{
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   "source": [
    "# vo006_IndexManipulation\n",
    "In this tutorial we demonstrate RVec helpers for index manipulation.\n",
    "\n",
    "\n",
    "\n",
    "\n",
    "**Author:** Stefan Wunsch  \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 08:28 PM.</small></i>"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "6640d3ec",
   "metadata": {},
   "source": [
    "We assume that we have multiple linked collections, the elements of which\n",
    "represent different objects."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "9b665a15",
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    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Muon 1 (pt, eta): 20, 1\n",
      "Muon 2 (pt, eta): 30, -2\n",
      "Muon 3 (pt, eta): 10, 0.5\n",
      "Muon 4 (pt, eta): 25, 2.5\n"
     ]
    }
   ],
   "source": [
    "ROOT::RVecF muon_pt = {20.0, 30.0, 10.0, 25.0};\n",
    "ROOT::RVecF muon_eta = {1.0, -2.0, 0.5, 2.5};\n",
    "\n",
    "for (size_t i = 0; i < muon_pt.size(); i++) {\n",
    "   std::cout << \"Muon \" << i + 1 << \" (pt, eta): \" << muon_pt[i] << \", \"\n",
    "             << muon_eta[i] << std::endl;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "4be77c8b",
   "metadata": {},
   "source": [
    "First, let's make a selection and write out all indices, which pass."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "fb8b746d",
   "metadata": {
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    }
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "input_line_53:2:20: error: cannot deduce 'auto' from unknown expression\n",
      " auto idx_select = Nonzero(muon_pt > 15 && abs(muon_eta) < 2.5);\n",
      "                   ^\n"
     ]
    }
   ],
   "source": [
    "auto idx_select = Nonzero(muon_pt > 15 && abs(muon_eta) < 2.5);"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "052c0743",
   "metadata": {},
   "source": [
    "Second, get indices that sort one of the collections in descending order."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "a2938e36",
   "metadata": {
    "collapsed": false,
    "execution": {
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    }
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "input_line_54:2:18: error: cannot deduce 'auto' from unknown expression\n",
      " auto idx_sort = Reverse(Argsort(muon_pt));\n",
      "                 ^\n"
     ]
    }
   ],
   "source": [
    "auto idx_sort = Reverse(Argsort(muon_pt));"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "269eb127",
   "metadata": {},
   "source": [
    "Finally, we find all indices present in both collections of indices retrieved\n",
    "from sorting and selecting.\n",
    "Note, that the order of the first list passed to the Intersect helper is\n",
    "contained."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "fc993457",
   "metadata": {
    "collapsed": false,
    "execution": {
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   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "input_line_55:2:13: error: cannot deduce 'auto' from unknown expression\n",
      " auto idx = Intersect(idx_sort, idx_select);\n",
      "            ^\n"
     ]
    }
   ],
   "source": [
    "auto idx = Intersect(idx_sort, idx_select);"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "739873af",
   "metadata": {},
   "source": [
    "Take from all lists the elements of the passing objects."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "4dbf3290",
   "metadata": {
    "collapsed": false,
    "execution": {
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    }
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "input_line_56:5:24: error: use of undeclared identifier 'idx'\n",
      "for (size_t i = 0; i < idx.size(); i++) {\n",
      "                       ^\n",
      "input_line_56:2:22: error: cannot deduce 'auto' from unknown expression\n",
      " auto good_muon_pt = Take(muon_pt, idx);\n",
      "                     ^\n"
     ]
    }
   ],
   "source": [
    "auto good_muon_pt = Take(muon_pt, idx);\n",
    "auto good_muon_eta = Take(muon_eta, idx);\n",
    "\n",
    "for (size_t i = 0; i < idx.size(); i++) {\n",
    "   std::cout << \"Selected muon \" << i + 1 << \" (pt, eta): \" << good_muon_pt[i]\n",
    "             << \", \" << good_muon_eta[i] << std::endl;\n",
    "}"
   ]
  }
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