Circular Queue Implementation Using Array in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Circular Queue Enqueue and Dequeue Operations

What is the printed state of the circular queue after the following operations?

Initialize queue with size 5.
Enqueue 10, 20, 30.
Dequeue one element.
Enqueue 40, 50, 60.
Print queue elements from front to rear.

DSA C#

class CircularQueue {
    private int[] queue;
    private int front, rear, size, count;

    public CircularQueue(int size) {
        this.size = size;
        queue = new int[size];
        front = 0;
        rear = -1;
        count = 0;
    }

    public void Enqueue(int item) {
        if (count == size) return; // full
        rear = (rear + 1) % size;
        queue[rear] = item;
        count++;
    }

    public int Dequeue() {
        if (count == 0) return -1; // empty
        int item = queue[front];
        front = (front + 1) % size;
        count--;
        return item;
    }

    public string PrintQueue() {
        if (count == 0) return "empty";
        string result = "";
        for (int i = 0; i < count; i++) {
            int index = (front + i) % size;
            result += queue[index] + " -> ";
        }
        return result + "null";
    }
}

var cq = new CircularQueue(5);
cq.Enqueue(10);
cq.Enqueue(20);
cq.Enqueue(30);
cq.Dequeue();
cq.Enqueue(40);
cq.Enqueue(50);
cq.Enqueue(60);
Console.WriteLine(cq.PrintQueue());

A 10 -> 20 -> 30 -> 40 -> 50 -> null

B 20 -> 30 -> 40 -> 50 -> null

C 30 -> 40 -> 50 -> 60 -> null

D 20 -> 30 -> 40 -> 50 -> 60 -> null

💡 Hint

Remember that dequeue removes the front element and enqueue adds at rear in a circular manner.

After enqueueing 10, 20, 30, the queue is: 10 -> 20 -> 30 -> null.
Dequeue removes 10.
Enqueue 40, 50, 60 fills the queue circularly.
Final queue: 20 -> 30 -> 40 -> 50 -> 60 -> null.

🧠 Conceptual

intermediate

Capacity Check in Circular Queue

Which condition correctly checks if the circular queue is full?

A count == size

B count == 0

C front == rear

D rear == size - 1

💡 Hint

Full means number of elements equals the queue size.

In a circular queue, the queue is full when the count of elements equals the size of the queue.

🔧 Debug

advanced

Identify the Bug in Circular Queue Dequeue Method

What error will occur when running the following Dequeue method if the queue is empty?

DSA C#

public int Dequeue() {
    int item = queue[front];
    front = (front + 1) % size;
    count--;
    return item;
}

A No error, returns default value

B InvalidOperationException

C IndexOutOfRangeException

D NullReferenceException

💡 Hint

Check what happens if Dequeue is called when count is zero.

No runtime error occurs because queue[front] (front=0 initially) accesses a valid array index and returns the default value (0). However, it corrupts the queue state by advancing front and setting count to -1.

📝 Syntax

advanced

Correct Circular Queue Enqueue Syntax

Which option correctly implements the enqueue operation in a circular queue?

A

rear = (rear + 1) / size;
queue[rear] = item;
count++;

B

rear = rear + 1 % size;
queue[rear] = item;
count++;

C

rear = (rear + 1) % size;
queue[rear] = item;
count++;

D

rear = rear + 1 % size;
queue[count] = item;
count++;

💡 Hint

Remember to use parentheses to ensure correct modulo operation.

Option C correctly increments rear circularly using modulo. Option C lacks parentheses causing wrong precedence. Option C uses division instead of modulo. Option C incorrectly uses count as index.

🚀 Application

expert

Final Queue State After Mixed Operations

Given a circular queue of size 4, what is the queue state after these operations?

Enqueue 1, 2, 3.
Dequeue.
Enqueue 4.
Dequeue.
Enqueue 5.
Print queue elements from front to rear.

DSA C#

class CircularQueue {
    private int[] queue;
    private int front, rear, size, count;

    public CircularQueue(int size) {
        this.size = size;
        queue = new int[size];
        front = 0;
        rear = -1;
        count = 0;
    }

    public void Enqueue(int item) {
        if (count == size) return;
        rear = (rear + 1) % size;
        queue[rear] = item;
        count++;
    }

    public int Dequeue() {
        if (count == 0) return -1;
        int item = queue[front];
        front = (front + 1) % size;
        count--;
        return item;
    }

    public string PrintQueue() {
        if (count == 0) return "empty";
        string result = "";
        for (int i = 0; i < count; i++) {
            int index = (front + i) % size;
            result += queue[index] + " -> ";
        }
        return result + "null";
    }
}

var cq = new CircularQueue(4);
cq.Enqueue(1);
cq.Enqueue(2);
cq.Enqueue(3);
cq.Dequeue();
cq.Enqueue(4);
cq.Dequeue();
cq.Enqueue(5);
Console.WriteLine(cq.PrintQueue());

A 2 -> 3 -> 4 -> 5 -> null

B 3 -> 4 -> 5 -> null

C 1 -> 2 -> 3 -> 4 -> null

D 4 -> 5 -> null

💡 Hint

Track front and rear carefully after each operation.

After enqueue 1,2,3: queue is 1->2->3.
Dequeue removes 1.
Enqueue 4: queue is 2->3->4.
Dequeue removes 2.
Enqueue 5: queue is 3->4->5.
Printed as 3 -> 4 -> 5 -> null.

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 the array space by reusing freed slots.

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

C. It automatically sorts the elements in the queue.

D. It increases the maximum size of the queue beyond the array length.

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 are empty slots at the front.

2. Step 2: Circular queue advantage

   A circular queue treats the array as circular, so when rear reaches the end, it wraps around to use empty slots at the front, efficiently using all available space.

3. Final Answer:

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

4. Quick Check:

   Circular queue reuses space = Advantage A [OK]

Hint: Circular queue reuses space freed at front [OK]

Common Mistakes:

• Thinking circular queue increases array size
• Believing it sorts elements automatically
• Assuming it stores different data types

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 indices?

easy

A. rear == n

B. rear == front

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

D. front == 0 && rear == n - 1

Solution

1. Step 1: Understand circular queue full condition

   In a circular queue, the queue is full when the next position of rear (circularly) is front.

2. Step 2: Express condition mathematically

   This means (rear + 1) modulo n equals front, ensuring no overlap and full queue detection.

3. Final Answer:

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

4. Quick Check:

   Full if next rear equals front = D [OK]

Hint: Full when next rear wraps to front [OK]

Common Mistakes:

• Using rear == front as full (actually empty)
• Checking rear == n ignoring wrap-around
• Using front == 0 && rear == n - 1 only (linear queue)

3. Consider a circular queue of size 5 (indices 0 to 4). Initially, front = 0 and rear = 3. After performing Enqueue(10) and Dequeue(), what are the values of front and rear?

medium

A. front = 0, rear = 4

B. front = 1, rear = 3

C. front = 0, rear = 3

D. front = 1, rear = 4

Solution

1. Step 1: Perform Enqueue operation

   Enqueue increments rear circularly: rear = (3 + 1) % 5 = 4.

2. Step 2: Perform Dequeue operation

   Dequeue increments front circularly: front = (0 + 1) % 5 = 1.

3. Final Answer:

   front = 1, rear = 4 -> Option D

4. Quick Check:

   Enqueue rear+1=4, Dequeue front+1=1 [OK]

Hint: Increment indices modulo size after enqueue/dequeue [OK]

Common Mistakes:

• Not wrapping indices with modulo
• Updating front on enqueue or rear on dequeue
• Confusing initial and final index values

4. Given this snippet of circular queue code, what is the error?

int n = 5;
int front = 0, rear = 0;
int[] queue = new int[n];

// Enqueue operation
rear = (rear + 1) % n;
queue[rear] = 10;

// Dequeue operation
front = (front + 1) % n;
int x = queue[front];

// Check if queue is empty
if (front == rear)
    Console.WriteLine("Queue is empty");

medium

A. The enqueue operation increments rear before inserting, causing off-by-one error.

B. The initial values of front and rear cause the first enqueue to overwrite data.

C. The empty check condition is incorrect; it should be (rear + 1) % n == front.

D. The dequeue operation should decrement front instead of incrementing.

Solution

1. Step 1: Analyze enqueue operation

   Incrementing rear before inserting means the first element is placed at index 1, skipping index 0, which wastes space and causes off-by-one errors.

2. Step 2: Understand correct enqueue logic

   Enqueue should insert at current rear, then increment rear after insertion or handle indices carefully to avoid skipping slots.

3. Final Answer:

   The enqueue operation increments rear before inserting, causing off-by-one error. -> Option A

4. Quick Check:

   Increment rear before insert = off-by-one error B [OK]

Hint: Insert before incrementing rear to avoid skipping slots [OK]

Common Mistakes:

• Confusing empty and full conditions
• Incrementing front on enqueue or rear on dequeue
• Ignoring initial index positions

5. You have a circular queue of size 6. Initially, front = 4 and rear = 2. The queue currently holds 4 elements. Which of the following sequences of operations will cause the queue to become full?

hard

A. Enqueue three times, then dequeue once, then enqueue once.

B. Dequeue once, then enqueue three times.

C. Enqueue three times without any dequeue.

D. Dequeue twice, then enqueue five times.

Solution

1. Step 1: Calculate current queue state

   Size is 6, front=4, rear=2, holding 4 elements means the queue is partially filled with wrap-around.

2. Step 2: Analyze each operation sequence

   A: Dequeue once (remove 1), enqueue three times (add 3) -> net +2 elements, total 6 (full). B: Enqueue three times (add 3), dequeue once (remove 1), enqueue once (add 1) -> net +3 elements, total 7 (overflow). C: Enqueue three times without dequeue would overflow (since only 2 slots free). D: Dequeue twice (remove 2), enqueue five times (add 5) -> net +3 elements, total 7 (overflow).

3. Step 3: Identify which sequence is valid without overflow

   Only Dequeue once, then enqueue three times. correctly balances operations to reach full without overflow errors.

4. Final Answer:

   Dequeue once, then enqueue three times. -> Option B

5. Quick Check:

   Dequeue 1 + Enqueue 3 fills queue = A [OK]

Hint: Track net enqueue minus dequeue to reach full capacity [OK]

Common Mistakes:

• Ignoring wrap-around effect on indices
• Assuming enqueue always succeeds without checking full
• Miscounting elements after operations