Java and Python MergeSort

Question

I am new to Java and I have been trying to do MergeSort, in a similar way I did it in Python. This is how I do it in Python:

def merge_sort(a):
    length = len(a)
    if length > 1:
        left = a[: len(a) // 2]
        right = a[len(a) // 2 :]

        merge_sort(left)
        merge_sort(right)

        merge(a, left, right)


def merge(a, left, right):
    length = len(a)
    left_half = len(left)
    right_half = len(right)
    i1, i2 = 0, 0
    for i in range(length):
        if i2 >= right_half or (i1 < left_half and left[i1] < right[i2]):
            a[i] = left[i1]
            i1 += 1
        else:
            a[i] = right[i2]
            i2 += 1

This is my Java code and it returns index error.

import java.util.Arrays;

public class Merge_sort1 {

    public static void main(String[] args) {

        int[] a = {5, 3, 0, 1, 9, 6, 7, 4, 8, 2};
        System.out.println("Array before sorting");
        System.out.println(Arrays.toString(a));

        mergeSort(a);

        System.out.println("Array after sorting");
        System.out.println(Arrays.toString(a));
    }

    public static void mergeSort(int[] a) {
        if (a.length > 1) {
            int mid = a.length / 2;
            int[] left = Arrays.copyOfRange(a, 0, mid - 1);
            int[] right = Arrays.copyOfRange(a, mid, a.length - 1);

            mergeSort(left);
            mergeSort(right);

            merge(a, left, right);
        }   
    }

    public static void merge(int[] a, int[] l, int[] r) {
        int i1 = 0;
        int i2 = 0;
        for(int i = 0; i < a.length; i++) {
            if(i2 >= r.length || (i1 < l.length && l[i1] < r[i2])) {
                a[i] = l[i1];
                i1++;
            a[i] = r[i2];
            i2++;
                }
            }
        }
    }

I would really appreciate it if anyone can explain to me what I'm doing wrong and the best way to fix it. Thank you

Answer 1

The copyOfRange call is erroneous; note that the second argument is exclusive rather than inclusive. Which means the second argument of the method minus 1 is the last element to be copied. So when calling copyOfRange(a, b, c) for example, the array a is copied from index b to c - 1, not c.

Also, the if statement in merge should be an if-else one (you forgot the else part).

So here is the code fixed, with proper output:

import java.util.Arrays;

public class Main {

    public static void main(final String[] args) {

        int[] a = {5, 3, 0, 1, 9, 6, 7, 4, 8, 2};
        System.out.println("Array before sorting");
        System.out.println(Arrays.toString(a));

        mergeSort(a);

        System.out.println("Array after sorting");
        System.out.println(Arrays.toString(a));
    }

    public static void mergeSort(int[] a) {
        if (a.length > 1) {
            int mid = a.length / 2;
            int[] left = Arrays.copyOfRange(a, 0, mid);
            int[] right = Arrays.copyOfRange(a, mid, a.length);

            mergeSort(left);
            mergeSort(right);

            merge(a, left, right);
        }   
    }

    public static void merge(int[] a, int[] l, int[] r) {
        int i1 = 0;
        int i2 = 0;
        for(int i = 0; i < a.length; i++) {
            if(i2 >= r.length || (i1 < l.length && l[i1] < r[i2])) {
                a[i] = l[i1];
                i1++;
            }
            else {
                a[i] = r[i2];
                i2++;
            }
        }
    }
}