Exception in thread

[Michael Statham]

I've written this code so far and only 1 error but it's driving me nuts, maybe lack of sleep or something far worse, who knows. If any one can help, thank you in advance.

  /*************************************************************************
 *  
 *
 *  Generate N random real numbers between 0 and 1 and quicksort them.
 *
 *  On average, this quicksort algorithm runs in time proportional to
 *  N log N, independent of the input distribution. 
 *
 *************************************************************************/
 
public class QuickSort {
    private static long comparisons = 0;
    private static long exchanges   = 0;
 
   public static void quicksort(double[] a) {
        shuffle(a);                        // to guard against worst-case
        quicksort(a, 0, a.length - 1);
    }
    public static void quicksort(double[] a, int left, int right) {
        if (right <= left) return;
        int i = partition(a, left, right);
        quicksort(a, left, i-1);
        quicksort(a, i+1, right);
    }
 
    private static int partition(double[] a, int left, int right) {
        int i = left - 1;
        int j = right;
        while (true) {
            while (less(a[++i], a[right]))      // find item on left to swap
                ;                               // a[right] acts as sentinel
            while (less(a[right], a[--j]))      // find item on right to swap
                if (j == left) break;           // don't go out-of-bounds
            if (i >= j) break;                  // check if pointers cross
            exch(a, i, j);                      // swap two elements into place
        }
        exch(a, i, right);                      // swap with partition element
        return i;
    }
 
    // is x < y ?
    private static boolean less(double x, double y) {
        comparisons++;
        return (x < y);
    }
 
    // exchange a[i] and a[j]
    private static void exch(double[] a, int i, int j) {
        exchanges++;
        double swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }
 
    // shuffle the array a
    private static void shuffle(double[] a) {
        int N = a.length;
        for (int i = 0; i < N; i++) {
            int r = i + (int) (Math.random() * (N-i));   // between i and N-1
            exch(a, i, r);
        }
    }
 
 
 
    // test client
    public static void main(String[] args) {
        int N = Integer.parseInt(args[0]);
 
        // generate N random real numbers between 0 and 1
        long start = System.currentTimeMillis();
        double[] a = new double[N];
        for (int i = 0; i < N; i++)
            a[i] = Math.random();
        long stop = System.currentTimeMillis();
        double elapsed = (stop - start) / 1000.0;
        System.out.println("Generating input:  " + elapsed + " seconds");
 
        // sort them
        start = System.currentTimeMillis();
        quicksort(a);
        stop = System.currentTimeMillis();
        elapsed = (stop - start) / 1000.0;
        System.out.println("Quicksort:   " + elapsed + " seconds");
 
        // print statistics
        System.out.println("Comparisons: " + comparisons);
        System.out.println("Exchanges:   " + exchanges);
    }
}
 

   

This is the error I'm getting any help would be appreciated in advance, thank you!

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
at QuickSort.main(QuickSort.java:77)

[Garrett Rowe]

How are you running it? I ran it using this command

java QuickSort 15

and this was my output:

Generating input:  0.0 seconds
Quicksort:   0.0 seconds
Comparisons: 68
Exchanges:   30

[Anton Uwe]

Perhaps you called this program without a parameter?

[Campbell Ritchie]

Welcome to the Ranch.

Go back to your Exception error message; it has a line no on ?77. Go to your line 77 and there you will find an array used. If the index is 0 it means you don't have a 1st member.
There are several possibilities for this error:-

1: You have an array declared but have never created it with a statement like: fooArray = new foo[20];

2: You have an array which you have never inserted any members into.

Are you using the args[] array without passing anything from the command line? That is a potent source of unexpected ArrayIndexEtcExceptions.

CR
[ February 11, 2007: Message edited by: Campbell Ritchie ]

[kiranmayi srivastava]

I think the error comes when there is no argument passed with the run command.
This can be handled this way:

Replace int N = Integer.parseInt(args[0]);
with the following code:

int N = 0;
if(args.length > 0)
N = Integer.parseInt(args[0]);

OR

int N = 0;
try
{
N = Integer.parseInt(args[0]);
}
catch (ArrayIndexOutOfBoundsException aiob)
{
System.out.println("No value recieved!!!");
}

[Ilja Preuss]

Originally posted by Campbell Ritchie:

There are several possibilities for this error:-

1: You have an array declared but have never created it with a statement like: fooArray = new foo[20];

2: You have an array which you have never inserted any members into.

Not quite - the first would cause a NullPointerException when trying to access the array, the latter doesn't make sense to me because you don't really *insert* members to arrays.

Really the only way this error can happen is when the array has a length of zero.