RE: [ROOT] Can't strip TChain

From: Philippe Canal (pcanal@fnal.gov)
Date: Thu Nov 18 2004 - 23:57:45 MET 

Hi Salvatore,

Which version of ROOT are using?

The problem is that you seemingly are setting the addresses of the
underlying original tree but not the addresses of the cloned tree:

    if ( chain->GetTreeNumber() != prevTreeNum ) {
      SetTheAddresses(chain->GetTree());
      prevTreeNum = chain->GetTreeNumber();
    }

In recent version of ROOT, it is sufficient to set the address of the
original chain (using SetBranchAddress).

Cheers,
Philippe.

-----Original Message-----
From: owner-roottalk@pcroot.cern.ch [mailto:owner-roottalk@pcroot.cern.ch]
On Behalf Of Salvatore Rappoccio
Sent: Thursday, November 18, 2004 2:34 PM
To: roottalk@pcroot.cern.ch
Subject: [ROOT] Can't strip TChain

Hi,

I am trying to read a subset of the events in my TChain object to another
file, given some selection criteria.

I can write the objects out without complaint, but when I try to read back
the events that I wrote from the second file of the TChain, I get nonsense
values.

Any ideas?

My code is:

{
  std::cout << "Getting trees" << std::endl;

  TChain * chain = GetChain();
  TTree * tskim = 0;
  int nskim = 0;
  int counter = 0;
  int prevTreeNum = -1;
  int prevRunNum = -1;
  // Load the tree and set the addresses for the second time
  chain->LoadTree(0);
  chain->GetEntry(0);
  
  if ( stripTree ) {
    std::cout << "Setting up strip tree" << std::endl;
    histf->cd();
    tskim = (TTree*)chain->GetTree()->CloneTree(0);
  }


  prevTreeNum=-1;

  //Loop over the events
  for(int ev=firstEvent; ev<nEvents; ev++) { 
    counter++;
    int lflag = -1;
    lflag = chain->LoadTree(ev);

    // If this tree hasn't already been set, set the branch addresses
    if ( chain->GetTreeNumber() != prevTreeNum ) {
      SetTheAddresses(chain->GetTree());
      prevTreeNum = chain->GetTreeNumber();
    }

    if ( stripTree )
      lflag = chain->GetEntry(ev,1);
    else
      lflag = chain->GetEntry(ev);

    if ( lflag < 0 ) {
      cout << "Error in loading trees" << endl;
      return;
    }
    
    // Decide to strip the event or not
    bool stripEvent = false;

    
    // If this event passes simple cuts, process the
    // event, returning strip status
    if ( PassCuts( ev, lflag ) ) {
      stripEvent = Process( ev, lflag );
      // If desired, strip this event and write to file
      if ( stripTree ) {
	tskim->Fill();
	nskim++;
	if ( nskim % 10000 == 0 ) {
	cout << "Writing to strip tree : nskim = " << nskim << endl;
	tskim->AutoSave();
	}
      }
    }
  }

  // Write stuff to output file
  histf->cd();

  // Strip tree
  if ( stripTree ) {
    cout << "Writing to strip tree : nskim = " << nskim << endl;
    tskim->AutoSave();
  }
  histf -> Close();

}


Thanks,
Sal Rappoccio

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