Types of Queue

Question

Explore the fascinating world of Types of Queues in computer science with our comprehensive guide. Learn about popular queue structures like FIFO and Priority Queues, their applications, and key concepts. Dive into the essential data structures that power efficient algorithms. Uncover the secrets of queue implementations and enhance your coding skills today.

Answer 1

A queue, mirroring its real-world counterpart, adheres to the FIFO (First-In-First-Out) principle, where the item that arrives first is the first to be processed or removed. In this data structure, elements are inserted at the rear end or tail, and deletion occurs from the front end or head. This parallels real-world scenarios, such as a ticket queue outside a cinema, where the first person in line receives a ticket before those who join later, aligning with the queue's orderly approach to processing elements.

Types of Queue

Before delving into the types of queues, let's begin with a brief introduction to the concept of queues in this article.

1. Simple Queue or Linear Queue

• A Simple Queue is a linear data structure that follows the First-In-First-Out (FIFO) principle.
• Elements are added at the rear (enqueue) and removed from the front (dequeue).
• Example operations: enqueue(), dequeue(), isEmpty(), isFull().

2. Circular Queue

• A Circular Queue is an extension of a Simple Queue with a circular structure.
• Enqueue and dequeue operations are performed in a circular fashion.
• Solves the limitation of space in a Simple Queue.
• Example operations: enqueue(), dequeue(), isFull(), isEmpty().

3. Priority Queue

• A Priority Queue assigns a priority to each element.
• Elements with higher priority are served before elements with lower priority.
• Operations include insert(), delete(), getHighestPriority().

4. Deque (Double Ended Queue)

• A Deque is a queue that allows insertion and deletion at both ends.
• It can be implemented as a linear structure or a circular structure.
• Operations: insertFront(), deleteFront(), insertRear(), deleteRear().

Operations Performed on Queue

1. Enqueue Operation

• Adds an element to the rear of the queue.
• In a circular queue, it may involve wrapping around to the front.

void enqueue(Queue* queue, int item);
 

2. Dequeue Operation

• Removes an element from the front of the queue.
• In a circular queue, it may involve wrapping around to the rear.

int dequeue(Queue* queue);
 

3. isEmpty Operation

• Checks if the queue is empty.

int isEmpty(Queue* queue);
 

4. isFull Operation

• Checks if the queue is full (for bounded queues).

int isFull(Queue* queue);
 

5. InsertFront Operation (Deque)

• Adds an element to the front of the deque.

void insertFront(Deque* deque, int item);
 

6. DeleteFront Operation (Deque)

• Removes an element from the front of the deque.

int deleteFront(Deque* deque);
 

7. InsertRear Operation (Deque)

• Adds an element to the rear of the deque.

void insertRear(Deque* deque, int item);
 

8. DeleteRear Operation (Deque)

• Removes an element from the rear of the deque.

int deleteRear(Deque* deque);
 

Ways to Implement a Queue

1. Array Implementation

• Use an array to store queue elements.
• Requires defining the maximum size of the queue.

2. Linked List Implementation

• Use a linked list to implement a dynamic queue.
• No need to define a maximum size.

3. Circular Array Implementation

• Similar to the array implementation but with circular indexing.
• Solves the issue of unused space in a simple array implementation.

Example Code

Here's a simplified example in C demonstrating a circular queue using an array:

#include <stdio.h>
#include <stdlib.h>

#define MAX_SIZE 5

typedef struct {
    int front, rear;
    int data[MAX_SIZE];
} CircularQueue;

void enqueue(CircularQueue* queue, int item) {
    if ((queue->rear + 1) % MAX_SIZE == queue->front) {
        printf("Queue is full. Cannot enqueue.\n");
        return;
    }
    queue->rear = (queue->rear + 1) % MAX_SIZE;
    queue->data[queue->rear] = item;
}

int dequeue(CircularQueue* queue) {
    if (queue->front == queue->rear) {
        printf("Queue is empty. Cannot dequeue.\n");
        return -1; // Assuming -1 represents an error or an invalid value
    }
    queue->front = (queue->front + 1) % MAX_SIZE;
    return queue->data[queue->front];
}

int main() {
    CircularQueue cq = {0, 0};

    enqueue(&cq, 1);
    enqueue(&cq, 2);
    enqueue(&cq, 3);

    printf("Dequeued: %d\n", dequeue(&cq));
    printf("Dequeued: %d\n", dequeue(&cq));

    enqueue(&cq, 4);
    enqueue(&cq, 5);
    enqueue(&cq, 6); // This will display an error message since the queue is full.

    return 0;
}
 

This example demonstrates a simple circular queue using an array, with enqueue and dequeue operations.

Answer 2

FAQs on Types of Queue

Q: What is a Queue?

A: A queue is a data structure that follows the First In, First Out (FIFO) principle. It means that the element that is added first is the one that is removed first.

Q: What are the common operations on a Queue?

A: Common operations on a queue include enqueue (adding an element to the end of the queue) and dequeue (removing an element from the front of the queue).

Q: What are the types of queues based on usage?

A: Queues can be categorized into two main types:

• Linear Queue: A simple queue where elements are added at the rear and removed from the front.
• Circular Queue: A variation of the linear queue where the rear and front are connected, forming a circle.

Q: What is a Priority Queue?

A: A priority queue is a type of queue in which each element has an associated priority, and elements with higher priority are dequeued before elements with lower priority.

Q: How is a Double-ended Queue (Deque) different from a regular queue?

A: A deque allows insertion and deletion at both ends, i.e., front and rear, providing more flexibility than a regular queue.

Q: What is a Blocking Queue?

A: A blocking queue is a queue that blocks or waits when trying to dequeue from an empty queue or enqueue into a full queue.

Q: How is a LIFO Queue different from a regular queue?

A: A LIFO (Last In, First Out) queue, also known as a stack, follows the principle of the last element added being the first one to be removed.

Q: Explain the concept of a Priority Deque.

A: A priority deque is a double-ended queue where each element has a priority, and elements with higher priority are dequeued first, regardless of whether the deque operation is performed at the front or rear.

Q: What is a Delay Queue?

A: A delay queue is a specialized queue where each element has an associated delay time, and elements are dequeued only when their delay time expires.

Q: Can you give an example of real-life applications of queues?

A: Queues are commonly used in computer science and real-world scenarios such as task scheduling, print job management, call centers, and traffic management systems.

Important Interview Questions and Answers on Types of Queue

Q: What is a Queue?

A queue is a linear data structure that follows the First In, First Out (FIFO) principle, where the element that is added first is the one that is removed first.

Q: What are the types of Queues?

There are various types of queues, including:

• Linear Queue: Elements are added at one end and removed from the other end.
• Circular Queue: The last element is connected to the first element to form a circle.
• Priority Queue: Elements have priorities and are processed based on their priority.
• Double-ended Queue (Deque): Elements can be added or removed from both ends.

Q: Explain a Linear Queue with Example Code in Python.

Here is the code.

class LinearQueue:
    def __init__(self):
        self.queue = []

    def enqueue(self, item):
        self.queue.append(item)

    def dequeue(self):
        if not self.is_empty():
            return self.queue.pop(0)
        else:
            return "Queue is empty"

    def is_empty(self):
        return len(self.queue) == 0

    def size(self):
        return len(self.queue)

# Example Usage
linear_queue = LinearQueue()
linear_queue.enqueue(10)
linear_queue.enqueue(20)
linear_queue.enqueue(30)
print("Dequeued:", linear_queue.dequeue())
print("Size of Queue:", linear_queue.size())
 

Q: Explain a Circular Queue with Example Code in Python.

Here is the code

class CircularQueue:
    def __init__(self, size):
        self.queue = [None] * size
        self.front = self.rear = -1
        self.size = size

    def enqueue(self, item):
        if (self.rear + 1) % self.size == self.front:
            print("Queue is full")
        elif self.front == -1:
            self.front = self.rear = 0
            self.queue[self.rear] = item
        else:
            self.rear = (self.rear + 1) % self.size
            self.queue[self.rear] = item

    def dequeue(self):
        if self.front == -1:
            print("Queue is empty")
        elif self.front == self.rear:
            item = self.queue[self.front]
            self.front = self.rear = -1
            return item
        else:
            item = self.queue[self.front]
            self.front = (self.front + 1) % self.size
            return item

    def is_empty(self):
        return self.front == -1

    def is_full(self):
        return (self.rear + 1) % self.size == self.front

# Example Usage
circular_queue = CircularQueue(3)
circular_queue.enqueue(10)
circular_queue.enqueue(20)
circular_queue.enqueue(30)
print("Dequeued:", circular_queue.dequeue())
 

Q: Explain a Priority Queue with Example Code in Python.

Here is the code

import heapq

class PriorityQueue:
    def __init__(self):
        self.queue = []

    def enqueue(self, item, priority):
        heapq.heappush(self.queue, (priority, item))

    def dequeue(self):
        if not self.is_empty():
            return heapq.heappop(self.queue)[1]
        else:
            return "Queue is empty"

    def is_empty(self):
        return len(self.queue) == 0

    def size(self):
        return len(self.queue)

# Example Usage
priority_queue = PriorityQueue()
priority_queue.enqueue("Task 1", 2)
priority_queue.enqueue("Task 2", 1)
priority_queue.enqueue("Task 3", 3)
print("Dequeued:", priority_queue.dequeue())
 

Q: Explain a Double-ended Queue (Deque) with Example Code in Python.

Here is the code

from collections import deque

class Deque:
    def __init__(self):
        self.deque = deque()

    def append_left(self, item):
        self.deque.appendleft(item)

    def append_right(self, item):
        self.deque.append(item)

    def pop_left(self):
        if not self.is_empty():
            return self.deque.popleft()
        else:
            return "Deque is empty"

    def pop_right(self):
        if not self.is_empty():
            return self.deque.pop()
        else:
            return "Deque is empty"

    def is_empty(self):
        return len(self.deque) == 0

    def size(self):
        return len(self.deque)

# Example Usage
deque_instance = Deque()
deque_instance.append_left(10)
deque_instance.append_right(20)
deque_instance.append_left(5)
print("Pop left:", deque_instance.pop_left())
print("Pop right:", deque_instance.pop_right())