Clone a TTree and modify the values stored in selected branches #askROOT

ROOT tutorial #3

Download the C codes and run the following commands:

$ alias rot='root -l -b -q'
$ rot create_tree.C+
$ rot clone_tree.C+

Tested in ROOT 6.06/00

clone_tree.C

#include "TFile.h"
#include "TTree.h"
#include "TRandom.h"

void clone_tree() {
    TFile * f1 = TFile::Open("giallo.root", "READ");
    TTree * t1 = (TTree*) f1->Get("tree1");
    gRandom->SetSeed(2);

    TFile * f2 = TFile::Open("giallo_clone.root", "RECREATE");
    TTree * t2 = t1->CloneTree(0);
    
    double var1;
    double var2;
    t1->SetBranchAddress("var1", &var1);
    t1->SetBranchAddress("var2", &var2);
    
    for (int i=0; i<t1->GetEntries(); i++) {
      t1->GetEntry(i);
      
      var2 /= 2.0;  // modify this branch
      
      t2->Fill();
    }
    
    t2->Write();
    f2->Close();
    f1->Close();
}

create_tree.C

#include "TFile.h"
#include "TTree.h"
#include "TRandom.h"

void create_tree() {

    TFile * f1 = TFile::Open("giallo.root", "RECREATE");
    gRandom->SetSeed(1);

    TTree * t1 = new TTree("tree1", "tree1");
    double var1;
    double var2;
    t1->Branch("var1", &var1);
    t1->Branch("var2", &var2);
    for (int i=0; i<100; i++) {
        var1 = gRandom->Rndm();
        var2 = var1 * 2.0;
        t1->Fill();
    }

    t1->Write();
    f1->Close();
}