# Circular Queue | Set 1 (Introduction and Array Implementation)

Prerequisite – Queues

Circular Queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called ‘Ring Buffer’.

In a normal Queue, we can insert elements until queue becomes full. But once queue becomes full, we can not insert the next element even if there is a space in front of queue.

Operations on Circular Queue:

• Front: Get the front item from queue.
• Rear: Get the last item from queue.
• enQueue(value) This function is used to insert an element into the circular queue. In a circular queue, the new element is always inserted at Rear position.
Steps:

1. Check whether queue is Full – Check ((rear == SIZE-1 && front == 0) || (rear == front-1)).
2. If it is full then display Queue is full. If queue is not full then, check if (rear == SIZE – 1 && front != 0) if it is true then set rear=0 and insert element.
• deQueue() This function is used to delete an element from the circular queue. In a circular queue, the element is always deleted from front position.
Steps:

1. Check whether queue is Empty means check (front==-1).
2. If it is empty then display Queue is empty. If queue is not empty then step 3
3. Check if (front==rear) if it is true then set front=rear= -1 else check if (front==size-1), if it is true then set front=0 and return the element.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

// C or C++ program for insertion and
// deletion in Circular Queue
#include<bits/stdc++.h>
using namespace std;

struct Queue
{
// Initialize front and rear
int rear, front;

// Circular Queue
int size;
int *arr;

Queue(int s)
{
front = rear = -1;
size = s;
arr = new int[s];
}

void enQueue(int value);
int deQueue();
void displayQueue();
};

/* Function to create Circular queue */
void Queue::enQueue(int value)
{
if ((front == 0 && rear == size-1) ||
(rear == front-1))
{
printf("\nQueue is Full");
return;
}

else if (front == -1) /* Insert First Element */
{
front = rear = 0;
arr[rear] = value;
}

else if (rear == size-1 && front != 0)
{
rear = 0;
arr[rear] = value;
}

else
{
rear++;
arr[rear] = value;
}
}

// Function to delete element from Circular Queue
int Queue::deQueue()
{
if (front == -1)
{
printf("\nQueue is Empty");
return INT_MIN;
}

int data = arr[front];
arr[front] = -1;
if (front == rear)
{
front = -1;
rear = -1;
}
else if (front == size-1)
front = 0;
else
front++;

return data;
}

// Function displaying the elements
// of Circular Queue
void Queue::displayQueue()
{
if (front == -1)
{
printf("\nQueue is Empty");
return;
}
printf("\nElements in Circular Queue are: ");
if (rear >= front)
{
for (int i = front; i <= rear; i++)
printf("%d ",arr[i]);
}
else
{
for (int i = front; i < size; i++)
printf("%d ", arr[i]);

for (int i = 0; i <= rear; i++)
printf("%d ", arr[i]);
}
}

/* Driver of the program */
int main()
{
Queue q(5);

// Inserting elements in Circular Queue
q.enQueue(14);
q.enQueue(22);
q.enQueue(13);
q.enQueue(-6);

// Display elements present in Circular Queue
q.displayQueue();

// Deleting elements from Circular Queue
printf("\nDeleted value = %d", q.deQueue());
printf("\nDeleted value = %d", q.deQueue());

q.displayQueue();

q.enQueue(9);
q.enQueue(20);
q.enQueue(5);

q.displayQueue();

q.enQueue(20);
return 0;
}

Output:

Elements in Circular Queue are: 14 22 13 -6
Deleted value = 14
Deleted value = 22
Elements in Circular Queue are: 13 -6
Elements in Circular Queue are: 13 -6 9 20 5
Queue is Full

Time Complexity: Time complexity of enQueue(), deQueue() operation is O(1) as there is no loop in any of the operation.

Applications:

1. Memory Management: The unused memory locations in the case of ordinary queues can be utilized in circular queues.
2. Traffic system: In computer controlled traffic system, circular queues are used to switch on the traffic lights one by one repeatedly as per the time set.
3. CPU Scheduling: Operating systems often maintain a queue of processes that are ready to execute or that are waiting for a particular event to occur.

This article is contributed by Akash Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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