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Introduction to Linked Queues

In one of the article Introduction to Linked Stacks, I said that representing data structures such as Stacks and Queues as arrays had one major problem that it can’t have more than a predefined number of elements. To overcome this we used linked lists to represent stacks. In this article we’ll use linked lists to represent queues.

Below are some graphics that illustrate the addition and retrieval of elements to and from the linked queue.

Addition of elements in the linked queue

FIG.: Addition of data to the linked queue

FIG.: Retrieval of elements from the linked queue

I don’t think there is anything more that needs to be discussed, so let’s have a look at the example program:

  // -- Linked Queues --
  // C++ Example Program to
  // illustrate the representation
  // of queues as linked lists
  #include<iostream.h>

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

  // declare global objects
  node *front=NULL;
  node *rear=NULL;

  // function prototypes
  void add(int);
  int retrieve();
  void free();

  void main(void)
  {
   int ch=0,num;

   while(ch!=3)
   {
    cout<<"1> Add";
    cout<<"\n2> Retrieve";
    cout<<"\n3> Quit\n";

    cin>>ch;

    switch(ch)
    {
     case 1:
       cout<<"enter element:";
       cin>>num;

       add(num);
       break;

     case 2:
       cout<<"\n\Retrieved: ";
       cout<<retrieve();
       cout<<"\n\n";
       break;
    }
   }

   // free up the memory
   free();
  }

  // function to add new nodes
  // to the linked queue
  void add(int inf)
  {
   node *temp;

   temp=new node;

   temp->info=inf;
   temp->link=NULL;

   if(front==NULL)
   {
    rear=front=temp;
    return;
   }

   rear->link=temp;
   rear=rear->link;
  }

  // function to retrieve
  // data from the linked
  // queue
  int retrieve()
  {
   node *temp;
   int inf;

   if(front==NULL)
   {
    cout<<"Queue Empty!\n";
    return NULL;
   }

   inf=front->info;
   temp=front;
   front=front->link;

   delete temp;
   return inf;
  }

  // free the dynamic memory
  // allocated in the form of
  // nodes of the linked queue
  void free(void)
  {
   // below is a bit confusing
   // part.
   // here all the nodes that
   // we have allocated are
   // being freed up
   node temp;
   while(front!=NULL)
   {
    // store the next node
    // to the one being deleted
    temp=*front;
    // delete the node
    delete front;

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

Good-Bye!

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