copytree.C

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/// \file
/// \ingroup tutorial_tree
/// \notebook -nodraw
/// Copy a subset of a Tree to a new Tree
///
/// The input file has been generated by the program in `$ROOTSYS/test/Event`
/// with `Event 1000 1 1 1`.
///
/// \macro_code
///
/// \author Rene Brun

// Load the library at macro parsing time: we need this to use its content in the code
R__LOAD_LIBRARY($ROOTSYS/test/libEvent.so)

void copytree()
{

   TString dir = "$ROOTSYS/test/Event.root";
   gSystem->ExpandPathName(dir);
   const auto filename = gSystem->AccessPathName(dir) ? "./Event.root" : "$ROOTSYS/test/Event.root";

   TFile oldfile(filename);
   TTree *oldtree;
   oldfile.GetObject("T", oldtree);

   // Deactivate all branches
   oldtree->SetBranchStatus("*", 0);

   // Activate only four of them
   for (auto activeBranchName : {"event", "fNtrack", "fNseg", "fH"})
      oldtree->SetBranchStatus(activeBranchName, 1);

   // Create a new file + a clone of old tree in new file
   TFile newfile("small.root", "recreate");
   auto newtree = oldtree->CloneTree();

   newtree->Print();
   newfile.Write();
}