Java - Sorting Grouped by Ties

Question

I'm writing a Java program in which I want to sort a set of items and get the N-highest elements of the set. The thing is, though, that I want the elements to be returned grouped by their rank -- so if I want the 3 highest elements, but there is a tie between two elements for third place, then the third result is a collection that contains the two tied elements.

I know I could write this myself, but I'm wondering if it's already been implemented somewhere else. Does anybody know of anything like this?

score 0 · Answer 1 · answered May 22 '11 at 12:27

0

Sounds like the Google Collection's MultiMap might be what you're after.

Use the "rank" as your key when inserting your elements. Then sort the keys.

answered May 22 '11 at 12:27

Jim

1,040
1
10
16

That won't work, because the "rank" of each object is unknown before the sorting is done. It's determined by the sorting, not the other way around. – Masterofpsi May 23 '11 at 01:17
Ah I see. Nothing I know of then. I'd do a custom implementation that sticks them in a list, sorts them, then walks though creating the required data structure - perhaps using the MultiMap. – Jim May 23 '11 at 20:55

score 0 · Accepted Answer · answered May 29 '11 at 23:12

This is what I ended up going with:

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import com.google.common.collect.Ordering;

public final class Sorting {
    private Sorting() {}

    public static <T extends Comparable<? super T>> List<List<T>> rank(
            Iterable<T> iterable, int nRanks) {
        if (nRanks < 0) {
            throw new IllegalArgumentException(Integer.toString(nRanks));
        }
        if (nRanks == 0) {
            return new ArrayList<List<T>>();
        }

        Iterator<T> iter = Ordering.natural().sortedCopy(iterable).iterator();
        List<List<T>> ret = new ArrayList<List<T>>();
        if (iter.hasNext()) {
            T prev = iter.next();
            List<T> group = new ArrayList<T>();
            group.add(prev);
            ret.add(group);

            int rank = 1;
            while (iter.hasNext()) {
                T next = iter.next();
                if (prev.compareTo(next) > 0) {
                    rank++;
                    if (rank > nRanks) {
                        break;
                    }

                    group = new ArrayList<T>();
                    ret.add(group);
                }
                group.add(next);
                prev = next;
            }
        }

        return ret;
    }
}

Java - Sorting Grouped by Ties

2 Answers2