why the value is null in this case?

18y4.8kVisual Basic .NET

I try to use C# to illustrate HashTable algorithm. My problem is: when I assign "hashtable[0] = p;" in method InsertNode-class LinkList, the properties of p is assigned to HashTable[0] correctly. But when I debug, I see the value of HashTable[0] is null in DisplayDictionary(){ PrintList(hashtable[0]); } method - class Dictionary inherit from LinkList class. and that's reason why I cannot display the value of HashTable[0] into screen.

Please tell me why the value of HashTable[0] is null when I call DisplayDictionary() method. Thank you in advance.
This is my code:

class Node
    {
        public string word;
        public string mean;
        public Node next;
        public Node()
        {
            next = null;
        }

        public Node GetNode(String word, String mean)
        {
            Node p;
            p = new Node();
            p.word = string.Copy(word);
            p.mean = string.Copy(mean);
            return p;
        }

        public int HF(string word)
        {
            char[] firstchar = word.ToUpper().ToCharArray(0, 1);
            int value = firstchar[0];
            return ((value - 65) % 26);
        }
    }
    class LinkList : Node
    {
        public Node[] hashtable = new Node[26];

        public LinkList() { }
        public void InsertNode(Node p)
        {
           // int i = HF(p.word);
           // int i = 0;
            p.next = hashtable[0];
            hashtable[0] = p;
        }

        public void PrintList(Node list)
        {
            Node temp = list;
            while (temp != null)
            {
                Console.WriteLine("word:{0}", temp.word);
                Console.WriteLine("mean:{0}", temp.mean);
                temp = temp.next;
            }
        }
    }
    class Dictionary : LinkList
    {
        public Dictionary() { }
        public void MakeDictionary()
        {
            string word, mean;
            word = "a";
            mean = "is a";
            //Node p = new Node();
            LinkList linklist = new LinkList();
            Node p = linklist.GetNode(word, mean);
            linklist.InsertNode(p);  // null
        }
        public void DisplayDictionary()
        {
           // for (int i = 0; i < 26; i++)
                PrintList(hashtable[0]);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary dict = new Dictionary();
            dict.MakeDictionary();
            dict.DisplayDictionary();
        }
    }
}

8 Replies

Know the answer? Post it — somebody with the same question will find it here.

Alan18y ago
In this particular case, yes, as the Dictionary class doesn't hide the inherited ancestor class methods with its own versions.
k18y ago
"this" or "base" is the same, right?
18y ago
Thank you very much for explanation, Alan
Alan18y ago
The string.Copy method creates a clone of the string being copied i.e. a new instance containing the same characters as the original string.

The clones of the 'word' and 'mean' strings are then assigned to the new node, p's, fields of those names.

This is probably being done to ensure that all nodes contain independent copies of these strings i.e.no node contains a reference to a string which is a field of another node.
18y ago
How this can be explained (highlighted in yellow)? What is the function of this?

public Node GetNode(String word, String mean)

{

Node p;

      p = new Node();

      p.word = string.Copy(word);

      p.mean = string.Copy(mean);

      return p;

}
Alan18y ago
Well, the Dictionary class is inheriting the InsertNode() method from LinkList and, since LinkList inherits in turn from Node, Dictionary is also inheriting the GetNode() method from the latter class.

So this is why preceding these methods with 'this' works. You could also just call them without preceding them with 'this.

It's the only way that you're going to be able to change the hashtable field (also inherited from LinkList) from within the Dictionary class.

Whilst there's nothing to stop you creating a separate LinkList object and calling its methods, it will have its own copy of hashtable and won't therefore affect the one which the Dictionary object inherits. This will apply wherever you put it.
k18y ago
Thanks Alan very much, but if I want to create object LinkList type to use its methods correctly (not using "this" anymore). Where can I declare it?
Alan18y ago
When you create the new Dictionary object it inherits the public field 'hashtable' from LinkList. However, all the elements of the hashtable array are null at this point.

When you then call the MakeDictionary() method, a new LinkList object is created and its InsertMode() method is called which sets hashtable[0] to a Node object. However, this LinkList object and its hashtable array are destroyed when the method ends because linklist is a local variable - it's not the same as the LinkList base class from which Dictionary is inheriting.

Thus, the Dictionary object's hashtable still has all its elements
set to null and so the DisplayDictionary() method displays nothing.

To remedy the matter, change the MakeDictionary method to the following:

        public void MakeDictionary()
        {
            string word, mean;
            word = "a";
            mean = "is a";
            //Node p = new Node();
            //LinkList linklist = new LinkList();
            Node p = this.GetNode(word, mean);
            this.InsertNode(p); // not null now
        }