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    "# tree111_copy\n",
    "Copy a subset of a Tree to a new Tree, one branch in a separate file.\n",
    "\n",
    "One branch of the new Tree is written to a separate file\n",
    "The input file has been generated by the program in `$ROOTSYS/test/Event`\n",
    "with the command `Event 1000 1 1 1`\n",
    "\n",
    "\n",
    "\n",
    "\n",
    "**Author:** Rene Brun  \n",
    "<i><small>This notebook tutorial was automatically generated with <a href= \"https://github.com/root-project/root/blob/master/documentation/doxygen/converttonotebook.py\">ROOTBOOK-izer</a> from the macro found in the ROOT repository  on Tuesday, May 19, 2026 at 08:18 PM.</small></i>"
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   "source": [
    "#ifdef ACTUAL_RUN  // -------- Second pass: dictionary already built --------\n",
    "\n",
    "#include \"./Event.h\"  // now safe to include Event, its dictionary is loaded\n",
    "\n",
    "#else  // -------- First pass: build dictionary + rerun macro --------"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "e6a29400",
   "metadata": {},
   "source": [
    " Definition of a helper function: "
   ]
  },
  {
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   "source": [
    "%%cpp -d\n",
    "void run()\n",
    "{\n",
    "   TFile oldfile(\"tree108.root\");\n",
    "   TTree *oldtree;\n",
    "   oldfile.GetObject(\"t4\", oldtree);\n",
    "\n",
    "   // Activate only four of them\n",
    "   for (auto activeBranchName : {\"event_split\", \"fNtrack\", \"fNseg\", \"fH\"}) {\n",
    "      oldtree->SetBranchStatus(activeBranchName, 1);\n",
    "   }\n",
    "\n",
    "   // Create a new file + a clone of old tree header. Do not copy events\n",
    "   TFile newfile(\"tree111.root\", \"recreate\");\n",
    "   auto newtree = oldtree->CloneTree(0);\n",
    "\n",
    "   // Divert branch fH to a separate file and copy all events\n",
    "   newtree->GetBranch(\"fH\")->SetFile(\"small_fH.root\");\n",
    "   newtree->CopyEntries(oldtree);\n",
    "\n",
    "   newtree->Print();\n",
    "   newfile.Write();\n",
    "}"
   ]
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   "source": [
    "TString tutdir = gROOT->GetTutorialDir();\n",
    "gROOT->ProcessLine(\".L \" + tutdir + \"/io/tree/Event.cxx+\");\n",
    "gROOT->ProcessLine(\"#define ACTUAL_RUN yes\");\n",
    "gROOT->ProcessLine(\"#include \\\"\" __FILE__ \"\\\"\");\n",
    "gROOT->ProcessLine(\"run()\");"
   ]
  }
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