Sorting an Array List using Merge Sort

Question

I am having trouble with converting my sorting class to sort objects with arraylists. It is currently sorting objects, but I am having trouble converting it to sorting Arraylists. Here is the code:

package Merge_Sort_Objects_ArrayList;
import java.util.ArrayList;
public class mergesort {

    /**
     * Merges two sorted portion of items array
     * pre: items[start.mid] is sorted.  items[mid+1.end] sorted.  start <= mid <= end
     * post: items[start.end] is sorted
     */

    private static void merge(ArrayList <Comparable> items, int start, int mid, int end){
            Comparable temp;
            int pos1 = start;
            int pos2 = mid + 1;
            int spot = start;
            ArrayList <Comparable> objectSort = items;

            while (!(pos1 > mid && pos2 > end)){
                if ((pos1 > mid) || ((pos2 <= end) &&(items[pos2].getRadius() < items[pos1].getRadius()))){
                    temp[spot] = items[pos2];
                    pos2 +=1;
                }else{
                    temp[spot] = items[pos1];
                    pos1 += 1;
                }
                spot += 1;
            }
            /* copy values from temp back to items */

            for (int i = start;  i <= end; i++){
                items[i] = temp[i];
            }
    }

    /**
     * mergesort items[start..end]
     * pre: start > 0, end > 0
     * post: items[start..end] is sorted low to high
     */
    public static void mergesort(ArrayList <Comparable> items, int start, int end){
        if (start < end){
            int mid = (start + end) / 2;
            mergesort(items, start, mid);
            mergesort(items, mid + 1, end);
            merge(items, start, mid, end);
        }
    }
}

now I have started to convert it, however I am stuck on this section right here:

  while (!(pos1 > mid && pos2 > end)){
            if ((pos1 > mid) || ((pos2 <= end) &&(items[pos2].getRadius() < items[pos1].getRadius()))){
                temp[spot] = items[pos2];
                pos2 +=1;
            }else{
                temp[spot] = items[pos1];
                pos1 += 1;
            }
            spot += 1;
        }
        /* copy values from temp back to items */

        for (int i = start;  i <= end; i++){
            items[i] = temp[i];
        }

Thank you in advance!

Clarify what you mean by "stuck." What trouble are you running into? What have you tried? It seems pretty straightforward to me. — Mark Peters
– Mark Peters, Commented May 31, 2012 at 3:27
What I am stuck on is converting that while statement into a while statement that uses an array list with no errors. The problem that I am having is I am unsure of how to start converting it. — Spartan
– Spartan, Commented May 31, 2012 at 3:29
Oh, so you have errors? What are the errors? And that would imply you've given it some effort, right? So why don't you show what you've tried for the while loop. Both of these things are pretty obviously relevant information. — Mark Peters
– Mark Peters, Commented May 31, 2012 at 3:30
Good point, sorry about that. The errors I am receiving are basically telling me that I need an array, but java comparable was found instead. However, I am trying to get the sorter read from an arraylist, not an array. Does that make any sense? I am asking how to do it. I really don't know how. I am just asking for some advice on how to tackle it since I have not had to do this before. — Spartan
– Spartan, Commented May 31, 2012 at 3:32
Ah, if you haven't used a List at all before, here's a tutorial that might be of some help: docs.oracle.com/javase/tutorial/collections/interfaces/…. The first step is learning the difference between a list and an array. — Mark Peters
– Mark Peters, Commented May 31, 2012 at 3:39

QuantumMechanic · Accepted Answer · 2012-05-31 03:36:39Z

1

Use the fact that

Foo[] array = ......;
Foo rhs = .....;
Foo lhs;
array[i] = rhs;
lhs = array[j];

is analogous to:

ArrayList<Foo> list = .....;
Foo rhs = ......;
Foo lhs;
list.set(i, rhs);
lhs = list.get(i);

answered May 31, 2012 at 3:36

QuantumMechanic

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1 Comment

Spartan Over a year ago

Thank you Quantum. Had some trouble at first reading the "foo" and such, but at the end it made perfect sense! Thanks!

Sumit · Accepted Answer · 2012-05-31 13:27:23Z

0

In case it is not for practice, you might look at Collections sort method for sorting the ArrayList.

answered May 31, 2012 at 13:27

Sumit

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