// Named macro to save filled objects to file
// Load in ROOT:           ".L make_tree.C"
// Execute after loading:  "make_tree()"
void make_tree(){
  //Input file
  TFile *ifile = new TFile("neppsr_basictutorial2.root");
  
  //Tree in input file
  TTree *itree = (TTree *)ifile->Get("tree1");

  //Create output file. If it already exists, recreate it.
  TFile *ofile = new TFile("outputfile.root","RECREATE");

  // Tell ROOT the addresses of some of the variables that 
  // exist in the input tree so that we may access them event-by-event
  Int_t event;
  Float_t ebeam, px,py,pz;
  itree->SetBranchAddress("ebeam", &ebeam);
  itree->SetBranchAddress("px", &px);
  itree->SetBranchAddress("py", &py);
  itree->SetBranchAddress("pz", &pz);

  // Define a histogram with 100 bins in the range 100.0 to 150.0
  TH1F *hmom = new TH1F("hmom", "Momentum magnitude",100,100.0,150.0);

  // Create a tree with 2 branches (variables)
  TTree *newtree = new TTree("newtree","New example tree");
  Float_t magmom, sqrt_ebeam;
  newtree->Branch("magmom",&magmom,"Momentum magnitude/F");
  newtree->Branch("sqrt_ebeam", &sqrt_ebeam, "Square root of ebeam/F");

  // Find out how many events are in the input tree
  int nentries = itree->GetEntries();
  
  //Loop over entries in the input tree
  for (Int_t i = 0; i < nentries; i++) {
    // Get 1 entry from the tree
    itree->GetEntry(i);

    // Perform calculations using the values of the variables in the entry
    // we got from the tree
    magmom = sqrt(px*px + py*py + pz*pz);
    sqrt_ebeam = sqrt(ebeam);

    // Fill the histogram with one of the results of above calculations
    hmom->Fill(magmom);

    // Fill the new tree with results of both calculations
    newtree->Fill();
  }

  // Make sure ROOT knows we want to write to the output file
  ofile->cd();
  
  // Write the histogram and tree to the file
  hmom->Write();
  newtree->Write();

  // Close the output file
  ofile->Close();
}