Understanding Queues in Programming
Concetps, Pseudocode, Examples

What is a Queue?

A Queue is a linear data structure that follows the FIFO (First In First Out) principle. The element that is inserted first is the one that gets removed first.

Imagine a line at a ticket counter — the person who comes first gets served first. This is exactly how a queue works in programming.

Key Operations in a Queue

• Enqueue: Add an element to the rear (end) of the queue
• Dequeue: Remove an element from the front of the queue
• Front: View the element at the front without removing it
• Rear: View the element at the end
• isEmpty: Check if the queue has no elements

Why Use Queues?

Queues are used in:

• Task scheduling (like CPU or printer jobs)
• Breadth-First Search (BFS) in graphs
• Handling requests in asynchronous operations

Example 1: Implementing a Simple Queue

Initialize Queue as empty list

Function enqueue(item):
    Add item to the end of Queue

Function dequeue():
    If Queue is not empty:
        Remove and return first element of Queue
    Else:
        Return "Queue Underflow"

Function front():
    If Queue is not empty:
        Return first element
    Else:
        Return "Queue is empty"

Function isEmpty():
    Return true if Queue is empty, else false

Queue: []
Enqueue 10 → Queue: [10]
Enqueue 20 → Queue: [10, 20]
Dequeue → Returned: 10, Queue: [20]
Front → 20
isEmpty → false

Question:

Why do we remove elements from the front instead of the rear in a queue?

Answer: Because a queue follows FIFO — the first element that comes in should be the first to go out, just like a queue in real life.

Example 2: Handling Overflow in Fixed-Size Queue

Suppose we want to limit the queue to a fixed capacity. We need to check for overflow before enqueuing new items.

Initialize Queue as empty list
Set MAX_SIZE = 3

Function enqueue(item):
    If length of Queue == MAX_SIZE:
        Return "Queue Overflow"
    Else:
        Add item to end of Queue

Function dequeue():
    If Queue is empty:
        Return "Queue Underflow"
    Else:
        Remove and return first element

Enqueue 1 → [1]
Enqueue 2 → [1, 2]
Enqueue 3 → [1, 2, 3]
Enqueue 4 → Queue Overflow
Dequeue → Returned 1, Queue: [2, 3]

Question:

How can we improve space usage when using a fixed-size queue?

Answer: Use a circular queue where the rear pointer wraps around to the front once it reaches the end, making better use of space.

Concept Extension: Circular Queue Overview

In a circular queue, the last position is connected back to the first to form a circle. This allows insertion at the beginning when there's space available due to prior deletions at the front.

Summary

• Queues are ideal when order matters (first-in, first-out)
• They support operations like enqueue, dequeue, front, and isEmpty
• Commonly used in scheduling, buffering, and BFS

Practice Exercise

Implement a pseudocode queue that supports the following operations: enqueue, dequeue, front, size, isEmpty, and displayQueue().

Try adding elements beyond the queue size and removing elements from an empty queue to see how it behaves.