Circular Queue Data Structure

In this tutorial, you will learn what a circular queue is. Additionally, you will discover the implementation of the circular queue in C, C++, Java, and Python.

What is a Circular Queue?

A circular queue is like a linear queue as it is likewise based on the FIFO (First In First Out) rule aside from that the last position is associated with the first position in a circular queue that forms a circle. It is otherwise called a Ring Buffer.

A circular queue avoids the wastage of space in a regular queue implementation using arrays.

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As you can see in the above picture, after a touch of enqueuing and dequeuing, the size of the queue has been reduced.

The indexes 0 and 1 can only be used after the queue is reset when all the elements have been dequeued.

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How Circular Queue Works

Circular Queue works by the process of circular increment for example at the point when we attempt to increment the pointer and we arrive at the finish of the queue we start from the beginning point of the queue.

Here, the circular increment is performed by modulo division with the queue size. That is,

if REAR + 1 == 5 (overflow!), REAR = (REAR + 1)%5 = 0 (start of queue)

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Circular Queues Operations

The circulars queue function as follows:

• two pointers FRONT and REAR
• FRONT track the primary element of the queue
• REAR track the last elements of the queue
• initially, set the value of FRONT and REAR to – 1

1. Enqueue Operation

• check if the queue is full
• for the first element, set the value of FRONT to 0
• circularly increase the REAR index by 1 (i.e. if the rear reaches the end, next it would be at the start of the queue)
• add the new element in the position pointed to by REAR

2. Dequeue Operation

• check if the queue is empty
• return the value pointed by FRONT
• circularly increase the FRONT index by 1
• for the last element, reset the values of FRONT and REAR to -1

In any case, the check for full line has another extra case:

However, the check for full queue has a new additional case:

• Case 1: FRONT = 0 && REAR == SIZE – 1
• Case 2: FRONT = REAR + 1

The subsequent case happens when REAR beginnings from 0 because of circular increment and when its worth is just 1 less than FRONT, the queue is full.

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Circular Queues Implementations in Python, Java, C, and C++

The most well-known queue implementation is using arrays, however, it can likewise be executed using lists.

Python

# Circular Queue implementation in Python


class MyCircularQueue():

    def __init__(self, k):
        self.k = k
        self.queue = [None] * k
        self.head = self.tail = -1

    # Insert an element into the circular queue
    def enqueue(self, data):

        if ((self.tail + 1) % self.k == self.head):
            print("The circular queue is full\n")

        elif (self.head == -1):
            self.head = 0
            self.tail = 0
            self.queue[self.tail] = data
        else:
            self.tail = (self.tail + 1) % self.k
            self.queue[self.tail] = data

    # Delete an element from the circular queue
    def dequeue(self):
        if (self.head == -1):
            print("The circular queue is empty\n")

        elif (self.head == self.tail):
            temp = self.queue[self.head]
            self.head = -1
            self.tail = -1
            return temp
        else:
            temp = self.queue[self.head]
            self.head = (self.head + 1) % self.k
            return temp

    def printCQueue(self):
        if(self.head == -1):
            print("No element in the circular queue")

        elif (self.tail >= self.head):
            for i in range(self.head, self.tail + 1):
                print(self.queue[i], end=" ")
            print()
        else:
            for i in range(self.head, self.k):
                print(self.queue[i], end=" ")
            for i in range(0, self.tail + 1):
                print(self.queue[i], end=" ")
            print()


# Your MyCircularQueue object will be instantiated and called as such:
obj = MyCircularQueue(5)
obj.enqueue(1)
obj.enqueue(2)
obj.enqueue(3)
obj.enqueue(4)
obj.enqueue(5)
print("Initial queue")
obj.printCQueue()

obj.dequeue()
print("After removing an element from the queue")
obj.printCQueue()

Java

// Circular Queue implementation in Java

public class CQueue {
  int SIZE = 5; // Size of Circular Queue
  int front, rear;
  int items[] = new int[SIZE];

  CQueue() {
    front = -1;
    rear = -1;
  }

  // Check if the queue is full
  boolean isFull() {
    if (front == 0 && rear == SIZE - 1) {
      return true;
    }
    if (front == rear + 1) {
      return true;
    }
    return false;
  }

  // Check if the queue is empty
  boolean isEmpty() {
    if (front == -1)
      return true;
    else
      return false;
  }

  // Adding an element
  void enQueue(int element) {
    if (isFull()) {
      System.out.println("Queue is full");
    } else {
      if (front == -1)
        front = 0;
      rear = (rear + 1) % SIZE;
      items[rear] = element;
      System.out.println("Inserted " + element);
    }
  }

  // Removing an element
  int deQueue() {
    int element;
    if (isEmpty()) {
      System.out.println("Queue is empty");
      return (-1);
    } else {
      element = items[front];
      if (front == rear) {
        front = -1;
        rear = -1;
      } /* Q has only one element, so we reset the queue after deleting it. */
      else {
        front = (front + 1) % SIZE;
      }
      return (element);
    }
  }

  void display() {
    /* Function to display status of Circular Queue */
    int i;
    if (isEmpty()) {
      System.out.println("Empty Queue");
    } else {
      System.out.println("Front -> " + front);
      System.out.println("Items -> ");
      for (i = front; i != rear; i = (i + 1) % SIZE)
        System.out.print(items[i] + " ");
      System.out.println(items[i]);
      System.out.println("Rear -> " + rear);
    }
  }

  public static void main(String[] args) {

    CQueue q = new CQueue();

    // Fails because front = -1
    q.deQueue();

    q.enQueue(1);
    q.enQueue(2);
    q.enQueue(3);
    q.enQueue(4);
    q.enQueue(5);

    // Fails to enqueue because front == 0 && rear == SIZE - 1
    q.enQueue(6);

    q.display();

    int elem = q.deQueue();

    if (elem != -1) {
      System.out.println("Deleted Element is " + elem);
    }
    q.display();

    q.enQueue(7);

    q.display();

    // Fails to enqueue because front == rear + 1
    q.enQueue(8);
  }

}

C

// Circular Queue implementation in C

#include <stdio.h>

#define SIZE 5

int items[SIZE];
int front = -1, rear = -1;

// Check if the queue is full
int isFull() {
  if ((front == rear + 1) || (front == 0 && rear == SIZE - 1)) return 1;
  return 0;
}

// Check if the queue is empty
int isEmpty() {
  if (front == -1) return 1;
  return 0;
}

// Adding an element
void enQueue(int element) {
  if (isFull())
    printf("\n Queue is full!! \n");
  else {
    if (front == -1) front = 0;
    rear = (rear + 1) % SIZE;
    items[rear] = element;
    printf("\n Inserted -> %d", element);
  }
}

// Removing an element
int deQueue() {
  int element;
  if (isEmpty()) {
    printf("\n Queue is empty !! \n");
    return (-1);
  } else {
    element = items[front];
    if (front == rear) {
      front = -1;
      rear = -1;
    } 
    // Q has only one element, so we reset the 
    // queue after dequeing it. ?
    else {
      front = (front + 1) % SIZE;
    }
    printf("\n Deleted element -> %d \n", element);
    return (element);
  }
}

// Display the queue
void display() {
  int i;
  if (isEmpty())
    printf(" \n Empty Queue\n");
  else {
    printf("\n Front -> %d ", front);
    printf("\n Items -> ");
    for (i = front; i != rear; i = (i + 1) % SIZE) {
      printf("%d ", items[i]);
    }
    printf("%d ", items[i]);
    printf("\n Rear -> %d \n", rear);
  }
}

int main() {
  // Fails because front = -1
  deQueue();

  enQueue(1);
  enQueue(2);
  enQueue(3);
  enQueue(4);
  enQueue(5);

  // Fails to enqueue because front == 0 && rear == SIZE - 1
  enQueue(6);

  display();
  deQueue();

  display();

  enQueue(7);
  display();

  // Fails to enqueue because front == rear + 1
  enQueue(8);

  return 0;
}

C++

// Circular Queue implementation in C++

#include <iostream>
#define SIZE 5 /* Size of Circular Queue */

using namespace std;

class Queue {
   private:
  int items[SIZE], front, rear;

   public:
  Queue() {
    front = -1;
    rear = -1;
  }
  // Check if the queue is full
  bool isFull() {
    if (front == 0 && rear == SIZE - 1) {
      return true;
    }
    if (front == rear + 1) {
      return true;
    }
    return false;
  }
  // Check if the queue is empty
  bool isEmpty() {
    if (front == -1)
      return true;
    else
      return false;
  }
  // Adding an element
  void enQueue(int element) {
    if (isFull()) {
      cout << "Queue is full";
    } else {
      if (front == -1) front = 0;
      rear = (rear + 1) % SIZE;
      items[rear] = element;
      cout << endl
         << "Inserted " << element << endl;
    }
  }
  // Removing an element
  int deQueue() {
    int element;
    if (isEmpty()) {
      cout << "Queue is empty" << endl;
      return (-1);
    } else {
      element = items[front];
      if (front == rear) {
        front = -1;
        rear = -1;
      }
      // Q has only one element,
      // so we reset the queue after deleting it.
      else {
        front = (front + 1) % SIZE;
      }
      return (element);
    }
  }

  void display() {
    // Function to display status of Circular Queue
    int i;
    if (isEmpty()) {
      cout << endl
         << "Empty Queue" << endl;
    } else {
      cout << "Front -> " << front;
      cout << endl
         << "Items -> ";
      for (i = front; i != rear; i = (i + 1) % SIZE)
        cout << items[i];
      cout << items[i];
      cout << endl
         << "Rear -> " << rear;
    }
  }
};

int main() {
  Queue q;

  // Fails because front = -1
  q.deQueue();

  q.enQueue(1);
  q.enQueue(2);
  q.enQueue(3);
  q.enQueue(4);
  q.enQueue(5);

  // Fails to enqueue because front == 0 && rear == SIZE - 1
  q.enQueue(6);

  q.display();

  int elem = q.deQueue();

  if (elem != -1)
    cout << endl
       << "Deleted Element is " << elem;

  q.display();

  q.enQueue(7);

  q.display();

  // Fails to enqueue because front == rear + 1
  q.enQueue(8);

  return 0;
}

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Circular Queues Complexity Analysis

The complexity of the enqueue and dequeue activities of a circular queue is O(1) for (array implementations).

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Applications of Circular Queues

CPU scheduling
Memory management
Traffic Management

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Data Structures (I)