Queue Implementation Using Array in DSA Python

Mental Model

A queue is like a line where the first person in is the first person out. Using an array, we keep track of where the line starts and ends.

Analogy: Imagine a line of people waiting to buy tickets. The first person to get in line is the first to get the ticket and leave the line. The array holds the people, and two markers show who is first and last.

front -> [ ] [ ] [ ] [ ] [ ] ← rear
indexes  0   1   2   3   4

Dry Run Walkthrough

Input: Start with empty queue of size 5, enqueue 10, enqueue 20, dequeue, enqueue 30

Goal: Show how elements enter and leave the queue using array indexes

Step 1: enqueue 10 at rear

front=0, rear=0, array=[10, _, _, _, _]

Why: Add first element at start of queue

Step 2: enqueue 20 at rear+1

front=0, rear=1, array=[10, 20, _, _, _]

Why: Add next element behind the first

Step 3: dequeue element at front

front=1, rear=1, array=[10, 20, _, _, _]

Why: Remove first element by moving front forward

Step 4: enqueue 30 at rear+1

front=1, rear=2, array=[10, 20, 30, _, _]

Why: Add new element behind current rear

Result:

front=1, rear=2, array=[10, 20, 30, _, _]
Queue elements: 20 -> 30 -> null

Annotated Code

DSA Python

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

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

    def is_full(self):
        return self.rear == self.size - 1

    def enqueue(self, value):
        if self.is_full():
            print("Queue is full")
            return
        if self.is_empty():
            self.front = 0
        self.rear += 1  # move rear forward to add new element
        self.queue[self.rear] = value

    def dequeue(self):
        if self.is_empty():
            print("Queue is empty")
            return None
        value = self.queue[self.front]
        if self.front == self.rear:  # only one element
            self.front = -1
            self.rear = -1
        else:
            self.front += 1  # move front forward to remove element
        return value

    def display(self):
        if self.is_empty():
            print("Queue is empty")
            return
        result = []
        for i in range(self.front, self.rear + 1):
            result.append(str(self.queue[i]))
        print(" -> ".join(result) + " -> null")

# Driver code
q = Queue(5)
q.enqueue(10)
q.enqueue(20)
q.dequeue()
q.enqueue(30)
q.display()

if self.is_full():

check if queue has no space left to add

if self.is_empty():

initialize front when adding first element

self.rear += 1 # move rear forward to add new element

advance rear pointer to next free slot

if self.is_empty():

check if queue is empty before removing

if self.front == self.rear: # only one element

reset queue when last element is removed

self.front += 1 # move front forward to remove element

advance front pointer to next element

for i in range(self.front, self.rear + 1):

iterate from front to rear to display queue

Output

20 -> 30 -> null

Complexity Analysis

Time: O(1) for enqueue and dequeue because we only move pointers without shifting elements

Space: O(n) because we use an array of fixed size n to store elements

vs Alternative: Compared to linked list queue, array queue uses fixed space and can be faster but may waste space if not circular

Edge Cases

enqueue when queue is full

prints 'Queue is full' and does not add element

DSA Python

if self.is_full():

dequeue when queue is empty

prints 'Queue is empty' and returns None

DSA Python

if self.is_empty():

dequeue last element

resets front and rear to -1 to mark empty queue

DSA Python

if self.front == self.rear:

When to Use This Pattern

When you need a first-in-first-out order and want fast add/remove at ends, use a queue with front and rear pointers to track positions in an array.

Common Mistakes

Mistake: Not updating front pointer on dequeue, causing repeated removal of same element
Fix: Advance front pointer by one after removing element

Mistake: Not resetting front and rear when queue becomes empty after dequeue
Fix: Set front and rear to -1 when last element is removed

Mistake: Trying to enqueue without checking if queue is full, causing index error
Fix: Check is_full before adding new element

Summary

Implements a queue using an array with front and rear pointers to add and remove elements in order.

Use when you want a simple queue with fixed size and fast enqueue/dequeue operations.

Remember to move front and rear pointers correctly and reset them when queue is empty.