Mergesort in java

Question

I am new to Java and have tried to implement mergesort in Java. However, even after running the program several times, instead of the desired sorted output, I am getting the same user given input as the output. I would be thankful if someone could help me understand this unexpected behaviour.

import java.io.*;
import java.util.Arrays;

public class MergeSort {

    public static void main(String[] args) throws IOException {
        BufferedReader R = new BufferedReader(new InputStreamReader(System.in));
        int arraySize = Integer.parseInt(R.readLine());
        int[] inputArray = new int[arraySize];
        for (int i = 0; i < arraySize; i++) {
            inputArray[i] = Integer.parseInt(R.readLine());
        }
        mergeSort(inputArray);
        
        for (int j = 0; j < inputArray.length; j++) {
            System.out.println(inputArray[j]);
        }
    }
    
    static void mergeSort(int[] A) {
        if (A.length > 1) {
            int q = A.length / 2;
            int[] leftArray = Arrays.copyOfRange(A, 0, q);
            int[] rightArray = Arrays.copyOfRange(A, q + 1, A.length);
            mergeSort(leftArray);
            mergeSort(rightArray);
            A = merge(leftArray, rightArray);
        }
    }
    
    static int[] merge(int[] l, int[] r) {
        int totElem = l.length + r.length;
        int[] a = new int[totElem];
        int i, li, ri;
        i = li = ri = 0;
        while (i < totElem) {
            if ((li < l.length) && (ri < r.length)) {
                if (l[li] < r[ri]) {
                    a[i] = l[li];
                    i++;
                    li++;
                } else {
                    a[i] = r[ri];
                    i++;
                    ri++;
                }
            } else {
                if (li >= l.length) {
                    while (ri < r.length) {
                        a[i] = r[ri];
                        i++;
                        ri++;
                    }
                }
                if (ri >= r.length) {
                    while (li < l.length) {
                        a[i] = l[li];
                        li++;
                        i++;
                    }
                }
            }
        }
        return a;
    }
}

Answer 1

When you rebind A in mergeSort():

        A = merge(leftArray,rightArray);

this has no effect in inputArray in main().

You need to return the sorted array from mergeSort() similarly to how you return it from merge().

static int[] mergeSort(int[] A) {
    ...
    return A;
}

and in main():

    int[] mergedArray = mergeSort(inputArray);

    for (int j = 0; j < mergedArray.length; j++) {
        System.out.println(mergedArray[j]);
    }

Answer 2

Here is a corrected version of your code:

import java.io.*;
import java.util.Arrays;


public class MergeSort {

    public static void main(String[] args) throws IOException{
        BufferedReader R = new BufferedReader(new InputStreamReader(System.in));
        int arraySize = Integer.parseInt(R.readLine());
        int[] inputArray = new int[arraySize];
        for (int i = 0; i < arraySize; i++) {
            inputArray[i] = Integer.parseInt(R.readLine());
        }
        mergeSort(inputArray);

        for (int j = 0; j < inputArray.length; j++) {
            System.out.println(inputArray[j]);
        }

    }

    static void mergeSort(int[] A) {
        if (A.length > 1) {
            int q = A.length/2;

//changed range of leftArray from 0-to-q to 0-to-(q-1)
            int[] leftArray = Arrays.copyOfRange(A, 0, q-1);
//changed range of rightArray from q-to-A.length to q-to-(A.length-1)
            int[] rightArray = Arrays.copyOfRange(A,q,A.length-1);

            mergeSort(leftArray);
            mergeSort(rightArray);

            merge(A,leftArray,rightArray);
        }
    }

    static void merge(int[] a, int[] l, int[] r) {
        int totElem = l.length + r.length;
        //int[] a = new int[totElem];
        int i,li,ri;
        i = li = ri = 0;
        while ( i < totElem) {
            if ((li < l.length) && (ri<r.length)) {
                if (l[li] < r[ri]) {
                    a[i] = l[li];
                    i++;
                    li++;
                }
                else {
                    a[i] = r[ri];
                    i++;
                    ri++;
                }
            }
            else {
                if (li >= l.length) {
                    while (ri < r.length) {
                        a[i] = r[ri];
                        i++;
                        ri++;
                    }
                }
                if (ri >= r.length) {
                    while (li < l.length) {
                        a[i] = l[li];
                        li++;
                        i++;
                    }
                }
            }
        }
        //return a;

    }

}

Answer 3

The problem is that java is pass by value and not pass by reference... When you are assigning to array A in the merge method you are changing a copy of the reference to A and not the reference to A itself. Therefore you need to pass A into your merge method and make a structural change to A.