Getting object list in Knapsack

Question

I've a problem similar to the 0-1 Knapsack.

The modification I'm trying to do is to get the list of selected object instead of cost. I tried with the following code:

public static int fillPackage(double weight,ArrayList<Item> item, int n) {
    //base case
    if(n == 0 || weight == 0)
        return 0;

    if(item.get(n - 1).getWeight() > weight)
        return fillPackage(weight, item, n - 1);    

    else {
        int include_cost = item.get(n - 1).getCost() + fillPackage((weight - item.get(n - 1).getWeight()), item, n - 1);
        int exclude_cost = fillPackage(weight, item, n - 1);
        if(include_cost > exclude_cost) {
            my_pack.add(item.get(n - 1));
            return include_cost;
        }
        else {
            return exclude_cost;
        }
    }   
}

Here my_pack is the ArrayList that is supposed to store the selected object. But, it is storing other objects too. Also, I can't use DP table method because my parameters are float.

Where should my_pack.add() line be put ?

Answer 1

Q:

where should my_pack.add() line be put ?

A:

Put my_pack.add() to nowhere to get right code.

---

Let me tell you how to do it. You know the way to solve 0-1 Knapsack is:

best_cost = max(included_cost, excluded_cost)

So similarly, you should think your problem in this way:

if included_cost > excluded_cost
    best_choice = included_best_choice + included_item
else
    best_choice = excluded_best_choice

I modified your code and it can solve your problem correctly(you can edit and run my test code). See my code below.

If you have any question, feel free to leave a comment and I will reply as soon as possible.

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

public class Package {

    static List<Item> my_pack;

    public static int fillPackage(double weight, ArrayList<Item> item, List<Item> optimalChoice, int n){
        //base case
        if(n == 0 || weight == 0)
            return 0;

        if(item.get(n-1).getWeight() > weight) {
            List<Item> subOptimalChoice = new ArrayList<>();
            int optimalCost =fillPackage(weight, item, subOptimalChoice, n-1);
            optimalChoice.addAll(subOptimalChoice);
            return optimalCost;
        }
        else{
            List<Item> includeOptimalChoice = new ArrayList<>();
            List<Item> excludeOptimalChoice = new ArrayList<>();
            int include_cost = item.get(n-1).getCost() + fillPackage((weight-item.get(n-1).getWeight()), item, includeOptimalChoice, n-1);
            int exclude_cost = fillPackage(weight, item, excludeOptimalChoice, n-1);
            if(include_cost > exclude_cost){
                optimalChoice.addAll(includeOptimalChoice);
                optimalChoice.add(item.get(n - 1));
                return include_cost;
            }
            else{
                optimalChoice.addAll(excludeOptimalChoice);
                return exclude_cost;
            }
        }
    }

    public static void main(String args[]) {
        ArrayList<Item> itemList = new ArrayList<>();
        itemList.add(new Item(2, 1));
        itemList.add(new Item(5, 6));
        itemList.add(new Item(3, 2));
        itemList.add(new Item(4, 4));
        itemList.add(new Item(7, 7));

        printOptimalChoice(itemList, 9);
        printOptimalChoice(itemList, 10);
        printOptimalChoice(itemList, 11);
    }

    private static void printOptimalChoice(ArrayList<Item> itemList, double weight) {
        my_pack = new ArrayList<>();
        fillPackage(weight, itemList, my_pack, itemList.size());
        System.out.println("Best choice for weight: " + weight);
        for(int i = 0; i < my_pack.size(); i++) {
            System.out.println(my_pack.get(i));
        }
    }
}

My test output:

Best choice for weight: 9.0
Item{weight=5.0, cost=6}
Item{weight=4.0, cost=4}
Best choice for weight: 10.0
Item{weight=5.0, cost=6}
Item{weight=4.0, cost=4}
Best choice for weight: 11.0
Item{weight=2.0, cost=1}
Item{weight=5.0, cost=6}
Item{weight=4.0, cost=4}

code of Item.class:

class Item {
    private double weight;
    private int cost;

    public Item(double weight, int cost) {
        this.weight = weight;
        this.cost = cost;
    }

    @Override
    public String toString() {
        return "Item{" +
                "weight=" + weight +
                ", cost=" + cost +
                '}';
    }

    public double getWeight() {
        return weight;
    }

    public void setWeight(double weight) {
        this.weight = weight;
    }

    public int getCost() {
        return cost;
    }

    public void setCost(int cost) {
        this.cost = cost;
    }
}