Stack & Queue • Data Structures

Circular Queue

A linear data structure that operates on FIFO but the last position is connected back to the first position to make a circle, overcoming the limitation of normal array-based queues.

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The Problem

In a standard Array-Queue, after enqueuing and dequeuing many times, the `front` and `rear` pointers hit the end of the array. Even if there are empty spots at the beginning, we can't use them!

The Solution

A Circular Queue uses modulo arithmetic (or explicit checks) to wrap the `rear` pointer back to `0` if it hits the end and the front has advanced.

Full vs Empty

Empty: `front == -1`. Full: `(rear + 1) % size == front` (or `front == 0 && rear == size-1` along with `rear == front-1`).

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Code Tracker

:::

1function enqueue(val) {

2 if (isFull()) return;

3 if (front === -1) { front = 0; rear = 0; }

4 else if (rear === size - 1 && front !== 0) rear = 0;

5 else rear++;

6 queue[rear] = val;

7}

8function dequeue() {

9 if (isEmpty()) return;

10 let val = queue[front];

11 if (front === rear) { front = -1; rear = -1; }

12 else if (front === size - 1) front = 0;

13 else front++;

14 return val;

15}