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

Answer 1

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 2

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 3

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++;
            }
        }
    }

}

Answer 4

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 5

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 6

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]);
    }