perChiara.C

#include <TTree.h>
#include <RooRealVar.h>
#include <RooArgSet.h>
#include <RooArgList.h>>

#include <TList.h>
#include <TObject.h>
#include <TKey.h>

TTree *dataset2tree(RooDataSet *dataset){

  const RooArgSet *args = dataset->get();
  RooArgList argList(*args);

  Double_t variables[50];
  Long64_t nEntries= dataset->numEntries();
  //nEntries=1;
  TTree *tree = new TTree("tree","tree");
  tree->SetDirectory(0);
  TIterator *it1=NULL; 
  it1 = argList.createIterator();
  int index1=0;
  for(RooRealVar *var = (RooRealVar *) it1->Next(); var!=NULL;
      var = (RooRealVar *) it1->Next(),index1++){
    TString name(var->GetName());
    name.ReplaceAll("-","_");
    tree->Branch(name, &(variables[index1]), name+"/D");
  }

  Double_t newweight;
  tree->Branch("newweight", &newweight, "newweight/D");
  //  tree->Print();

  for(Long64_t jentry=0; jentry<nEntries; jentry++){
    
    TIterator *it=NULL; 
    RooArgList argList1(*(dataset->get(jentry)));
    it = argList1.createIterator();
    //(dataset->get(jentry))->Print();
    int index=0;
    for(RooRealVar *var = (RooRealVar *) it->Next(); var!=NULL;
	var = (RooRealVar *) it->Next(), index++){
      variables[index]=var->getVal();
      //var->Print();
      if(var->GetName() == "evweight") newweight = evweight*2;
    }
   
    delete it;
    tree->Fill();
  }
  tree->ResetBranchAddresses();
  //  tree->Print();
  return tree;
}