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Circular Queue Data Structure

Circular Queue

Circular queue is a modified form of queue in which the front and rear pointers move circularly.
Instead of starting from -1, now front and rear start from 0.

Representation of circular queue:

circular queue

class Queue{
   int arr[50]
   int front, rear;
   int size;
};

  Queue(int n)
    {
    front = 0;
    rear  = 0;
    size =n; 
    }

Operations

To make front and rear move circularly, after rear reaches (size-1 ) position, it must be reset to 0.
To do this, '%' is used.
Working of mod operator can be seen:
Let size = 5.
| Rear = | (Rear+1)%size |
|:–:|:–:|
| 0 | (0+1)%7 = 1 |
| 1 | (1+1)%7 =2|
| 2 | (2+1)%7 =3|
| : | : |
| 4 | (4+1)%5 =0 |

Hence when rear points to (size-1), it gets reset to 0.

Working of circular queue :

working of circular queue

Enqueue

  • Elements are inserted from the rear end as usual.
  • If rear reaches (size-1), it is reset to zero
  • Insertion can't be done at the index where front is pointing

Only (size-1) spaces can be utilized in Circular queue.

void enqueue(int x)
 {
     if((rear+1)%size == front)   
          print("Queue is Full");
     else 
    { 
     rear=(rear+1)%size;      
     arr[rear]=x;     
    }
}

Dequeue

int dequeue()
 {   
  int x=-1;     
if(front==rear)  
      print("Queue is Empty") 
else
    {     
       front=(front+1)%size; 
        x=arr[front];  
   }    
   return x; 
   }

Display

void Display() 
{    int i= front+1; 
     do    
     {           
       print(arr[i]);  
       i=(i+1)%size;   
     }while(i!=(rear+1)%size); 
}

Complexity for all operations on circular queue is O(1).

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