Sort two arrayLists concurrently

Question

Say I have two ArrayLists:

name: [Four, Three, One, Two]
num:  [4, 3, 1, 2]

If I do: Arrays.sort(num), then I have:

name: [         


        

Answer 1

If you don't have repeated elements, then you could just use a sorted Map like a TreeMap instead:

int[] num = {4, 3, 1, 2};
String[] name = {"Four", "Three", "One", "Two"};
TreeMap<Integer,String> sortedMap = new TreeMap<Integer,String>();
for (int i=0; i<num.length; i++) sortedMap.put(num[i], name[i]);
// Resulting sortedMap: {1=One, 2=Two, 3=Three, 4=Four}

If you do have repeated elements then this won't work because the keys of the map must be unique.

Answer 2

You should somehow associate name and num fields into one class and then have a list of instances of that specific class. In this class, provide a compareTo() method which checks on the numerical values. If you sort the instances, then the name fields will be in the order you desire as well.

class Entity implements Comparable<Entity> {
    String name;
    int num;
    Entity(String name, int num) {
        this.name = name;
        this.num = num;
    }
    @Override
    public int compareTo(Entity o) {
        if (this.num > o.num)
            return 1;
        else if (this.num < o.num)
            return -1;
        return 0;
    }
}

Test code could be like this:

public static void main(String[] args) {
    List<Entity> entities = new ArrayList<Entity>();
    entities.add(new Entity("One", 1));
    entities.add(new Entity("Two", 2));
    entities.add(new Entity("Three", 3));
    entities.add(new Entity("Four", 4));
    Collections.sort(entities);

    for (Entity entity : entities)
        System.out.print(entity.num + " => " + entity.name + " ");
}

Output:

1 => One 2 => Two 3 => Three 4 => Four

Answer 3

Instead of sorting the actual arrays you can have an array with just indices

a[i] = i for i = 0..n

and you can sort this array based on your numeruc array with a custom comparator. e.g.

bool compare( int a, int b ) { return num[a] < num[b]; }

Thus you have both arrays sorted by using these indices.

Answer 4

In some cases it doesn't make much sense to create a new class just to do multiple sorts based off a given list. I've created a function that does this, but I've posted the code in a another SO post so I wont repeat it. Below is an example of how to use it.

---

Usage

Here is an example of how you can use the function to sort multiple lists of arbitrary types:

// The key can be any type that implements Comparable, Dupes are allowed
List<Integer> key = Arrays.asList(4, 3, 1, 2, 1);

// List Types do not need to be the same
List<String> list1 = Arrays.asList("Four", "Three", "One", "Two", "One");
List<Character> list2 = Arrays.asList('d', 'c', 'a', 'b', 'a');

// Sorts key, list1, list2 using key as the sorting key.
keySort(key, key, list1, list2);

Output:

key:   [1, 1, 2, 3, 4]
list1: [One, One, Two, Three, Four]
list2: [a, a, b, c, d]