method const vs non-const

graph_lenght.cpp

  friend void get_edge_value(const Graph &g, int node_x, int node_y){
    if (g.mapping[node_x][node_y]==1) {
      list<Edge>::iterator it;
      for (it = g.list_of_edges.begin(); it != g.list_of_edges.end(); it++) {
        if ( (it->begin == node_y && it->end ==  node_x) || (it->begin == node_x && it->end ==  node_y)){
          cout << it->distance;
        }
      }
    }
  }