Queue - Data Structures and Algorithms

Queue is an abstract data structure, somewhat similar to Stacks. Unlike stacks, a queue is open at both its ends. One end is always used to insert data (enqueue) and the other is used to remove data (dequeue). Queue follows First-In-First-Out methodology, i.e., the data item stored first will be accessed first.

 

Queue Example

 

A real-world example of queue can be a single-lane one-way road, where the vehicle enters first, exits first. More real-world examples can be seen as queues at the ticket windows and bus-stops.

1. Queue Representation

As we now understand that in queue, we access both ends for different reasons. The following diagram given below tries to explain queue representation as data structure: 

 

Queue Example

 

As in stacks, a queue can also be implemented using Arrays, Linked-lists, Pointers and Structures. For the sake of simplicity, we shall implement queues using one-dimensional array.

2. Basic Operations

Queue operations may involve initializing or defining the queue, utilizing it, and then completely erasing it from the memory. Here we shall try to understand the basic operations associated with queues:

  • enqueue():  add (store) an item to the queue.

  • dequeue():  remove (access) an item from the queue.

Few more functions are required to make the above-mentioned queue operation efficient. These are: 

  • peek():  Gets the element at the front of the queue without removing it.

  • isfull():  Checks if the queue is full.

  • isempty():  Checks if the queue is empty.

In queue, we always dequeue (or access) data, pointed by front pointer and while enqueing (or storing) data in the queue we take help of rear pointer.

Let's first learn about supportive functions of a queue ?

2.1 peek()

This function helps to see the data at the front of the queue. The algorithm of peek() function is as follows: 

Algorithm

 

begin procedure peek

   return queue[front]

end procedure

 

Implementation of peek() function in C programming language:

Example

 

int peek() {

   return queue[front];

}

 

2.2 isfull()

As we are using single dimension array to implement queue, we just check for the rear pointer to reach at MAXSIZE to determine that the queue is full. In case we maintain the queue in a circular linked-list, the algorithm will differ. Algorithm of isfull() function ?

Algorithm

 

begin procedure isfull

 

   if rear equals to MAXSIZE

      return true

   else

      return false

   endif

  

end procedure

 

Implementation of isfull() function in C programming language:

Example

 

bool isfull() {

   if(rear == MAXSIZE - 1)

      return true;

   else

      return false;

}

 

2.3 isempty()

Algorithm of isempty() function:

Algorithm

 

begin procedure isempty

 

   if front is less than MIN  OR front is greater than rear

      return true

   else

      return false

   endif

  

end procedure

 

If the value of front is less than MIN or 0, it tells that the queue is not yet initialized, hence empty.

Here's the C programming code: 

Example

 

bool isempty() {

   if(front < 0 || front > rear)

      return true;

   else

      return false;

}

 

3. Enqueue Operation

Queues maintain two data pointers, front and rear. Therefore, its operations are comparatively difficult to implement than that of stacks.

The following steps should be taken to enqueue (insert) data into a queue:

  • Step 1  Check if the queue is full.

  • Step 2  If the queue is full, produce overflow error and exit.

  • Step 3  If the queue is not full, increment rear pointer to point the next empty space.

  • Step 4  Add data element to the queue location, where the rear is pointing.

  • Step 5  return success.

 

Insert Operation

 

Sometimes, we also check to see if a queue is initialized or not, to handle any unforeseen situations.

Algorithm for enqueue operation

 

procedure enqueue(data)     

  

   if queue is full

      return overflow

   endif

  

   rear ? rear + 1

   queue[rear] ? data

   return true

  

end procedure

 

Implementation of enqueue() in C programming language:

Example

 

int enqueue(int data)     

   if(isfull())

      return 0;

  

   rear = rear + 1;

   queue[rear] = data;

  

   return 1;

end procedure

 

4. Dequeue Operation

Accessing data from the queue is a process of two tasks ? access the data where front is pointing and remove the data after access. The following steps are taken to perform dequeue operation ?

  • Step 1  Check if the queue is empty.

  • Step 2 If the queue is empty, produce underflow error and exit.

  • Step 3 If the queue is not empty, access the data where front is pointing.

  • Step 4 Increment front pointer to point to the next available data element.

  • Step 5 Return success.

 

Remove Operation

 

Algorithm for dequeue operation

 

procedure dequeue

  

   if queue is empty

      return underflow

   end if

 

   data = queue[front]

   front ? front + 1

   return true

 

end procedure

 

Implementation of dequeue() in C programming language 

Example:

 

int dequeue() {

if(isempty())

return 0;

 

int data = queue[front];

front = front + 1;

 

return data;

}