Circular Queue Implementation Using Array in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C++

class CircularQueue {
public:
    int front = [1];
    int rear = -1;
    int size;
    int* arr;

    CircularQueue(int s) {
        size = s;
        arr = new int[size];
        front = -1;
        rear = -1;
    }
};

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

Asize

B0

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front to 0 causes confusion between empty and full states.

The front pointer 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 C++

bool isFull() {
    return ((rear + 1) % size) == [1];
}

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

Afront

Brear

C-1

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with rear or size instead of front.

Not using modulo operator.

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

3fill in blank

hard

Fix the error in the enqueue function to correctly update the rear pointer.

DSA C++

void enqueue(int value) {
    if (isFull()) {
        std::cout << "Queue is full" << std::endl;
        return;
    }
    rear = [1];
    arr[rear] = value;
    if (front == -1) front = 0;
}

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

A(rear + 1) % size

Brear + 1

Crear - 1

Drear % size

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing rear without modulo causes index out of bounds.

Using rear + 1 without modulo.

Rear must move forward circularly using modulo to wrap around the array.

4fill in blank

hard

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

DSA C++

int dequeue() {
    if (front == -1) {
        std::cout << "Queue is empty" << std::endl;
        return -1;
    }
    int value = arr[front];
    if (front == rear) {
        front = [1];
        rear = [2];
    } else {
        front = (front + 1) % size;
    }
    return value;
}

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

A-1

B0

Cfront

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Not resetting front and rear to -1 causes incorrect empty state.

Setting front or rear to 0 instead of -1.

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

5fill in blank

hard

Fill in the blank to implement a function that prints the current elements in the circular queue.

DSA C++

void display() {
    if (front == -1) {
        std::cout << "Queue is empty" << std::endl;
        return;
    }
    int i = front;
    while (true) {
        std::cout << arr[i] << " ";
        if (i == [1]) break;
        i = (i + 1) % size;
    }
    std::cout << std::endl;
}

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

A0

Bfront

Csize

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Stopping at front or size instead of rear.

Not using modulo for index increment.

The loop stops when the index reaches rear, printing all elements from front to rear 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 eliminates the need for front and rear pointers

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

C. It automatically sorts the elements in the queue

D. It efficiently uses the array space by reusing freed slots

Solution

1. Step 1: Understand linear queue limitation

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

2. Step 2: Circular queue advantage

   Circular queue connects the end of the array back to the front, allowing reuse of empty slots freed by dequeuing.

3. Final Answer:

   It efficiently uses the array space by reusing freed slots -> Option D

4. Quick Check:

   Space reuse = Advantage [OK]

Hint: Circular queue reuses space freed by dequeued elements [OK]

Common Mistakes:

• Thinking circular queue sorts elements
• Believing circular queue removes need for pointers
• Assuming it supports multiple data types inherently

2. Which of the following is the correct condition to check if a circular queue is full when using an array of size n with front and rear pointers?

easy

A. front == (rear + 1) % n

B. rear == n - 1

C. front == rear

D. rear == (front + 1) % n

Solution

1. Step 1: Understand full condition in circular queue

   In circular queue, queue is full when next position of rear is front, i.e., (rear + 1) modulo n equals front.

2. Step 2: Match condition with options

   front == (rear + 1) % n matches this condition: front == (rear + 1) % n.

3. Final Answer:

   front == (rear + 1) % n -> Option A

4. Quick Check:

   Full condition = front == (rear + 1) % n [OK]

Hint: Full when front is next to rear in circular manner [OK]

Common Mistakes:

• Using rear == n - 1 which is for linear queue
• Confusing front == rear as full (actually empty)
• Swapping front and rear in condition

3. Consider a circular queue of size 5 (indices 0 to 4). Initially, front = -1 and rear = -1. After the following operations:
enqueue(10); enqueue(20); enqueue(30); dequeue(); enqueue(40); enqueue(50); dequeue(); enqueue(60);
What will be the values of front and rear?

medium

A. front = 2, rear = 1

B. front = 2, rear = 0

C. front = 3, rear = 0

D. front = 1, rear = 4

Solution

1. Step 1: Trace each operation and update pointers

   Start: front = -1, rear = -1
   enqueue(10): front=0, rear=0
   enqueue(20): rear=1
   enqueue(30): rear=2
   dequeue(): front=1
   enqueue(40): rear=3
   enqueue(50): rear=4
   dequeue(): front=2
   enqueue(60): rear=(4+1)%5=0

2. Step 2: Final pointer values

   front = 2, rear = 0 which corresponds to front = 2, rear = 0.

3. Final Answer:

   front = 2, rear = 0 -> Option B

4. Quick Check:

   Trace pointers carefully [OK]

Hint: Track front and rear after each enqueue/dequeue [OK]

Common Mistakes:

• Not updating rear circularly after last index
• Confusing front and rear updates on dequeue
• Ignoring initial front = -1 condition

4. The following code snippet is part of a circular queue implementation. Identify the bug:

void enqueue(int value) {
  if ((rear + 1) % size == front) {
    cout << "Queue is full" << endl;
    return;
  }
  if (front == -1) front = 0;
  rear = (rear + 1) % size;
  arr[rear] = value;
}

medium

A. No bug, code is correct

B. The front pointer should be updated after rear

C. The initial check for empty queue should be before full check

D. The condition to check full queue is incorrect

Solution

1. Step 1: Analyze full queue condition

   The condition (rear + 1) % size == front correctly checks if queue is full in circular queue.

2. Step 2: Check front pointer update and enqueue steps

   If queue is empty (front == -1), front is set to 0 before inserting.
   rear is updated circularly and value inserted at arr[rear].

3. Final Answer:

   No bug, code is correct -> Option A

4. Quick Check:

   Full check and pointers update are correct [OK]

Hint: Full check uses modulo; front updated only if empty [OK]

Common Mistakes:

• Misplacing front update after rear
• Incorrect full condition without modulo
• Checking empty after full condition

5. You have a circular queue of size 6. After performing the following operations:
enqueue(5); enqueue(10); enqueue(15); dequeue(); dequeue(); enqueue(20); enqueue(25); enqueue(30); enqueue(35);
What will be the sequence of elements in the queue from front to rear?

hard

A. 20 -> 25 -> 30 -> 35

B. 10 -> 15 -> 20 -> 25 -> 30 -> 35

C. 15 -> 20 -> 25 -> 30 -> 35

D. 5 -> 10 -> 15 -> 20 -> 25 -> 30

Solution

1. Step 1: Trace each operation and queue content

   Start empty.
   enqueue(5): queue = [5]
   enqueue(10): queue = [5,10]
   enqueue(15): queue = [5,10,15]
   dequeue(): removes 5, queue = [10,15]
   dequeue(): removes 10, queue = [15]
   enqueue(20): queue = [15,20]
   enqueue(25): queue = [15,20,25]
   enqueue(30): queue = [15,20,25,30]
   enqueue(35): queue = [15,20,25,30,35]

2. Step 2: Check queue size and elements

   Queue size is 6, current elements are 5.
   From front to rear: 15 -> 20 -> 25 -> 30 -> 35

3. Final Answer:

   15 -> 20 -> 25 -> 30 -> 35 -> Option C

4. Quick Check:

   After dequeues, front moves; final elements start from 15 [OK]

Hint: Remove dequeued elements, then list remaining from front to rear [OK]

Common Mistakes:

• Forgetting to remove dequeued elements
• Listing elements before dequeue operations
• Assuming queue is full and ignoring last enqueue