add example of custom c++ function

Author: N-Coder

docs/examples/cpp-interaction.ipynb

@@ -235,6 +235,36 @@
   {
    "cell_type": "code",
    "execution_count": 5,
+   "id": "sharing-sugar",
+   "metadata": {
+    "collapsed": false,
+    "execution": {
+     "iopub.execute_input": "2021-09-14T13:18:50.317866Z",
+     "iopub.status.busy": "2021-09-14T13:18:50.317492Z",
+     "iopub.status.idle": "2021-09-14T13:18:50.319354Z",
+     "shell.execute_reply": "2021-09-14T13:18:50.318988Z"
+    },
+    "pycharm": {
+     "name": "#%%\n"
+    }
+   },
+   "outputs": [],
+   "source": [
+    "%%cppdef\n",
+    "\n",
+    "// implemented in C++ for efficiency\n",
+    "int avg_width(const ogdf::GraphAttributes &GA) {\n",
+    "    int sum = 0;\n",
+    "    for (auto n : GA.constGraph().nodes) {\n",
+    "        sum += GA.width(n);\n",
+    "    }\n",
+    "    return sum / GA.constGraph().numberOfNodes();\n",
+    "}"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
    "id": "brutal-egyptian",
    "metadata": {
     "execution": {
@@ -249,7 +279,8 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "The node widths are 20 20 20 20 40 20 20 20 20 20 20 20 20 20 20 20\n"
+      "The node widths are 20 20 20 20 40 20 20 20 20 20 20 20 20 20 20 20\n",
+      "The average width is 22\n"
      ]
     },
     {
@@ -273,13 +304,14 @@
    ],
    "source": [
     "print(\"The node widths are\", GA.width())\n",
+    "print(\"The average width is\", cpp.avg_width(GA)) # call your own C++ functions from python\n",
     "\n",
     "dict(zip(G.nodes, GA.width()))"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 6,
+   "execution_count": 7,
    "id": "covered-effect",
    "metadata": {
     "execution": {
@@ -411,7 +443,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 7,
+   "execution_count": 8,
    "id": "wrapped-mexican",
    "metadata": {
     "execution": {