Circular Queue Implementation Using Array in DSA Rust - Build from Scratch

Circular Queue Implementation Using Array

📖 Scenario: Imagine you are managing a small parking lot with limited spaces arranged in a circle. Cars enter and leave in order, and you want to keep track of which spots are occupied using a circular queue.

🎯 Goal: You will build a simple circular queue using an array in Rust. This queue will help manage the parking spots by adding cars (enqueue) and removing cars (dequeue) in a circular manner.

📋 What You'll Learn

Create an array to hold the queue elements with a fixed size of 5

Create variables to track the front and rear positions of the queue

Implement enqueue operation to add elements in a circular way

Implement dequeue operation to remove elements in a circular way

Print the queue elements after operations to show the current state

💡 Why This Matters

🌍 Real World

Circular queues are used in real-world systems like traffic lights, CPU scheduling, and buffering data streams where resources are reused in a loop.

💼 Career

Understanding circular queues helps in roles involving system design, embedded programming, and performance optimization where efficient use of fixed memory is critical.

1

Create the Circular Queue Array and Initialize Pointers

Create a fixed-size array called queue of type i32 with 5 elements all set to 0. Also, create two variables called front and rear and set both to -1 to indicate the queue is empty.

DSA Rust

let mut queue: [i32; 5] = [0; 5];
let mut front: i32 = -1;
let mut rear: i32 = -1;
// Your code here

Hint

Use let mut to create mutable variables. Initialize queue with 5 zeros. Set front and rear to -1 to show the queue is empty.

2

Add a Variable for Queue Size

Create a variable called size and set it to 5 to represent the maximum number of elements the queue can hold.

DSA Rust

fn main() {
    let mut queue: [i32; 5] = [0; 5];
    let mut front: i32 = -1;
    let mut rear: i32 = -1;
    // Your code here
}

Hint

Use let size: i32 = 5; to store the queue capacity.

3

Implement Enqueue and Dequeue Operations

Inside main, write code to enqueue the numbers 10, 20, and 30 into the queue using circular logic. Then dequeue one element from the queue. Update front and rear accordingly.

DSA Rust

fn main() {
    let mut queue: [i32; 5] = [0; 5];
    let mut front: i32 = -1;
    let mut rear: i32 = -1;
    let size: i32 = 5;

    // Your code here
}

Hint

Use modulo % with size to wrap rear and front around the array. Set queue[rear] to the new value when enqueueing. When dequeueing, set queue[front] to 0 and move front forward.

4

Print the Current State of the Queue

Print the queue array to show the current elements after enqueueing and dequeueing.

DSA Rust

fn main() {
    let mut queue: [i32; 5] = [0; 5];
    let mut front: i32 = -1;
    let mut rear: i32 = -1;
    let size: i32 = 5;

    // Enqueue 10
    if front == -1 {
        front = 0;
    }
    rear = (rear + 1) % size;
    queue[rear as usize] = 10;

    // Enqueue 20
    rear = (rear + 1) % size;
    queue[rear as usize] = 20;

    // Enqueue 30
    rear = (rear + 1) % size;
    queue[rear as usize] = 30;

    // Dequeue one element
    if front != -1 {
        queue[front as usize] = 0;
        if front == rear {
            front = -1;
            rear = -1;
        } else {
            front = (front + 1) % size;
        }
    }

    // Your code here
}

Hint

Use println!("{:?}", queue); to print the array showing the queue state.

Practice

1. What is the main advantage of using a circular queue over a simple linear queue when implemented using an array?

easy

A. It efficiently uses all available space by wrapping around the array.

B. It allows storing different data types in the same queue.

C. It automatically sorts the elements in the queue.

D. It uses less memory than a linear queue.

Solution

1. Step 1: Understand linear queue limitation

   In a linear queue using an array, once the rear reaches the end, no more elements can be added even if there is free space at the front.

2. Step 2: Circular queue advantage

   A circular queue connects the end of the array back to the front, allowing reuse of empty spaces and efficient use of the array.

3. Final Answer:

   It efficiently uses all available space by wrapping around the array. -> Option A

4. Quick Check:

   Circular queue = efficient space use [OK]

Hint: Circular queue reuses space by wrapping indices [OK]

Common Mistakes:

• Thinking circular queue sorts elements
• Believing it uses less memory than linear queue
• Assuming it stores different data types

2. Which of the following Rust code snippets correctly initializes a circular queue array of size 5 with front and rear pointers set to -1?

easy

A. let mut queue: [i32; 5]; let front: i32; let rear: i32;

B. let queue = Vec::new(); let front = 0; let rear = 0;

C. let mut queue: [i32; 5] = [1; 5]; let mut front: usize = 0; let mut rear: usize = 0;

D. let mut queue: [i32; 5] = [0; 5]; let mut front: i32 = -1; let mut rear: i32 = -1;

Solution

1. Step 1: Check array initialization

   let mut queue: [i32; 5] = [0; 5]; let mut front: i32 = -1; let mut rear: i32 = -1; initializes a fixed-size array of 5 integers with all zeros, which is correct for a circular queue.

2. Step 2: Check front and rear initialization

   let mut queue: [i32; 5] = [0; 5]; let mut front: i32 = -1; let mut rear: i32 = -1; sets front and rear to -1, indicating the queue is empty, which is standard practice.

3. Final Answer:

   let mut queue: [i32; 5] = [0; 5]; let mut front: i32 = -1; let mut rear: i32 = -1; -> Option D

4. Quick Check:

   Correct array and pointers initialization = let mut queue: [i32; 5] = [0; 5]; let mut front: i32 = -1; let mut rear: i32 = -1; [OK]

Hint: Initialize array with zeros and front/rear as -1 [OK]

Common Mistakes:

• Using Vec instead of fixed-size array
• Not initializing front and rear to -1
• Leaving variables uninitialized

3. Consider this Rust code snippet for a circular queue of size 5. After these operations, what is the state of the queue array?

let mut queue = [0; 5];
let mut front: i32 = -1;
let mut rear: i32 = -1;

// Enqueue 10
if rear == -1 {
    front = 0;
    rear = 0;
    queue[rear as usize] = 10;
} else {
    rear = (rear + 1) % 5;
    queue[rear as usize] = 10;
}

// Enqueue 20
rear = (rear + 1) % 5;
queue[rear as usize] = 20;

// Dequeue
front = (front + 1) % 5;

// Enqueue 30
rear = (rear + 1) % 5;
queue[rear as usize] = 30;

medium

A. [10, 20, 30, 0, 0]

B. [0, 20, 0, 30, 0]

C. [10, 20, 0, 30, 0]

D. [0, 0, 20, 30, 0]

Solution

1. Step 1: Trace enqueue and dequeue operations

   Initially front = -1, rear = -1, queue = [0,0,0,0,0]. - Enqueue 10: front=0, rear=0, queue[0]=10 -> [10,0,0,0,0] - Enqueue 20: rear=(0+1)%5=1, queue[1]=20 -> [10,20,0,0,0] - Dequeue: front=(0+1)%5=1 (10 removed logically) - Enqueue 30: rear=(1+1)%5=2, queue[2]=30 -> [10,20,30,0,0]

2. Step 2: Determine final queue state

   Queue array is [10, 20, 30, 0, 0]. Although 10 is logically dequeued, it remains in the array until overwritten.

3. Final Answer:

   [10, 20, 0, 30, 0] -> Option C

4. Quick Check:

   After dequeue, front moves, old value stays [OK]

Hint: Remember dequeue moves front, old data stays until overwritten [OK]

Common Mistakes:

• Assuming dequeued elements are erased immediately
• Not applying modulo for rear and front
• Mixing up front and rear positions

4. The following Rust code for enqueue in a circular queue has a bug. What is the error?

fn enqueue(queue: &mut [i32; 5], front: &mut i32, rear: &mut i32, value: i32) {
    if (*rear + 1) % 5 == *front {
        println!("Queue is full");
        return;
    }
    if *rear == -1 {
        *front = 0;
        *rear = 0;
    } else {
        *rear = (*rear + 1) % 5;
    }
    queue[*rear as usize] = value;
}

medium

A. The condition to check if queue is full is incorrect.

B. The initial check for empty queue should be *front == -1, not *rear == -1.

C. The queue array should be mutable but is not.

D. The modulo operation should use queue length minus one.

Solution

1. Step 1: Analyze empty queue check

   The code checks if *rear == -1 to detect empty queue, but standard practice is to check if *front == -1 because front indicates the start of queue.

2. Step 2: Understand impact of wrong check

   If *rear == -1 but *front is not -1, the queue might be incorrectly treated as empty, causing logic errors.

3. Final Answer:

   The initial check for empty queue should be *front == -1, not *rear == -1. -> Option B

4. Quick Check:

   Empty queue check uses front pointer [OK]

Hint: Empty queue means front == -1, not rear [OK]

Common Mistakes:

• Checking rear instead of front for empty queue
• Incorrect modulo usage
• Not passing mutable references properly

5. You have a circular queue of size 4 implemented using an array. Initially, front = 2 and rear = 1 (meaning the queue is full). You perform one dequeue operation followed by two enqueue operations with values 50 and 60. What will be the final positions of front and rear pointers?

hard

A. front = 3, rear = 3

B. front = 3, rear = 0

C. front = 3, rear = 1

D. front = 2, rear = 3

Solution

1. Step 1: Understand initial full queue state

   Queue size = 4, front = 2, rear = 1 means the queue is full because (rear + 1) % size == front ( (1+1)%4=2 ).

2. Step 2: Perform dequeue operation

   Dequeue moves front forward: front = (2 + 1) % 4 = 3. Rear remains 1.

3. Step 3: Perform first enqueue (50)

   rear = (1 + 1) % 4 = 2, insert 50 at index 2.

4. Step 4: Perform second enqueue (60)

   rear = (2 + 1) % 4 = 3, insert 60 at index 3.

5. Final Answer:

   front = 3, rear = 3 -> Option A

6. Quick Check:

   After dequeue and two enqueues, pointers wrap correctly [OK]

Hint: Update front on dequeue, rear on enqueue with modulo [OK]

Common Mistakes:

• Not applying modulo after pointer increment
• Confusing front and rear updates
• Assuming queue is empty after dequeue