Quicksort. Exception in thread “main” java.lang.StackOverflowError

Question

Good day! I have here a Java program that does the quicksort. It reads a file then sorts the first 10,000 words in it. I followed the pseudocode of Thomas Cormen in his Introduction to Algorithms, Second Ed.

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

public class SortingAnalysis {

    public static int partition(String[] A, int p, int r) {
        String x = A[r];
        int i = p-1;
        for (int j=p; j < r-1; j++) {
            int comparison = A[j].compareTo(x);
            if (comparison<=0) {
                i=i+1;
                A[i] = A[j];
            }       
        }
        A[i+1] = A[r];
        return i+1;
    }

    public static void quickSort(String[] a, int p, int r) {
        if (p < r) {
            int q = partition(a, p, r);
            quickSort(a, p, q-1);
            quickSort(a, q+1, r);
        }
    }

    public static void main(String[] args) {
        final int NO_OF_WORDS = 10000;
        try {
            Scanner file = new Scanner(new File(args[0]));
            String[] words = new String[NO_OF_WORDS];
            int i = 0;
            while(file.hasNext() && i < NO_OF_WORDS) {
                words[i] = file.next();
                i++;
            }
            long start = System.currentTimeMillis();
            quickSort(words, 0, words.length-1);
            long end = System.currentTimeMillis();
            System.out.println("Sorted Words: ");
            for(int j = 0; j < words.length; j++) {
                System.out.println(words[j]);
            }
            System.out.print("Running time: " + (end - start) + "ms");

        }
        catch(SecurityException securityException) {
            System.err.println("Error");
            System.exit(1);
        }
        catch(FileNotFoundException fileNotFoundException) {
            System.err.println("Error");
            System.exit(1);
        }
    }
}

However, when I run the code, the console says
Exception in thread "main" java.lang.StackOverflowError at SortingAnalysis.partition and quickSort

I thought that the error was just because of the large size (ie, 10000) so I decreased it to 100 instead. However, it still doesn't sort the first 100 words from a file, rather, it displays the 100th word 100 times.
Please help me fix the code. I'm new in Java and I need help from you guys. Thank you very much!

EDIT: I now edited my code. It doesn't have an error now even when the NO_OF_WORDS reaches 10000. The problem is it halts the wrong sequence.

Answer 1

You have two problems:

• the loop in partition() should run to j <= r - 1 , you are jumping out early.

• You are not swapping elements. Try the following code:

   public static int partition(String[] A, int p, int r) { String x = A[r]; int i = p - 1; for (int j = p; j <= r - 1; j++) { int comparison = A[j].compareTo(x); if (comparison <= 0) { i = i + 1; swap(A, i, j); } } swap(A, i + 1, r); return i + 1; } public static void swap(String[] a, int i, int j) { String temp = a[i]; a[i] = a[j]; a[j] = temp; } 

Answer 2

Looking at the Quicksort algo of wikipedia , the partition algo is the following :

// left is the index of the leftmost element of the array
// right is the index of the rightmost element of the array (inclusive)
//   number of elements in subarray = right-left+1
function partition(array, 'left', 'right', 'pivotIndex')
  'pivotValue' := array['pivotIndex']
  swap array['pivotIndex'] and array['right']  // Move pivot to end
  'storeIndex' := 'left'
  for 'i' from 'left' to 'right' - 1  // left ≤ i < right
      if array['i'] < 'pivotValue'
          swap array['i'] and array['storeIndex']
          'storeIndex' := 'storeIndex' + 1
  swap array['storeIndex'] and array['right']  // Move pivot to its final place
  return 'storeIndex'

In your method you don't use the pivotIndex value, you base your pivotValue on the right index. You need to add this parameter to your method.

Following the wiki algo it should be like this :

public static int partition(String[] A, int p, int r, int pivotIdx) {
    String x = A[pivotIdx];
    String tmp = A[pivotIdx];
    A[pivotIdx] = A[r];
    A[r]=tmp;
    int i = p;
    for (int j=p; j < r; j++) {
        int comparison = A[j].compareTo(x);
        if (comparison<=0) {
            tmp=A[i];
            A[i] = A[j];
            A[j]=tmp;
            i++;
        }       
    }
    tmp=A[i];
    A[i] = A[r];
    A[r]=tmp;
    return i;
}