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Sunday, July 15, 2007

Introduction to Linked Lists

We have been using arrays to store similar data linearly. While arrays are simple to understand and easy to implement in common situations, they do suffer from some drawbacks which are listed below:

  • Arrays have fixed dimensions, even if we dynamically allocate the dimension it remains constant throughout. So there is a limit to the number of elements it can store.

  • Operations such as insertion and deletion are pretty much difficult to implement and increases the overhead because these operations require elements in the array to be physically shifted.

Linked lists overcome these drawbacks and are commonly used to store linear data.

Actually elements of linked lists (called as nodes) store two information, data and the link (pointer) pointing to the next elements (node).

The elements (nodes) are linked sequentially with the help of link pointers. So we can say that linked lists are collection of nodes which have data and are linked sequentially so that all the nodes or elements are grouped together.

In programming sense, linked lists are classes whose general form is:

  class node
  {
   public:
     data-type info;
     node *link;
  };

Here info stores the actual data while link stores the memory address of the next node, which forms the link between the nodes.

Graphical Representation of a Node of a Linked List FIG.: Graphical representation of a node

Following figure illustrates the growing of linked lists, the node which has its link as NULL is the last element in the linked list.

Growing of linked list FIG.: Linked lists grows like this

Let us now discuss a bit about how a linked list grows:

  1. We have a pointer that stores the memory address of the first element in the linked list, represented as the start pointer (of type node). It is NULL to begin with as we don’t have any element in the list.

  2. As an element is added, the start pointer is made to point at it and since for now the first element is the last element therefore its link is made to be NULL

  3. After the addition of each element the link pointer of the previously last element is made to point at new last element. This step continues…

In this way the number of nodes in a linked list can grow or shrink over time as far as memory permits.

Below is the example program that illustrates linked lists. This program is made as simple as possible and therefore it only performs the basic action (addition of element) in the linked list.

  // -- Linked Lists --
  // ------------------
  // Example program in C++
  // to illustrate the most simple
  // linked list
  // NOTE: It is designed so that it
  // could only add nodes to the
  // list and display them.
  #include<iostream.h>

  // node class, this will
  // represent the nodes or elements
  // of the linked list
  class node
  {
   public:
     int info;
     node *link;
  };

  // declare global objects
  node *start=NULL;

  // function prototypes
  void add(int inf);
  void display(void);

  void main(void)
  {
   int ch;

   // input elements
   while(ch!=0)
   {
    cout<<"enter element to be added:";
    cout<<"\nenter 0 to stop...\n";
    cin>>ch;

    if(ch!=0) add(ch);
    cout<<"\n\n";
   }

   cout<<"elements are...\n";
   display();

   // below is a bit confusing
   // part.
   // here all the nodes that
   // we have allocated are
   // being freed up
   node temp;
   while(start!=NULL)
   {
    // store the next node
    // to the one being deleted
    temp=*start;
  
    // delete the node
    delete start;

    // retrieve the next node
    // to be deleted
    start=temp.link;
    }
   }

   void add(int inf)
   {
    node *temp1;
    node *temp2;

    // if the element to be added
    // is the first element
    if(start==NULL)
    {
    // allocate a new node
    temp1=new node;
    temp1->info=inf;
    temp1->link=NULL;

    // make start point at it
    start=temp1;
   }
   // if not
   else
   {
    temp1=start;

    // find out the last element
    while(temp1->link!=NULL)
    temp1=temp1->link;

    // allocate new node
    temp2=new node;
    temp2->info=inf;
    temp2->link=NULL;

    // make the last element
    // of the list to point
    // at the newly created node
    temp1->link=temp2;
   }
  }

  void display(void)
  {
   node *temp;
   temp=start;

   // traverse or process
   // through each element
   // and keep printing
   // the information
   while(temp!=NULL)
   {
    cout<<temp->info<<endl;
    temp=temp->link;
   }
  }

Good-Bye!

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