🚶‍♂️🚶‍♂️🚶‍♂️ DSA using Python: QUEUE 🚶‍♀️🚶‍♀️🚶‍♀️

Queue

🧾 What is a Queue?

A Queue is a linear data structure that follows the FIFO (First In, First Out) rule.
This means the element that is inserted first will be removed first — just like a line of people waiting for a ticket.

🧠 Real-life Example:

Like standing in a queue at a bus stop — the person who comes first gets on the bus first.

✅ Basic Queue Operations

Operation	Meaning
enqueue()	Add (insert) an element to the rear (end) of the queue
dequeue()	Remove an element from the front of the queue
peek() or front()	Look at the front element without removing it
isEmpty()	Check if the queue has no elements
isFull() (for fixed-size queues)	Check if the queue is full

💡 Queue Implementation in Python using List

🔷 1. enqueue() – Add element to the end

queue = [] queue.append(10) queue.append(20) queue.append(30) print("Queue after enqueue:", queue)

📌 Output:

Queue after enqueue: [10, 20, 30]

🔷 2. dequeue() – Remove element from the front

if queue: removed = queue.pop(0) print("Removed element:", removed) print("Queue after dequeue:", queue) else: print("Queue is empty")

📌 Output:

Removed element: 10 Queue after dequeue: [20, 30]

🔷 3. peek() – View the front element

if queue: print("Front element is:", queue[0]) else: print("Queue is empty")

📌 Output:

Front element is: 20

🔷 4. isEmpty() – Check if queue is empty

if not queue: print("Queue is empty") else: print("Queue is not empty")

📌 Output:

Queue is not empty

🔷 5. isFull() – (Used only when queue size is fixed)

Let’s say we fix size to 3:

MAX_SIZE = 3
if len(queue) == MAX_SIZE:
    print("Queue is full")
else:
    print("Queue is not full")

📌 Output (if queue = [20, 30]):

Queue is not full

🧠 Summary:

• Queue = first in, first out (FIFO)

• Main operations: enqueue, dequeue, peek, isEmpty, isFull

• You can use list, or for efficiency, use collections.deque in Python

Types of Queue

🔢 Types of Queues (with Syntax & Examples)

Queues are not limited to just one form. There are 5 main types of queues:

Simple Queue

✅ 1. Simple Queue (or Linear Queue)

📘 Definition:

A simple queue allows insertion at the rear and deletion from the front. It follows the FIFO rule.

🧑‍💻 Syntax & Example in Python:

queue = [] queue.append(10) # Enqueue queue.append(20) print("Queue:", queue) queue.pop(0) # Dequeue print("After Dequeue:", queue)

📌 Output:

Queue: [10, 20] After Dequeue: [20]

Circular Queue

🔁 2. Circular Queue

📘 Definition:

In a circular queue, the last position is connected back to the first to make a circle.
It helps in efficient use of memory when the queue is full but has empty space due to dequeued elements.

🧑‍💻 Syntax & Example using List (basic logic):

size = 5 queue = [None] * size front = rear = -1 # Enqueue example rear = (rear + 1) % size queue[rear] = 10 # Dequeue example front = (front + 1) % size queue[front] = None

✔️ Real implementation is done with modulus operator % to loop around the queue.

Deque

🔂 3. Deque (Double-Ended Queue)

📘 Definition:

A Deque allows insertion and deletion from both front and rear.
There are two types:

• Input Restricted Deque (insert at one end only)

• Output Restricted Deque (delete at one end only)

🧑‍💻 Syntax & Example using collections.deque:

from collections import deque dq = deque() dq.append(10) # Add to rear dq.appendleft(20) # Add to front print("Deque:", dq) dq.pop() # Remove from rear dq.popleft() # Remove from front print("After operations:", dq)

📌 Output:

Deque: deque([20, 10])
After operations: deque([])


Priority Queue

🛑 4. Priority Queue

📘 Definition:

In a priority queue, each element has a priority.
Elements with higher priority are dequeued first, even if they were added later.

🧑‍💻 Syntax & Example using queue.PriorityQueue:

from queue import PriorityQueue pq = PriorityQueue() pq.put((1, "Low Priority")) pq.put((0, "High Priority")) # 0 = highest priority while not pq.empty(): print(pq.get())

📌 Output:

(0, 'High Priority')
(1, 'Low Priority')

Double Ended Priority Queue

🔢 5. Doubly Ended Priority Queue (DEPQ)

📘 Definition:

A DEPQ allows insertion and deletion at both ends with priorities assigned to elements.
(Not commonly used in basic programming; used in advanced cases like AI, scheduling, etc.)

🧠 Summary Table