Random pivot quicksort in Java [duplicate]

Question

Possible Duplicate:
Quick Sort with random pivot in Java

The below written code of the Quicksort uses the first element of the array as the pivot and then sorts the array. Now I want to randomly pick up the pivot instead of first one and then sort the array and I am stuck please tell me what changes I can make in the below code to get the perfect results.

import java.util.*;
import javax.swing.JOptionPane;

public class Quicksort {

public static void main(String[] args) {
    String arraylength = JOptionPane.showInputDialog("Enter the length of the array.");

    int a = Integer.parseInt(arraylength);
    if (a == 0) {
        System.out.println("Null Length");
    } else {
        int[] list = new int[a];


        for (int i = 0; i < a; i++) {
            String input = JOptionPane.showInputDialog("Input the number.");
            int c = Integer.parseInt(input);
            list[i] = c;
        }

        System.out.println("Before");
        for (int i = 0; i < list.length; i++) {
            System.out.print(list[i] + " ");
        }
        partition(list, 0, list.length - 1);


        System.out.println("\nAfter partitionaing");
        for (int i = 0; i < list.length; i++) {
            System.out.print(list[i] + " ");
        }
        quickSort(list, 0, list.length - 1);

        System.out.println("\nAfter Sorting");
        for (int i = 0; i < list.length; i++) {
            System.out.print(list[i] + " ");
        }
    }
}

private static int partition(int[] list, int first, int last) {
    int pivot = list[first];
    int low = first + 1;
    int high = last;

    while (high > low) {

        while (low < high && list[low] < pivot) {
            low++;
        }


        while (low < high && list[high] >= pivot) {
            high--;
        }


        if (high > low) {
            int temp = list[high];
            list[high] = list[low];
            list[low] = temp;
        }
    }
    while (high > first && list[high] >= pivot) {
        high--;
    }

    if (pivot > list[high]) {
        list[first] = list[high];
        list[high] = pivot;
        return high;
    } else {
        return first;
    }

}

private static void quickSort(int[] list, int first, int last) {
    if (last > first) {
        int pivotIndex = partition(list, first, last);
        quickSort(list, first, pivotIndex - 1);
        quickSort(list, pivotIndex + 1, last);
    }
}
}

Answer 1

Try something like this:-

  int pivot = arr[left + rnd.nextInt(right - left)]; //rnd is a class Random object, which you could set as a private static final field.

  import java.util.Random;


  public class QuickSort {

/**
 * @param args
 */
public static void main(String[] args) {

      int i;
      int array[] = {10,9,1,2,3,4,100,200,300,400};  
      System.out.println(" Quick Sort\n\n");
      System.out.println("Values Before the sort:\n");
      for(i = 0; i < array.length; i++)
      System.out.print( array[i]+"  ");
      System.out.println();
      quickSort(array,0,array.length-1);
      System.out.print("Values after the sort:\n");
      for(i = 0; i <array.length; i++)
      System.out.print(array[i]+"  ");
      System.out.println(); 

}

public static int partition(int arr[], int left, int right)
{
      int i = left, j = right;
      int tmp;



      int pivot = arr[left + rnd.nextInt(right - left)];

      while (i <= j) {
            while (arr[i] < pivot)
                  i++;
            while (arr[j] > pivot)
                  j--;
            if (i <= j) {
                  tmp = arr[i];
                  arr[i] = arr[j];
                  arr[j] = tmp;
                  i++;
                  j--;
            }
      };

      return i;
}

public static void quickSort(int arr[], int left, int right) {
      int index = partition(arr, left, right);
      if (left < index - 1)
            quickSort(arr, left, index - 1);
      if (index < right)
            quickSort(arr, index, right);
}

 }

Answer 2

Use int rand = (int) (st + (Math.random()*((end-st)+1))); Below is the code

private E[] tofind;
private int sameCounter=0;
private int comparisions;
public void quickSort()
{
    quickInnerSort(0, tofind.length-1);
    System.out.println("jatin");
}
    /**
 * 
 * @param st index of the starting point of the array
 * @param end index of the ending point of the array
 * @param k rank of the element to be found out
 */
private void quickInnerSort(int st, int end)
{
    if(st>end)   
        return;

    //printWithComparisions();
    int cut = partition(st,end);        
    //check k lies in which partition

    quickInnerSort(st,cut-1);
    quickInnerSort(cut+1,end);


}

/**
 * 
 * @param st index of the array from where to partition
 * @param end index of the array till where to partition
 * @return index of the random number chosen to calculate
 */
private int partition(int st, int end)
{

    int rand = (int) (st + (Math.random()*((end-st)+1)));
    //System.out.println("rand ="+tofind[rand]+" index at "+rand);
    E x = tofind[rand];
    sameCounter=0;
    swap(end,rand);
    E x = tofind[end];
    int i = st-1;
    for(int j=st;j<=end-1;j++)
    {
        if(tofind[j].compareTo(x)==0)
            sameCounter++;
        if(tofind[j].compareTo(x)<0)
        {
            i = i+1;
            swap(i,j);
        }

    }
    swap(i+1,end);        
    return i+1;
}

Answer 3

You can use the Random class from java:

Random rand = new Random();
int num = begin_sub_array + rand.nextInt(end_sub_array - begin_sub_array);

This will generate a value from beginning of the sub array (begin_sub_array) to end of the sub array (end_sub_array). You just have to take the variable num, and pass as the pivot in your quicksort algorithm.

int pivot = list[num];