Question

I’m trying out an implementation of QuickSort but getting an

 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at com.JavaReference.QuickSort.swap(QuickSort.java:50)
at com.JavaReference.QuickSort.randPartition(QuickSort.java:20)
at com.JavaReference.QuickSort.randSort(QuickSort.java:12)
at com.JavaReference.QuickSort.randSort(QuickSort.java:13)
at com.JavaReference.QuickSort.randSort(QuickSort.java:13)
at com.JavaReference.QuickSort.randSort(QuickSort.java:13)
at com.JavaReference.QuickSort.randSort(QuickSort.java:13)
at com.JavaReference.QuickSort.randSort(QuickSort.java:8)
at com.JavaReference.QuickSort.main(QuickSort.java:59)

Here’s my source code[here.]

Im a newbie to programming so any advice on where Im going wrong would be appreciated.

EDIT:Added entire stacktrace

Answer 1

My suspect is line 30:

            int i =left-1;

Since left is initially 0, i will become -1. Then on line 35 you call

                            swap(a,i,j);

and bang.

OK, from the stack trace it seems my first guess was wrong.

Second guess: from the stack trace it shows that the array is partitioned 4 times, and then at the 5th attempt the exception comes. It is thrown on line 50:

int temp= array[a];

a is the first parameter to swap. The call to swap on line 20 was

swap(a,right,randPivot);

thus right is -1 at that point. This value comes from here (line 13):

randSort(a,left,pivot-1);

If pivot is 0 at this point, shit happens. And it can become 0 since it is taken as a random value between left and right, inclusive. (And actually, that is a mistake, as for effective partitioning, pivot should fall between left and right, noninclusive.) Currently, the probability of pivot becoming 0 increases as the leftmost partition becomes smaller. You need to introduce a check for this (or, more generally, to detect array partitions of size 1, which can’t be partitioned further), to stop the recursion in time.