Update robin_neumann_dirichlet.ipynb (#40)

Author: jorgensd

chapter3/robin_neumann_dirichlet.ipynb

@@ -179,7 +179,10 @@
     "    facets = dolfinx.mesh.locate_entities(mesh, fdim, locator)\n",
     "    facet_indices.append(facets)\n",
     "    facet_markers.append(np.full(len(facets), marker))\n",
-    "facet_tag = dolfinx.MeshTags(mesh, fdim, np.array(np.hstack(facet_indices),dtype=np.int32), np.array(np.hstack(facet_markers),dtype=np.int32))"
+    "facet_indices = np.array(np.hstack(facet_indices), dtype=np.int32)\n",
+    "facet_markers = np.array(np.hstack(facet_markers), dtype=np.int32)\n",
+    "sorted_facets = np.argsort(facet_indices)\n",
+    "facet_tag = dolfinx.MeshTags(mesh, fdim, facet_indices[sorted_facets], facet_markers[sorted_facets])"
    ]
   },
   {
@@ -411,4 +414,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}