Array implementation of queue (Simple)

In queue, insertion and deletion happen at the opposite ends, so implementation is not as simple as stack.
To implement a queue using array, create an array arr of size n and take two variables front and rear both of which will be initialized to 0 which means the queue is currently empty. Element rear is the index upto which the elements are stored in the array and front is the index of the first element of the array. Now, some of the implementation of queue operations are as follows:

  1. Enqueue: Addition of an element to the queue. Adding an element will be performed after checking whether the queue is full or not. If rear < n which indicates that the array is not full then store the element at arr[rear] and increment rear by 1 but if rear == n then it is said to be an Overflow condition as the array is full.
  2. Dequeue: Removal of an element from the queue. An element can only be deleted when there is at least an element to delete i.e. rear > 0. Now, element at arr[front] can be deleted but all the remaining elements have to shifted to the left by one position in order for the dequeue operation to delete the second element from the left on another dequeue operation.
  3. Front: Get the front element from the queue i.e. arr[front] if queue is not empty.
  4. Display: Print all element of the queue. If the queue is non-empty, traverse and print all the elements from index front to rear.

Below is the implementation of a queue using an array:

C++

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// C++ program to implement a queue using an array
#include <bits/stdc++.h>
using namespace std;
  
struct Queue {
    int front, rear, capacity;
    int* queue;
    Queue(int c)
    {
        front = rear = 0;
        capacity = c;
        queue = new int;
    }
  
    ~Queue() { delete[] queue; }
  
    // function to insert an element
    // at the rear of the queue
    void queueEnqueue(int data)
    {
        // check queue is full or not
        if (capacity == rear) {
            printf("\nQueue is full\n");
            return;
        }
  
        // insert element at the rear
        else {
            queue[rear] = data;
            rear++;
        }
        return;
    }
  
    // function to delete an element
    // from the front of the queue
    void queueDequeue()
    {
        // if queue is empty
        if (front == rear) {
            printf("\nQueue is  empty\n");
            return;
        }
  
        // shift all the elements from index 2 till rear
        // to the left by one
        else {
            for (int i = 0; i < rear - 1; i++) {
                queue[i] = queue[i + 1];
            }
  
            // decrement rear
            rear--;
        }
        return;
    }
  
    // print queue elements
    void queueDisplay()
    {
        int i;
        if (front == rear) {
            printf("\nQueue is Empty\n");
            return;
        }
  
        // traverse front to rear and print elements
        for (i = front; i < rear; i++) {
            printf(" %d <-- ", queue[i]);
        }
        return;
    }
  
    // print front of queue
    void queueFront()
    {
        if (front == rear) {
            printf("\nQueue is Empty\n");
            return;
        }
        printf("\nFront Element is: %d", queue[front]);
        return;
    }
};
  
// Driver code
int main(void)
{
    // Create a queue of capacity 5
    Queue q(4);
  
    // print Queue elements
    q.queueDisplay();
  
    // inserting elements in the queue
    q.queueEnqueue(20);
    q.queueEnqueue(30);
    q.queueEnqueue(40);
    q.queueEnqueue(50);
  
    // print Queue elements
    q.queueDisplay();
  
    // insert element in the queue
    q.queueEnqueue(60);
  
    // print Queue elements
    q.queueDisplay();
  
    q.queueDequeue();
    q.queueDequeue();
  
    printf("\n\nafter two node deletion\n\n");
  
    // print Queue elements
    q.queueDisplay();
  
    // print front of the queue
    q.queueFront();
  
    return 0;
}

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JAVA

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// Java program to implement a queue using an array
class Queue {
    private static int front, rear, capacity;
    private static int queue[];
  
    Queue(int c)
    {
        front = rear = 0;
        capacity = c;
        queue = new int[capacity];
    }
  
    // function to insert an element
    // at the rear of the queue
    static void queueEnqueue(int data)
    {
        // check queue is full or not
        if (capacity == rear) {
            System.out.printf("\nQueue is full\n");
            return;
        }
  
        // insert element at the rear
        else {
            queue[rear] = data;
            rear++;
        }
        return;
    }
  
    // function to delete an element
    // from the front of the queue
    static void queueDequeue()
    {
        // if queue is empty
        if (front == rear) {
            System.out.printf("\nQueue is empty\n");
            return;
        }
  
        // shift all the elements from index 2 till rear
        // to the right by one
        else {
            for (int i = 0; i < rear - 1; i++) {
                queue[i] = queue[i + 1];
            }
  
            // store 0 at rear indicating there's no element
            if (rear < capacity)
                queue[rear] = 0;
  
            // decrement rear
            rear--;
        }
        return;
    }
  
    // print queue elements
    static void queueDisplay()
    {
        int i;
        if (front == rear) {
            System.out.printf("\nQueue is Empty\n");
            return;
        }
  
        // traverse front to rear and print elements
        for (i = front; i < rear; i++) {
            System.out.printf(" %d <-- ", queue[i]);
        }
        return;
    }
  
    // print front of queue
    static void queueFront()
    {
        if (front == rear) {
            System.out.printf("\nQueue is Empty\n");
            return;
        }
        System.out.printf("\nFront Element is: %d", queue[front]);
        return;
    }
}
  
public class StaticQueueinjava {
  
    // Driver code
    public static void main(String[] args)
    {
        // Create a queue of capacity 4
        Queue q = new Queue(4);
  
        // print Queue elements
        q.queueDisplay();
  
        // inserting elements in the queue
        q.queueEnqueue(20);
        q.queueEnqueue(30);
        q.queueEnqueue(40);
        q.queueEnqueue(50);
  
        // print Queue elements
        q.queueDisplay();
  
        // insert element in the queue
        q.queueEnqueue(60);
  
        // print Queue elements
        q.queueDisplay();
  
        q.queueDequeue();
        q.queueDequeue();
        System.out.printf("\n\nafter two node deletion\n\n");
  
        // print Queue elements
        q.queueDisplay();
  
        // print front of the queue
        q.queueFront();
    }
}

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Output:

Queue is Empty
 20 <--  30 <--  40 <--  50 <-- 
Queue is full
 20 <--  30 <--  40 <--  50 <-- 

after two node deletion

 40 <--  50 <-- 
Front Element is: 40

Time Complexity of Enqueue : O(1)
Time Complexity of Dequeue : O(n)

Optimizations :
We can implement both enqueue and dequeue operations in O(1) time. To achieve this, we can either use Linked List Implementation of Queue or circular array implementation of queue.



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