RE: [ROOT] how to modify data in a tree ?

From: Philippe Canal (pcanal@fnal.gov)
Date: Mon May 24 2004 - 18:33:11 MEST 

Hi,

TTree are designed for write-once, read many times.  Modifying values
inside a TTree is not supported.

Instead we recommend to either create a new TTree containing the modify
values and to be used as a 'Friend' of the old tree.  Alternatively,
you can use CloneTree to duplicate the TTree into a new file and modify
the values during the duplication.

Cheers,
Philippe.

-----Original Message-----
From: owner-roottalk@pcroot.cern.ch
[mailto:owner-roottalk@pcroot.cern.ch]On Behalf Of Krzysztof Syryczynski
Sent: Monday, May 24, 2004 7:37 AM
To: roottalk@pcroot.cern.ch
Subject: [ROOT] how to modify data in a tree ?


root 4.00/03   CINT 5.15.128   Red Hat 7.2  

Hello

I'd like to modify values written in one of the branches.
Unfortunately I cannot find a tree method like "SetEntry(i)".
Fill() adds new data instead of changing existing.
How to correct the following lines:

  TFile *fH = new TFile("mlpHiggs.root","update");
  TTree *tH = (TTree *)fH->Get("sig_filtered");
  Float_t nch;
  tH->SetBranchAddress("nch",    &nch);
  tH->SetBranchStatus("*",0);
  tH->SetBranchStatus("nch",1);
  const int ent=tH->GetEntriesFast();
  for (Int_t i=0;i<ent;i++) { tH->GetEntry(i); nch=7; tH->Fill(); }
  tH->StartViewer();
  tv__tree->Draw("nch","","", 12345678, 0);
  fH->Write();

Cheers
Krzysztof Syryczynski

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