Reversing the order of sub-array using Recursion

Question

I am trying to reverse the order of a sub-array between the indices of start and end strictly using recursion. For example, if the subarray is 1,2,3,4 , it will become 4,3,2,1.

However, I am getting the following runtime error:

java.lang.ArrayIndexOutOfBoundsException: -1

at finalExam.reverse(finalExam.java:13)

at finalExam.reverse(finalExam.java:17)

I am not sure how to fix this problem.

Thanks.

 double[] reverse (double[] a, int start, int end) {
 if (start == end) {return a;}
 else {
 a[start] = a[end];
 a[end] = a[start];}


 return reverse (a, start+1, end-1);
}

You could do a check at the beginning of the method to see if the index you have reached is greater than the length of the array. — Brady Sheehan
– Brady Sheehan, Commented Apr 15, 2014 at 1:47
Classic tail recursion, meaning you shouldn't write it as recursion but as a simple loop. (A good compiler may save you from this mistake, or may not.) — keshlam
– keshlam, Commented Apr 15, 2014 at 1:58
"finalExam.java" so... we're helping you with a test? Is this permitted by your course? — DrLivingston
– DrLivingston, Commented Apr 15, 2014 at 1:58
Has your professor shown you programming with boxes or hand simulating your assignments? Your indices aren't your only problem, try doing this on paper step by step. — DrLivingston
– DrLivingston, Commented Apr 15, 2014 at 2:00
This was a question on a final exam which was completed a few hours ago on paper. The course is now over. — tim
– tim, Commented Apr 15, 2014 at 2:03

Amulya Khare · Accepted Answer · 2014-04-15 02:08:53Z

1

(Since you mention the exam is over). Here are the problems with your code:

Your check should be start >= end
Your code for swapping two numbers is incorrect.

Here is the correct solution:

public static double[] reverse (double[] a, int start, int end) {
    if (start >= end) {
        return a;
    }
    else {
        // this code will swap two elements
        double temp = a[start];
        a[start] = a[end];
        a[end] = temp;
    }
    return reverse (a, start+1, end-1);
}

answered Apr 15, 2014 at 2:08

Amulya Khare

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