Queue Using Array in Python

A Queue is one of the common linear data structures obeying the FIFO (First In, First Out) rule. Similar to a ticket counter queue in real life, the first inserted item will be deleted first.

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While we typically implement queues with collections.deque or using the queue module in Python, it is useful to learn how to implement a queue with arrays (Python lists) so that we can better grasp its basic working.

Key Properties of Queue

1. FIFO Order → First element inserted is the first to be removed.
2. Two main operations:

• Enqueue → Insert element at the rear.
• Dequeue → Remove element from the front

3. Overflow & Underflow Conditions:

• Overflow → Queue is full and cannot accept new elements.
• Underflow → Queue is empty, and we try to remove an element.

Queue Operations Using Array (List in Python)

Let’s implement a Queue class step by step.

class Queue:
    def __init__(self, capacity):
        self.queue = []
        self.capacity = capacity
    
    # Enqueue operation
    def enqueue(self, item):
        if len(self.queue) == self.capacity:
            print("Queue Overflow! Cannot insert", item)
        else:
            self.queue.append(item)
            print(f"Enqueued: {item}")
    
    # Dequeue operation
    def dequeue(self):
        if len(self.queue) == 0:
            print("Queue Underflow! No elements to remove.")
        else:
            removed = self.queue.pop(0)  # Removes front element
            print(f"Dequeued: {removed}")
    
    # Peek operation (get front element)
    def peek(self):
        if len(self.queue) == 0:
            print("Queue is empty")
        else:
            print("Front Element:", self.queue[0])
    
    # Display queue
    def display(self):
        print("Current Queue:", self.queue)

Example Usage

# Create a queue of size 5
q = Queue(5)

q.enqueue(10)
q.enqueue(20)
q.enqueue(30)
q.display()
q.dequeue()
q.display()
q.peek()

Output

Enqueued: 10
Enqueued: 20
Enqueued: 30
Current Queue: [10, 20, 30]
Dequeued: 10
Current Queue: [20, 30]
Front Element: 20

Limitations of Queue Using Array

1. Shifting Problem → When we dequeue, elements shift left (pop(0)), which takes O(n) time.
2. Fixed Size → Queue has limited capacity when implemented with arrays.
3. Inefficient for Large Data → Not suitable for high-performance systems.

Alternative Implementations

• Circular Queue → Avoids shifting problem by connecting end to front.
• Linked List Based Queue → Dynamic size without overflow (until memory is available).
• Python’s deque Module → Provides highly optimized queue operations.

mplementing a queue using arrays in Python is a great way to understand its core operations and limitations. Once you master this, you can easily extend the concept to circular queues, priority queues, and deque structures.