Sorting an array and keeping original index

Question

I am trying to make a program that will sort the array but keep track of their original indexes. I don't want to change the original array so I copied it into list[][].

   public static void main(String[] args) {

    int array[] = {17, 10, 8, 13, 5, 7, 8, 30};
    int list[][] = new int[8][2];
    int temp1, temp2, index, max;

    for(int i=0;i<array.length; i++){
        list[i][0]=array[i];
        list[i][1]=i;
    }

        for(int i=0; i <array.length-1; i++){
            max = list[i][0];
            index = i;

            for(int j = i+1; j<array.length;j++){
            if(max<list[j][0]){
                max = list[j][0];
                index = j;

            }
        }
            temp1 = list[i][0];
            temp2 = list[i][1];

            list[i][0]=max;
            list[i][1] = index;

            list[index][0]=temp1;
            list[index][1]=temp2;




        }
        for(int n=0; n<list.length;n++){
                System.out.println(list[n][0] + " " + list[n][1]);

        }



}

So it should print:

30 7

17 0

13 3

10 1

8 2

8 6

7 5

5 4

But when I run it, it prints:

30 7

17 7

13 3

10 7

8 6

8 7

7 7

5 4

Any suggestions?

Why not make the second array a tuple containing both the number and the original index? — EJoshuaS - Stand with Ukraine, Feb 22 '17 at 18:56

score 10 · Answer 1 · answered Feb 22 '17 at 19:27

Why not some nice and clean OOP?

class Element implements Comparable<Element> {

    int index, value;

    Element(int index, int value){
        this.index = index;
        this.value = value;
    }

    public int compareTo(Element e) {
        return this.value - e.value;
    }
}

Usage:

int array[] = {17, 10, 8, 13, 5, 7, 8, 30};

// Init the element list
List<Element> elements = new ArrayList<Element>();
for (int i = 0; i < array.length; i++) {
    elements.add(new Element(i, array[i]));
}

// Sort and print
Collections.sort(elements);
Collections.reverse(elements); // If you want reverse order
for (Element element : elements) {
    System.out.println(element.value + " " + element.index);
}

score 3 · Answer 2 · edited May 23 '17 at 11:53

Personally, I perfer OOP - like in Oneiros' answer. Based on his answer I altered the code a tiny bit for better cohesion.

Required imports

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

Oneiros' class - altered

class Element{
    private int index;
    private int value;

    public Element(int index, int value){
        this.index = index;
        this.value = value;
    }

    public int getValue(){
        return this.value;
    }

    @Override
    public String toString(){
        return value + " " + index;
    }

    public void print(){
        System.out.println(this);
    }
}

The Comparator

class NumberComparator implements Comparator<Element>{
    @Override
    public int compare(Element a, Element b) {
        return b.getValue() - a.getValue(); // ASC: a - b or DESC: b - a
    }
}

The main difference is that the Comparator is now a class itself. This allows for higher exchangeability, in case you want to use an other Comparator.

Usage

public class Main {
    // Holds the sorted numbers and their corresponding indexes
    private static List<Element> elements = new ArrayList<>();

    public static void main(String[] args) {
        // Original array
        int[] original = {17, 10, 8, 13, 5, 7, 8, 30};

        // Add elements to list for sorting
        for (int i = 0; i < original.length; i++) {
            elements.add(new Element(i, original[i]));
        }

        // Actually sort the list
        elements.sort(new NumberComparator());

        // Print
        elements.forEach(System.out::println);
    }
}

RamPrakash · Answer 3 · 2017-02-22T19:55:54.827

Another way of implementing it.

import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class MyMath {

    public static void main(String[] args) {
        int array[] = { 17, 10, 8, 13, 5, 7, 8, 30 };

        Map<Integer, Integer> m1= new HashMap<Integer, Integer>();
        int k=0;
        for(int i:array){
            m1.put(k++, i);
        }
        m1= MyMath.sortByValues(m1);
        for (int key: m1.keySet()){
             System.out.println(m1.get(key) + " " + key);  
        }  
    }   


    public static <K extends Comparable,V extends Comparable> Map<K,V> sortByValues(Map<K,V> map){
        List<Map.Entry<K,V>> entries = new LinkedList<Map.Entry<K,V>>(map.entrySet());

        Collections.sort(entries, new Comparator<Map.Entry<K,V>>() {

            @Override
            public int compare(Entry<K, V> o1, Entry<K, V> o2) {
                return o2.getValue().compareTo(o1.getValue());
            }
        });

         Map<K,V> sortedMap = new LinkedHashMap<K,V>();

        for(Map.Entry<K,V> entry: entries){
            sortedMap.put(entry.getKey(), entry.getValue());
        }

        return sortedMap;
    }

}

Sorting an array and keeping original index

3 Answers3