[ROOT] filter out the chain

From: Dmitriy Bandourine (bandurin@fnal.gov)
Date: Fri Feb 01 2002 - 04:50:04 MET 

Hi rooters,

I need to filter out the events containing in the chain
of root-files using some function INSIDE the loop over events
and then save them in new tree with analogous branch structure.
The method with 
TTree *tree = chain.CloneTree(Tcut);
does not suit me because, as mentioned above, I need to use a
selection function, which return value depends on the event parameters.
 
My script looks like this:
#=========================================== 
void filter()
{
TChain *chain = new TChain("Global");
chain->Add("file1.root");
chain->Add("file2.root");
chain->Add("file3.root"); 

  cout << " Creating new root-file ..."<< endl;
  TFile *newFile = new TFile("small.root","recreate");
  cout << " Creating new tree ..."<< endl;
  TTree *tree = chain.CloneTree(0);

  if (fChain == 0) return;
  Int_t nentries = Int_t(fChain->GetEntries());

  Int_t nbytes = 0, nb = 0;
  for (Int_t jentry=0; jentry<nentries;jentry++) {
     Int_t ientry = LoadTree(jentry); //in case of a TChain, 
     nb = fChain->GetEntry(jentry);   nbytes += nb;

    if ( SelectionFunction() ) tree->Fill();
  
  }// end loop over events

  newFile->cd();
  tree->Write();
}
#==============================================

For 3.01.06 in this case I get: 
Warning in <TChain::Fill()>: may not use this method
and nothing written to "tree".
For 3.02.07 I get: 
*** Break *** segmentation violation 

How to initialize properly "tree"?
Obviously, the string:
 TTree *tree = chain.CloneTree(0);
does not help


Thanks in advance,
Dmitri

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