Queue

QUEUE

What is Queue ?

Whenever I am asked about a queue i am reminded of the boring queue that I use to stand in to pay my bills .And yes its same in programming languages too though here its not boring . If I am first standing in a Queue I would be the first one to pay my bill and get out of the line . This makes the queue a FIFO(First In First Out) operation,the first element added to queue would be the first one to be removed .

(Example of a Queue at a booth)

(FIFO operations done on a queue)

It is just so simple to understand always remember the general idea behind it and you could use in programming.
Now I would put some light on the FIFO operations in programming .
There are two types of operations that can be performed on a queue:-
1. Enqueue - Insertion/Pushing of element 
2. Dequeue - Deletion/Poping of element 

Considering a array the enqueue always done at end and dequeue from the front

I have taken two counters(front & rear) representing front of the queue and rear/end of the queue

ENQUEUE operation on the  queue(Insertion of new element)

In order to insert a element in a queue we need to check if the queue is full or not in order to do so we check if rear pointer has reached the INIT_CAPACITY of our array by

rear<INIT_CAPACITY-1

If condition is satisfied we insert an element in the array by

arr[++rear]=data

else we print "Queue is Full"

DEQUEUE operation from the  queue(Delete a element from queue)

In order to delete a element from a queue we need to check if the queue is empty or not because if we delete a element from a empty queue it would throw an exception that should be handled . 

In order to do so we check if our pointer has reached front=-1 or does the front pointer has a higher value than rear pointer 

i.e.

front==-1 || front>rear

If condition satisfies we handle the exception as queue is empty .

If not we remove the element from the queue by returning the element 

return arr[front++]

 

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/* * Queue using an array * @ Author Karan Deep Singh * */ public class Queue { private static int INIT_CAPACITY = 5; int arr[]; int front; int rear; public Queue() { arr = new int[INIT_CAPACITY]; front = 0; rear = -1; } // Enqueue/Insert elements in queue by checking if queue is full or not private void enqueue(int data) { if(rear<INIT_CAPACITY-1) arr[++rear] = data; else System.out.println("Queue is Full"); } //Dequeue/Delete elements from queue by checking if queue is empty or not private int dequeue()throws Exception { if(front==-1 || front > rear) throw new Exception("Queue is Empty"); return arr[front++]; } public static void main(String[] args) throws Exception { Queue queue = new Queue(); queue.enqueue(10); queue.enqueue(12); queue.enqueue(13); queue.enqueue(14); queue.enqueue(15); queue.enqueue(16); queue.enqueue(17); queue.enqueue(18); System.out.println(queue.dequeue()); System.out.println(queue.dequeue()); System.out.println(queue.dequeue()); System.out.println(queue.dequeue()); System.out.println(queue.dequeue()); System.out.println(queue.dequeue()); } }