Circular Queue Implementation Using Array in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the front index of the circular queue.

DSA Rust

struct CircularQueue {
    front: i32,
    rear: i32,
    size: usize,
    data: Vec<i32>,
}

impl CircularQueue {
    fn new(capacity: usize) -> Self {
        CircularQueue {
            front: [1],
            rear: -1,
            size: capacity,
            data: vec![0; capacity],
        }
    }
}

Drag options to blanks, or click blank then click option'

A-1

Busize::MAX

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front to 0 causes confusion between empty and first element.

Using a large number like usize::MAX is invalid for indexing.

The front index is initialized to -1 to indicate the queue is empty initially.

2fill in blank

medium

Complete the code to check if the circular queue is full.

DSA Rust

impl CircularQueue {
    fn is_full(&self) -> bool {
        (self.rear + 1) % (self.size as i32) == [1]
    }
}

Drag options to blanks, or click blank then click option'

A-1

Bself.front

C0

Dself.rear

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing to 0 or -1 instead of front causes wrong full detection.

Using rear instead of front leads to incorrect logic.

The queue is full when the next position of rear equals front.

3fill in blank

hard

Fix the error in the enqueue method to correctly update the rear index.

DSA Rust

impl CircularQueue {
    fn enqueue(&mut self, value: i32) -> Result<(), &str> {
        if self.is_full() {
            return Err("Queue is full");
        }
        self.rear = [1];
        self.data[self.rear as usize] = value;
        if self.front == -1 {
            self.front = 0;
        }
        Ok(())
    }
}

Drag options to blanks, or click blank then click option'

A(self.rear + 1) % (self.size as i32)

Bself.rear + 1

Cself.front + 1

Dself.rear - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing rear without modulo causes index out of bounds.

Using front instead of rear leads to wrong pointer update.

The rear index must wrap around using modulo to stay within array bounds.

4fill in blank

hard

Fill both blanks to correctly update the front index in the dequeue method and handle empty queue.

DSA Rust

impl CircularQueue {
    fn dequeue(&mut self) -> Result<i32, &str> {
        if self.front == -1 {
            return Err("Queue is empty");
        }
        let value = self.data[self.front as usize];
        if self.front == self.rear {
            self.front = [1];
            self.rear = [2];
        } else {
            self.front = (self.front + 1) % (self.size as i32);
        }
        Ok(value)
    }
}

Drag options to blanks, or click blank then click option'

A-1

B0

Cself.size as i32

Dself.rear

Attempts:

3 left

💡 Hint

Common Mistakes

Setting front or rear to 0 causes incorrect empty state.

Not resetting both pointers leads to stale indices.

When the last element is dequeued, both front and rear reset to -1 to mark empty queue.

5fill in blank

hard

Fill all three blanks to implement the display method that prints queue elements in order.

DSA Rust

impl CircularQueue {
    fn display(&self) {
        if self.front == -1 {
            println!("Queue is empty");
            return;
        }
        let mut i = self.front;
        loop {
            print!("{} ", self.data[i as usize]);
            if i == self.rear {
                break;
            }
            i = (i + [1]) % (self.size as i32);
        }
        println!();
    }
}

Drag options to blanks, or click blank then click option'

Aself.size as i32

B0

C1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes infinite loop as index never changes.

Using size or -1 causes incorrect indexing.

Increment index by 1 modulo size to traverse queue elements circularly.

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