How to select and process events from a TTree that pass a graphical cut

ROOT
1

Dear root users, I’m facing the following problem. I have a TTree containing events and I would like to process only the events that pass a certain cut and then put these events in to a different TTree. So far, following this topic " root.cern.ch/root/roottalk/roottalk03/4281.html " , I was able to wrote this code:


#include "TFile.h"
#include "TTree.h"
#include "TH2.h"
#include "TEventList.h"
#include "TCutG.h"

#include <iostream>
#include <iostream>
#include <cstdlib>
#include <cmath>

using namespace std;

int CutSelector(){ 
 TFile *rf = new TFile("NNProva.root");
//  TFile *wf = new TFile("NNProvaCut.root","RECREATE");
 
 TTree *readTree = (TTree*)rf->Get("outTree");
/* TTree *writeTree = new TTree("writeTree","Cloned Tree");*/ 
 
  TCutG *cutg = new TCutG("gamma_cut",5);
  cutg->SetVarX("TOF");
  cutg->SetVarY("Slow Total Ratio");
  cutg->SetTitle("Graph");
  cutg->SetPoint(0,10.9311,0.437895);
  cutg->SetPoint(1,10.3443,0.365263);
  cutg->SetPoint(2,11.029,0.308421);
  cutg->SetPoint(3,13.0829,0.374737);
  cutg->SetPoint(4,10.9311,0.437895);
  
  readTree->Draw(">>elist","gamma_cut");
  TEventList *elist = (TEventList*)gDirectory->Get("elist");
  
  cout<<elist->GetN()<<endl; //It writes the number of events that passed the cut
  
  readTree->SetEventList(elist);
  
//   wf->Write();
//   wf->Close();  
}

it creates the TEventList “elist” of the events that pass the TCutG “gamma_cut”. Now, how can I access to the events of the TTree that are listed in “elist” so that I can process them (e.g. add another branch with a simple 1/0 tag) and put in another TTree? I’ve read the documentation for the TEventList, TEntryList, TTree classes, but I didn’t found the right method, so clearly I’m missing something.

Also, is this method convenient or should I try to do this task in a different, easier way?

Cheers,
Bart

2

Hi,

In the documentation of TEntryList (which should be prefered over TEventList), you can find:[code] void loopChain() {
TFile *fe = TFile::Open(“myelist.root”);
TEntryList myelist = (TEntryList)fe->Get(“myelist”);
TChain *ch = new TChain(“ntuple”);
ch->Add(“hsimple.root”);
ch->Add(“hsimple2.root”);
Long64_t listEntries = myelist->GetN();
Long64_t chainEntries = ch->GetEntries();
Int_t treenum = 0;
ch->SetEntryList(myelist);

    for (entry=start;entry < end;entry++) {
       entryNumber = treechain->GetEntryNumber(entry);
       if (entryNumber < 0) break;
       localEntry = fTree->LoadTree(entryNumber);
       if (localEntry < 0) break;
       ....
       then either call branch->GetEntry(localEntry);
       or  entryNumber->GetEntry(entryNumber);
       In the later case the LoadTree is then somewhat redudant.
       ...
    }
 }

[/code]

Cheers,
Philippe.