How do I execute insertion sorting algorithm properly in a doubly circular linked list in java?

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We were tasked to apply the insertion sorting algorithm to an unsorted doubly circular linked list. I tried this algorithm given below;

Java

<pre> public void insertSort(){
        Node cur = head;
        Node pointer; // this will hold the last position of where the sorting begins
        Node handler = head;
        
        do{
            pointer = cur;
            do{
                if(cur.data < cur.prev.data){
                    handler.data = cur.data;
                    cur.data = cur.prev.data;
                    cur.prev.data = handler.data;                   
                    
                }
                cur = cur.prev;
            }while(cur != head);
            cur = pointer;
            cur = cur.next;     
        }while(cur != head);
    }

Above is the method to be called in sorting my doubly circular linked list.

What I have tried:

I tried inputting 10,9,8,7,6,5,4,3,2,1. But the output will be;

Java

1-->1-->2-->3-->4-->5-->6-->7-->8-->9-->HEAD

It looks like the first node on the unsorted list is removed and then the first node of the sorted list is doubled. Just like what happened. 10 is not on the output and 1 is doubled. I do not know where I went wrong since I tried this algorithm manually using pen and paper but everything works just fine.

Included below is the full code;

Java

public class Circular {
    
    private Node head;
    private Node tail;
    
    public Circular(){
        this.head = null;
        this.tail = null;
    }
    
    public void addNode(int data){
        Node node = new Node(data);
        if(head == null){
            tail = node;
            tail.prev = tail;
            head = tail;
            return;
        }
        
            node.prev = tail.prev;
            node.next = tail;
            tail.prev.next = node;
            tail.prev = node;
            head = tail;

    }
    
    public void printForward(){
        Node node = head;
        
        if(head != null){
            do{
                System.out.print(node.data + "-->");
                node = node.next;
            }while(node != head);
        }
        System.out.print("HEAD\n");
    }
    
    public void insertSort(){
        Node cur = head;
        Node pointer; // this will hold the last position of where the sorting begins
        Node handler = head;
        
        do{
            pointer = cur;
            do{
                if(cur.data < cur.prev.data){
                    handler.data = cur.data;
                    cur.data = cur.prev.data;
                    cur.prev.data = handler.data;                   
                    System.out.print(cur.data + "<-->" + cur.next.data + "\n");
                }
                cur = cur.prev;
            }while(cur != tail);
            cur = pointer;
            cur = cur.next;     
        }while(cur != head);
    }
    private class Node{
        int data;
        Node next;
        Node prev;
        
        
        public Node(int data){
            this.data = data;
        }
    }
}
public class Main {
    public static void main(String [] args){
        Circular bubble = new Circular();
        int num;
       System.out.println("Enter 10 numbers in ascending order: ");
        for(int i = 0; i < 10; i++){
            Scanner scanner = new Scanner(System.in);
            num = scanner.nextInt();
            bubble.addNode(num);
        }
        
        bubble.printForward();
        bubble.insertSort();
        System.out.println("\n");
        bubble.printForward();

}
}

Posted 22-Sep-22 18:15pm

Paul Lawrence Perez

Updated 22-Sep-22 18:49pm

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OriginalGriff · Accepted Answer · 2022-09-22T18:49:00

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!