TEventList *elist2 = (TEventList*)gDirectory->Get("eventlist2");

(and/or delete the first TEventList)

4

Hi pcanal,
thanks for solving the problem. The solution is actually adding

  fChain->SetBranchAddress("event", &event, &b_event);

after the reset of the first TChain. The problem of the elist1==elist2 was a minor one (actually in my original macro the 2 TEventList were different). Finally, I can’t add the 2 files in a single chain because the 2 files contains the same events with different value for just few branches (its a systematic error study) so I need to run independently on the 2 chains to fill 2 histos with the same variable to compare the difference.

Thanks again for helping

Attilio