I'm getting arrayindexoutofbounds exception for no reason at all. Please help.

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Java

//Rearrange the input integer array using the following logic.

//Input : {1, 7, 8, 3, 5, 4}
//Output : {3, 5, 8, 7, 4, 1}

//Input : {1, 7, 8, 3, 4}
//Output : {1, 4, 8, 7, 3}

//Special note : You CANNOT declare a single extra array.


import java.util.*;
class arrayRearrange
{
    static int arr[];
    static int n = 0;
    static int c = 0, d = 0, a = 0, b = 0;
    static void input()
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the array size :");
        n = sc.nextInt();
        arr = new int [n];
        System.out.println("Enter the elements :");
        for(int i = 0; i < n; i++)
        {
            arr[i] = sc.nextInt();
        }
    }
    
    static void sort()
    {
        for(int i = 0; i < n; i++)
        {
            if(arr[i] > c)
            {
                c = arr[i];
                int t = arr[n/2];
                arr[n/2] = c;
                arr[i] = t;
            }
        }
        
        d = c;
        c = 0;
        
        a = n/2 - 1;
        b = n/2 + 1;
        
        while(a >= 0 || b <= n - 1)
        {
            for(int i = 0; i < n; i++)
            {
                if(arr[i] < d && arr[i] > c)
                {
                    c = arr[i];
                }
                
                arr[i] = arr[b];
                arr[b] = c;
                d = c;
                
                if(a < 0)
                    break;
                
                if(arr[i] < d && arr[i] > c)
                {
                    c = arr[i];
                }
                
                arr[i] = arr[a];
                arr[a] = c;
                d = c;
                
                a--; b++;
            }
        }
        
        System.out.println("The rearranged array : ");
        for(int i = 0; i < n; i++)
        {
            System.out.print(arr[i] + " ");
        }
    }
    
    public static void main(String args[])
    {
        input();
        sort();
    }
}

I have the following issue :

The program says (while running it) that the variable b is out of range.
However, when I try to debug, it seems to me that everything is perfectly alright.
Please help.

What I have tried:

I have no idea what to do. To be, the logic seems to be alright. I can find no reason for this exception. Please help.

Posted 23-Dec-21 7:56am

Abitatha Roy (Abitatha Roy)

Updated 23-Dec-21 8:41am

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Richard MacCutchan 24-Dec-21 4:51am

"it seems to me that everything is perfectly alright."
Obviously it is not. The statement while(a >= 0 || b <= n - 1) is probably the problem. Change the || to &&, and see what happens.

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OriginalGriff · Answer 1 · 2021-12-23T08:07:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

Java

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Luc Pattyn · Answer 2 · 2021-12-23T08:41:00

... for no reason at all
Sure, when humans and computers disagree, it is always the human who is right. Or is it?

C#

while(a >= 0 || b <= n - 1)

a (which is decreasing) and b (which is increasing) both need to be in the range [0, n-1] so why would you use an OR here, rather than an AND operator? This makes no sense to me.

C#

for(...) {               
    ...
    if(a < 0) break;
    ...
    a--; b++;
}

You feel a need to test a for its lower boundary, however you don't test b for its upper boundary? If you have a good reason not to include that second test, it would be wise to add a comment explaining that. However I think you're wrong here.

:)

I'm getting arrayindexoutofbounds exception for no reason at all. Please help.

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