Learn everything you need to know about the queue data structure, from implementation to applications. This comprehensive guide will help you master this powerful tool for solving real-world problems.
A queue is a useful data structure in programming. It is similar to the ticket queue outside a cinema hall, where the first person entering the queue is the first person who gets the ticket.
Queue follows the First In First Out (FIFO) rule - the item that goes in first is the item that comes out first.
FIFO Representation of Queue
In the above image, since 1 was kept in the queue before 2, it is the first to be removed from the queue as well. It follows the FIFO rule.
In programming terms, putting items in the queue is called enqueue, and removing items from the queue is called dequeue.
We can implement the queue in any programming language like C, C++, Java, Python or C#, but the specification is pretty much the same.
A queue is an object (an abstract data structure - ADT) that allows the following operations:
Queue operations work as follows:
Enqueue and Dequeue Operations
We usually use arrays to implement queues in Java and C/++. In the case of Python, we use lists.
Python:
# Queue implementation in Python
class Queue:
def __init__(self):
self.queue = []
# Add an element
def enqueue(self, item):
self.queue.append(item)
# Remove an element
def dequeue(self):
if len(self.queue) < 1:
return None
return self.queue.pop(0)
# Display the queue
def display(self):
print(self.queue)
def size(self):
return len(self.queue)
q = Queue()
q.enqueue(1)
q.enqueue(2)
q.enqueue(3)
q.enqueue(4)
q.enqueue(5)
q.display()
q.dequeue()
print("After removing an element")
q.display()
Java:
// Queue implementation in Java
public class Queue {
int SIZE = 5;
int items[] = new int[SIZE];
int front, rear;
Queue() {
front = -1;
rear = -1;
}
boolean isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
boolean isEmpty() {
if (front == -1)
return true;
else
return false;
}
void enQueue(int element) {
if (isFull()) {
System.out.println("Queue is full");
} else {
if (front == -1)
front = 0;
rear++;
items[rear] = element;
System.out.println("Inserted " + 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++;
}
System.out.println("Deleted -> " + element);
return (element);
}
}
void display() {
/* Function to display elements of Queue */
int i;
if (isEmpty()) {
System.out.println("Empty Queue");
} else {
System.out.println("\nFront index-> " + front);
System.out.println("Items -> ");
for (i = front; i <= rear; i++)
System.out.print(items[i] + " ");
System.out.println("\nRear index-> " + rear);
}
}
public static void main(String[] args) {
Queue q = new Queue();
// deQueue is not possible on empty queue
q.deQueue();
// enQueue 5 elements
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
q.enQueue(5);
// 6th element can't be added to because the queue is full
q.enQueue(6);
q.display();
// deQueue removes element entered first i.e. 1
q.deQueue();
// Now we have just 4 elements
q.display();
}
}
C Programming:
// Queue implementation in C
#include <stdio.h>
#define SIZE 5
void enQueue(int);
void deQueue();
void display();
int items[SIZE], front = -1, rear = -1;
int main() {
//deQueue is not possible on empty queue
deQueue();
//enQueue 5 elements
enQueue(1);
enQueue(2);
enQueue(3);
enQueue(4);
enQueue(5);
// 6th element can't be added to because the queue is full
enQueue(6);
display();
//deQueue removes element entered first i.e. 1
deQueue();
//Now we have just 4 elements
display();
return 0;
}
void enQueue(int value) {
if (rear == SIZE - 1)
printf("\nQueue is Full!!");
else {
if (front == -1)
front = 0;
rear++;
items[rear] = value;
printf("\nInserted -> %d", value);
}
}
void deQueue() {
if (front == -1)
printf("\nQueue is Empty!!");
else {
printf("\nDeleted : %d", items[front]);
front++;
if (front > rear)
front = rear = -1;
}
}
// Function to print the queue
void display() {
if (rear == -1)
printf("\nQueue is Empty!!!");
else {
int i;
printf("\nQueue elements are:\n");
for (i = front; i <= rear; i++)
printf("%d ", items[i]);
}
printf("\n");
}
C++:
// Queue implementation in C++
#include <iostream>
#define SIZE 5
using namespace std;
class Queue {
private:
int items[SIZE], front, rear;
public:
Queue() {
front = -1;
rear = -1;
}
bool isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
bool isEmpty() {
if (front == -1)
return true;
else
return false;
}
void enQueue(int element) {
if (isFull()) {
cout << "Queue is full";
} else {
if (front == -1) front = 0;
rear++;
items[rear] = element;
cout << endl
<< "Inserted " << element << endl;
}
}
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++;
}
cout << endl
<< "Deleted -> " << element << endl;
return (element);
}
}
void display() {
/* Function to display elements of Queue */
int i;
if (isEmpty()) {
cout << endl
<< "Empty Queue" << endl;
} else {
cout << endl
<< "Front index-> " << front;
cout << endl
<< "Items -> ";
for (i = front; i <= rear; i++)
cout << items[i] << " ";
cout << endl
<< "Rear index-> " << rear << endl;
}
}
};
int main() {
Queue q;
//deQueue is not possible on empty queue
q.deQueue();
//enQueue 5 elements
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
q.enQueue(5);
// 6th element can't be added to because the queue is full
q.enQueue(6);
q.display();
//deQueue removes element entered first i.e. 1
q.deQueue();
//Now we have just 4 elements
q.display();
return 0;
}
As you can see in the image below, after a bit of enqueuing and dequeuing, the size of the queue has been reduced.
Limitation of a queue
And we can only add indexes 0 and 1 only when the queue is reset (when all the elements have been dequeued).
After REAR reaches the last index, if we can store extra elements in the empty spaces (0 and 1), we can make use of the empty spaces. This is implemented by a modified queue called the circular queue.
The complexity of enqueue and dequeue operations in a queue using an array is O(1)
. If you use pop(N)
in python code, then the complexity might be O(n)
depending on the position of the item to be popped.
#datastructures #algorithms