Question

Okay, I know this question doesn’t show research effort, but I’ve been going through this code so many times and I couldn’t figure out was I was doing wrong. I know there are many Mergesort implementation examples but I wanted to do it my way. Any help is appreciated, thanks.

import java.util.Scanner;
public class MergeSort
{
    public static int[] mergeSort(int[] arr)
    {
        if (arr.length > 1)
        {
            int[] arr1 = splitLeft(arr);
            int[] arr2 = splitRight(arr);
            arr1 = mergeSort(arr1);
            arr2 = mergeSort(arr2);
            return merge(arr1, arr2);
        }
        else
            return arr;
    }

    public static int[] splitLeft(int[] arr)
    {
        int middle = arr.length / 2;
        int[] newarr = new int[middle];
        for (int i = 0; i < middle; i++)
            newarr[i] = arr[i];
        return newarr;
    }

    public static int[] splitRight(int[] arr)
    {
        int middle = arr.length / 2;
        int[] newarr = new int[arr.length - middle];
        for (int i = 0; i + middle < arr.length; i++)
            newarr[i] = arr[i + middle];
        return newarr;
    }

    public static int[] merge(int[] arr1, int[] arr2)
    {       
        int[] sorted = new int[arr1.length+arr2.length];

        int i1 = 0;
        int i2 = 0;
        int i = 0;

        while (i1 < arr1.length && i2 < arr2.length)
        {
            if (arr1[i1] < arr2[i2])
            {
                sorted[i] = arr1[i1];
                i1++;
            }

            else
            {
                sorted[i] = arr2[i2];
                i2++;
            }
        i++;
        }

        while (i1 < arr1.length) 
        {
            sorted[i] = arr1[i1];
            i1++;
            i++;
        }

        while (i2 < arr2.length) 
        {
            sorted[i] = arr1[i2];
            i2++;
            i++;
        }
        return sorted;
    }

    public static int getNum(int x)
    {
        int num = (int)(Math.random()*x + 1);
        return num;
    }

    public static void printArr(int[] arr)
    {
        System.out.println();
        for (int i = 0; i < arr.length; i++)
            System.out.println(arr[i]);
    }

    static Scanner reader = new Scanner(System.in);
    public static void main(String [ ] args)
    {
        int i;

        System.out.println("Type the length of the array");
        int n = reader.nextInt();

        System.out.println("Type the range of the random numbers generator");
        int range = reader.nextInt();

        int[]arr = new int[n];

        for (i = 0; i < n; i++)
            arr[i] = getNum(range);

        printArr(arr);

        int[] sorted = new int[n];
        sorted = mergeSort(arr);

        printArr(sorted);
    }
}

Answer 1

I think the problem is in your splitRight function. Consider this code:

for (int i = middle; i < arr.length; i++)
    newarr[i] = arr[i];

This tries to copy the ith element from arr to the ith position of newarr, but this is incorrect. For example, if the array arr has ten elements, you want to copy element 5 of arr to position 0 of newArr, element 6 of arr to position 1 of newarr, etc.

To fix this, consider trying something like this:

for (int i = 0; i + middle < arr.length; i++)
    newarr[i] = arr[i + middle];

Hope this helps!