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

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 2

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 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.

Answer 4

The problem lies here:

A = merge(leftArray,rightArray);

Now your merge array does this:

static int[] merge(int[] l, int[] r) {
    int[] a = new int[totElem];
    // bunch of code
    return a;
}

When you started, A was a reference to inputArray. But then you reassigned it to be whatever came out of merge. Unfortunately, that doesn't touch what inputArray is in the main method. That basically says "Oh look at all the work you did... throw it away!"

You could change that with something like

static int[] mergeSort(int[] A) {
    // A = merge... // not this
    return merge... // use this
}

Then in your main method, you can do

int[] merged = mergeSort(inputArray);
for(int i : merged) System.out.println(i);

Answer 5

public class MergeSort{
public static void sort(int[] in){
    if(in.length <2) return; //do not need to sort
    int mid = in.length/2;
    int left[] = new int[mid];
    int right[] = new int[in.length-mid];
    for(int i=0; i<mid; i++){ //copy left
        left[i] = in[i];
    }
    for(int i=0; i<in.length-mid; i++){ //copy right
        right[i] = in[mid+i];
    }
    sort(left);
    sort(right);
    merge(left, right, in);
}

private static void merge(int[] a, int[] b, int[] all){
    int i=0, j=0, k=0;
    while(i<a.length && j<b.length){ //merge back
        if(a[i] < b[j]){
            all[k] = a[i];
            i++;
        }else{
            all[k] = b[j];
            j++;
        }
        k++;
    }
    while(i<a.length){ //left remaining
        all[k++] = a[i++];
    }
    while(j<b.length){ //right remaining
        all[k++] = b[j++];
    }
}

public static void main(String[] args){
    int[] a = {2,3,6,4,9,22,12,1};
    sort(a);    
    for(int j=0; j<a.length; j++){
        System.out.print(a[j] + " ");
    }   
 }
}

Answer 6

Assuming your merge function is correct:

static int[] 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);
        return merge(mergeSort(leftArray),mergeSort(rightArray));
    }
    else
        return A;
}

Since we need to return the array, we return A unmodified if it only has one element, otherwise we merge the results of sorting the left and right parts of the array.

Answer 7

public void sort(int[] randomNumbersArrray)
{
    copy = randomNumbersArrray.clone();
    mergeSort(0 , copy.length - 1);
}

private void mergeSort(int low, int high)
{
    if(low < high)
    {
        int mid = ((low + high) / 2);
        mergeSort(low, mid); //left side
        mergeSort(mid + 1, high); // right side
        merge(low, mid, high); //combine them
    }

}


private void merge(int low, int mid, int high)
{
    int temp[] = new int[high - low + 1];
    int left = low;
    int right = mid + 1;
    int index = 0;

    // compare each item for equality
    while(left <= mid && right <= high)
    {
        if(copy[left] < copy[right])
        {
            temp[index] = copy[left];
            left++;
        }else
        {
            temp[index] = copy[right];
            right++;
        }
        index++;
    }

    // if there is any remaining loop through them
    while(left <= mid || right <= high)
    {
        if( left <= mid)
        {
            temp[index] = copy[left];
            left++;
            index++;
        }else if(right <= high)
        {
                temp[index] = copy[right];
                right++;
                index++;
        }
    }
    // copy back to array
    for(int i = 0; i < temp.length; i++)
    {
        copy[low + i] = temp[i];
    }
}

Answer 8

package Sorting;

public class MergeSort {

private int[] original;
private int len;
public MergeSort(int length){

    len = length;
    original = new int[len];

    original[0]=10;
    original[1]=9;
    original[2]=8;
    original[3]=7;
    original[4]=6;
    original[5]=5;
    original[6]=4;
    original[7]=3;
    original[8]=2;
    original[9]=1;

    int[] aux = new int[len];
    for(int i=0;i<len;++i){
        aux[i]=original[i];
    }

    Sort(aux,0,len);


}

public void Sort(int[] aux,int start, int end){

    int mid = start + (end-start)/2;

    if(start < end){
        Sort(aux, start, mid-1);
        Sort(aux, mid, end);
        Merge(aux, start, mid, end);
    }
}

public void Merge(int[] aux, int start, int mid, int end){// while array passing be careful of shallow and deep copying

    for(int i=start; i<=end; ++i)
        auxilary[i] = original[i];

    int i = start;
    int k = start;
    int j = mid+1;
    while(i < mid && j <end){
        if(aux[i] < aux[j]){
            original[k++] = aux[i++];
        }
        else{
            original[k++] = aux[j++];
        }   
    }
    if(i == mid){
        while(j < end){
            original[k++] = aux[j++];
        }
    }
    if(j == end){
        while(i < mid){
            original[k++] = aux[i++];
        }
    }
}
public void disp(){
    for(int i=0;i<len;++i)
        System.out.print(original[i]+" ");
}
public static void main(String[] args) {

    MergeSort ms = new MergeSort(10);

    ms.disp();

}

}

Answer 9

The above codes are a little confused Never use variables with names: "k", "j", "m",... this makes the code very confusing

Follows the code in an easier way...

import java.util.Arrays;

public class MergeSort {

    public static void main(String[] args) {
        Integer[] itens = {2,6,4,9,1,3,8,7,0};

        Integer[] tmp = new Integer[itens.length];
        int left = 0;
        int right = itens.length - 1;

        mergeSort(itens, tmp, left, right);

        System.out.println(Arrays.toString(itens));
    }

    private static void mergeSort(Integer[] itens, Integer[] tmpArray, int left, int right) {

        if(itens == null || itens.length == 0 || left >= right){
            return;
        }

        int midle = (left + right) / 2;

        mergeSort(itens, tmpArray, left, midle);
        mergeSort(itens, tmpArray, midle + 1, right);

        merge(itens, tmpArray, left, midle + 1, right);
    }

    private static void merge(Integer[] itens, Integer[] tmpArray, int left, int right, int rightEnd) {
        int leftEnd = right - 1;
        int tmpIndex = left;

        while (left <= leftEnd && right <= rightEnd){
            if (itens[left] < itens[right] ){
                tmpArray[tmpIndex++] = itens[left++];
            } else {
                tmpArray[tmpIndex++] = itens[right++];
            }
        }

        while (left <= leftEnd) { // Copy rest of LEFT half
            tmpArray[tmpIndex++] = itens[left++];
        }
        while (right <= rightEnd) { // Copy rest of RIGHT half
            tmpArray[tmpIndex++] = itens[right++];
        }
        while(rightEnd >= 0){ // Copy TEMP back
            itens[rightEnd] = tmpArray[rightEnd--];
        }
    }
}

Answer 10

Might as well add my take on this: Takes two int arrays and merges them. Where 'result' is an array of size a.length + b.length

public static void merge( int[] a, int[] b, int[] result )
{
  int i = 0, j = 0;

  while ( true )
  {
    if ( i == a.length )
    {
      if ( j == b.length )
        return;

      result[ i + j ] = b[ j ]; 
      j++;
    }
    else if ( j == b.length )
    {
      result[ i + j ] = a[ i ];
      i++;
    }
    else if ( a[ i ] < b[ j ] )
    {
      result[ i + j ] = a[ i ];
      i++;
    }
    else
    {
      result[ i + j ] = b[ j ];
      j++;
    }
  }
}

Answer 11

public class MyMergeSort {

    private int[] array;
    private int[] tempMergArr;
    private int length;

    public static void main(String a[]){

        int[] inputArr = {45,23,11,89,77,98,4,28,65,43};
        MyMergeSort mms = new MyMergeSort();
        mms.sort(inputArr);
        for(int i:inputArr){
            System.out.print(i);
            System.out.print(" ");
        }
    }

    public void sort(int inputArr[]) {
        this.array = inputArr;
        this.length = inputArr.length;
        this.tempMergArr = new int[length];
        doMergeSort(0, length - 1);
    }

    private void doMergeSort(int lowerIndex, int higherIndex) {

        if (lowerIndex < higherIndex) {
            int middle = lowerIndex + (higherIndex - lowerIndex) / 2;
            // Below step sorts the left side of the array
            doMergeSort(lowerIndex, middle);
            // Below step sorts the right side of the array
            doMergeSort(middle + 1, higherIndex);
            // Now merge both sides
            mergeParts(lowerIndex, middle, higherIndex);
        }
    }

    private void mergeParts(int lowerIndex, int middle, int higherIndex) {

        for (int i = lowerIndex; i <= higherIndex; i++) {
            tempMergArr[i] = array[i];
        }
        int i = lowerIndex;
        int j = middle + 1;
        int k = lowerIndex;
        while (i <= middle && j <= higherIndex) {
            if (tempMergArr[i] <= tempMergArr[j]) {
                array[k] = tempMergArr[i];
                i++;
            } else {
                array[k] = tempMergArr[j];
                j++;
            }
            k++;
        }
        while (i <= middle) {
            array[k] = tempMergArr[i];
            k++;
            i++;
        }

    }
}

Answer 12

very simple and easy to understand

static void sort(char[] data) {
    int length = data.length;
    if (length < 2)
        return;
    int mid = length / 2;
    char[] left = new char[mid];
    char[] right = new char[length - mid];

    for(int i=0;i<mid;i++) {
        left[i]=data[i];
    }

    for(int i=0,j=mid;j<length;i++,j++) {
        right[i]=data[j];
    }

    sort(left);
    sort(right);
    merge(left, right, data);

}

static void merge(char[] left, char[] right, char[] og) {
    int i = 0, j = 0, k = 0;
    while(i < left.length && j < right.length) {
        if (left[i] < right[j]) {
            og[k++] = left[i++];
        } else {
            og[k++] = right[j++];
        }
    }
    while (i < left.length) {
        og[k++] = left[i++];
    }
    while (j < right.length) {
        og[k++] = right[j++];
    }

}

Answer 13

I have a parallel version of Merge Sort. I am benefiting from the RecursiveAction and ForkJoinPool. Note that the number of workers can be set as a constant. However, I am setting it as the number of available processors on the machine.

import java.util.Arrays;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;

public class ParallelMergeSorter {

    private int[] array;

    public ParallelMergeSorter(int[] array) {
        this.array = array;
    }

    public int[] sort() {
        int numWorkers = Runtime.getRuntime().availableProcessors(); // Get number of available processors
        ForkJoinPool pool = new ForkJoinPool(numWorkers);
        pool.invoke(new ParallelWorker(0, array.length - 1));
        return array;
    }

    private class ParallelWorker extends RecursiveAction {
        private int left, right;

        public ParallelWorker(int left, int right) {
            this.left = left;
            this.right = right;
        }

        protected void compute() {
            if (left < right) {
                int mid = (left + right) / 2;
                ParallelWorker leftWorker = new ParallelWorker(left, mid);
                ParallelWorker rightWorker = new ParallelWorker(mid + 1, right);
                invokeAll(leftWorker, rightWorker);
                merge(left, mid, right);
            }
        }

        private void merge(int left, int mid, int right) {
            int[] leftTempArray = Arrays.copyOfRange(array, left, mid + 1);
            int[] rightTempArray = Arrays.copyOfRange(array, mid + 1, right + 1);
            int leftTempIndex = 0, rightTempIndex = 0, mergeIndex = left;
            while (leftTempIndex < mid - left + 1 || rightTempIndex < right - mid) {
                if (leftTempIndex < mid - left + 1 && rightTempIndex < right - mid) {
                    if (leftTempArray[leftTempIndex] <= rightTempArray[rightTempIndex]) {
                        array[mergeIndex] = leftTempArray[leftTempIndex];
                        leftTempIndex++;
                    } else {
                        array[mergeIndex] = rightTempArray[rightTempIndex];
                        rightTempIndex++;
                    }
                } else if (leftTempIndex < mid - left + 1) {
                    array[mergeIndex] = leftTempArray[leftTempIndex];
                    leftTempIndex++;
                } else if (rightTempIndex < right - mid) {
                    array[mergeIndex] = rightTempArray[rightTempIndex];
                    rightTempIndex++;
                }
                mergeIndex++;
            }
        }
    }

}