private int sort_apps (App a, App b) {

            if (a.popularity > b.popularity)
                return 1;
            else if (a.popularity == b.popularity)
                return 0;
            else
                return -1;

        }

        public SList<App> get_sorted_apps () {

            var sorted_apps = new SList<App> ();

            foreach (ArrayList<App> category in apps.values) {
                foreach (App app in category) {

                    sorted_apps.append (app);

                }
            }

            sorted_apps.sort_with_data ((CompareDataFunc<App>) sort_apps);
            return sorted_apps;

        }