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Question

I'm having an issue inserting a new object into a linked list. I use a custom method called Insert that takes the object type and the index where it should be placed. However, when I insert the object, the list doesn't update. Any idea why?

The list:

        GenericLinkedList<string> bet = new GenericLinkedList<string>();
        bet.Add("a");
        bet.Add("b");
        bet.Add("c");
        bet.Add("d");

        //bet.Remove();
        bet.Insert("x", 2);

The Generic Linked List Class:

public class GenericLinkedList<T> where T : IComparable<T> 
{

    public GenericLinkedList()
    {
        count = 0;
        head = null;
    }

    #region Nested Node Class
    private class Node<T>
    {
        private T data;
        public T Data
        {
            get { return data; }
            set { data = value; }
        }

        private Node<T> next;
        public Node<T> Next
        {
            get { return next; }
            set { next = value; }
        }

        public Node<T> Previous { get; set; }


        public Node(T obj_t)
        {
            next = null;
            data = obj_t;
        }
        public Node()
        {
            next = null;
        }


    }
    #endregion

    private Node<T> head;
    private Node<T> tail;


    private int count;
    public int Count
    {
        get
        {
            int num;
            if (count < 0)
                num = 0;
            else {
                num = count;
            }
            return num;
        }
    }

}

The Insert Method:

    public void Insert(T data, int index)
    {
        Node<T> replacedItem = new Node<T>();
        Node<T> newItem = new Node<T>(data);

        if (head == null)
        {
            head = new Node<T>(data);
            tail = head;
        }
        else
        {
            Node<T> current = head;
            Node<T> tempNode = new Node<T>();


            if (index > 0 && index <= count + 1)
            {
                int c = 0;
                while (current != null)
                {
                    if (c == index)
                    {
                        tempNode = current;
                        current = newItem;
                        current.Next = tempNode;
                    }
                    else
                    {
                        current = current.Next;
                    }
                    c++;
                }
            }
            count++; 
        }
    }

The Add Method:

  public void Add(T data)
    {
        count++;

        if (head == null)
        {
            head = new Node<T>(data);
            tail = head;
        }
        else
        {
            tail.Next = new Node<T>(data);
            tail.Next.Previous = tail;
            tail = tail.Next;
        }
    }

Answer 1

Need to add count++ in case head is null. Also count++ should go inside if(index>0&&index<=count+1)

public void Insert(T data, int index)
{
    Node<T> replacedItem = new Node<T>();
    Node<T> newItem = new Node<T>(data);

    if (head == null)
    {
        head = new Node<T>(data);
        tail = head;
        count++;
    }
    else
    {
        Node<T> current = head;
        Node<T> tempNode = new Node<T>();


        if (index > 0 && index <= count + 1)
        {
            int c = 0;
            while (current != null)
            {
                if (c == index)
                {
                    tempNode = current;
                    current = newItem;
                    current.Next = tempNode;
                }
                else
                {
                    current = current.Next;
                }
                c++;
            }
            count++;
        }             
    }
}

See if this helps.

Answer 2

You not correctly add new item in your Insert method
in this place

tempNode = current; // save pointer to current item
current = newItem; // save in current new item
current.Next = tempNode; //set next to counter

but you don't change link from previous element, so when you again go from head you just miss your added item.

You need change this code something like

public void Insert(T data, int index)
{
    Node<T> replacedItem = new Node<T>();
    Node<T> newItem = new Node<T>(data);

    if (head == null)
    {
        head = new Node<T>(data);
        tail = head;
        count++;
    }
    else
    {
        Node<T> current = head;
        Node<T> tempNode = new Node<T>();


        if (index > 0 && index <= count + 1)
        {
            int c = 0;
            while (current != null)
            {
                if (c == index)
                {
                    tempNode = current;
                    current = newItem;

                    //update link from previous element
                    if(tempNode.Previous != null) 
                       tempNode.Previous.Next = current;

                    current.Next = tempNode;
                    current.Previous = tempNode.Previous;
                    tempNode.Previous = current;
                    count++;
                    break;
                }
                else
                {
                    current = current.Next;
                }
                c++;
            }
        }             
    }
}