Question

java quicksort help?

Answer 1

// The "MyQuickSort" class.
//S

import java.awt.*;
import java.util.*;
import java.lang.*;
import java.io.*;
public class Quicksort
{

/**
* @param args
*/
static int arraylength;
public static void main(String[] args) throws IOException
{

BufferedReader in = new BufferedReader (new FileReader ("list_to_sort.txt"));
PrintWriter out = new PrintWriter (new FileWriter ("sorted.txt"));


int temp;
arraylength = Integer.parseInt (in.readLine ());
int[] sort = new int [arraylength];
for (int i = 0 ; i < arraylength ; i++)
{
sort [i] = Integer.parseInt (in.readLine ());
}

sort=quicksort(sort,0,arraylength-1);
for (int i = 0 ; i < arraylength ; i++)
{
out.println (sort [i]);
System.out.println(sort [i]);
}
in.close ();
out.close ();

}
public static int[] quicksort(int [] array, int beg,int end )
{
if (beg < end)
{
int pivotindex=(int)(Math.random()*end)+beg;

int pivotNewIndex = partition(beg, end,array, pivotindex);

// Recursively sort elements smaller than the pivot
quicksort(array,beg , pivotNewIndex - 1);

// Recursively sort elements at least as big as the pivot
quicksort(array, pivotNewIndex + 1, end);
}
return array;
}
public static int partition(int beg, int end, int [] qsort,int pivotindex)
{

int pivot=qsort[pivotindex];
int temp;
int indexstore;
temp=qsort[end];
qsort[end]=qsort[pivotindex];
qsort[pivotindex]=temp;

temp=0;
indexstore=beg;
for (int i=beg; i<end; i++)
{
if (qsort[i] < pivot)
{
temp=qsort[indexstore];
qsort[indexstore]=qsort[i];
qsort[i]=temp;
}
indexstore+=1;
qsort[i]=qsort[indexstore];
}
temp=qsort[indexstore];
qsort[indexstore]=qsort[end];
qsort[end]=temp;
return indexstore;
}

}

Answer 2

1. You are multiplying Math.random() by the wrong value. (beg might not be 0)
2. When should you update indexstore?
3. Why are you clobbering qsort[i] every time?

Answer 3

thnk u, tht wasnt the problem, but thnk u