Question

One famous programmer said “why anybody need DB, just give me hash table!”. I have list of grammar symbols together with their frequencies. One way it’s a map: symbol#->frequency. The other way its a [binary] relation. Problem: get top 5 symbols by frequency.

More general question. I’m aware of [binary] relation algebra slowly making inroad into CS theory. Is there java library supporting relations?

Answer 1

 List<Entry<String, Integer>> myList = new ArrayList<...>();
 for (Entry<String, Integer> e : myMap.entrySet())
       myList.add(e);

 Collections.sort(myList, new Comparator<Entry<String, Integer>>(){

    int compare(Entry a, Entry b){
       // compare b to a to get reverse order
       return new Integer(b.getValue()).compareTo(new Integer(a.getValue());
    }
 });

 List<Entry<String, Integer>> top5 = myList.sublist(0, 5);

More efficient:

 TreeSet<Entry<String, Integer>> myTree = new TreeSet<...>(
    new  Comparator<Entry<String, Integer>>(){

      int compare(Entry a, Entry b){
         // compare b to a to get reverse order
         return new Integer(b.getValue()).compareTo(new Integer(a.getValue());
      }
    });
 for (Entry<String, Integer> e : myMap.entrySet())
       myList.add(e);

 List<Entry<String, Integer>> top5 = new ArrayList<>();
 int i=0;
 for (Entry<String, Integer> e : myTree) {
     top5.add(e);
     if (i++ == 4) break;
 }