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#

private int front = [1];

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

A-1

B0

C1

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front to 0 causes confusion with a valid index.

Using null is invalid for int type.

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#

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

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

Afront

B-1

Crear

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with rear instead of front.

Using size directly instead of modulo operation.

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

3fill in blank

hard

Fix the error in the Enqueue method to update rear correctly.

DSA C#

public void Enqueue(int item) {
    if (IsFull()) {
        throw new InvalidOperationException("Queue is full");
    }
    rear = [1];
    queue[rear] = item;
    if (front == -1) front = 0;
}

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

Arear + 1

B(rear + 1) % size

Crear - 1

Drear % size

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing rear without modulo causes index out of range.

Decrementing rear is incorrect for enqueue.

rear must be updated using modulo to wrap around the array correctly.

4fill in blank

hard

Fill both blanks to correctly update front and return the dequeued item.

DSA C#

public int Dequeue() {
    if (IsEmpty()) {
        throw new InvalidOperationException("Queue is empty");
    }
    int item = queue[front];
    if (front == rear) {
        front = [1];
        rear = [2];
    } else {
        front = (front + 1) % size;
    }
    return item;
}

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

A-1

B0

Cfront

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Setting front or rear to 0 causes incorrect empty state.

Not resetting pointers leads to errors on next operations.

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

5fill in blank

hard

Fill both blanks to implement the Peek method that returns the front item without removing it.

DSA C#

public int Peek() {
    if ([1]()) {
        throw new InvalidOperationException("Queue is empty");
    }
    return queue[[2]];
}

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

AIsEmpty

Bfront

Crear

DIsFull

Attempts:

3 left

💡 Hint

Common Mistakes

Using IsFull() instead of IsEmpty() for empty check.

Returning rear element instead of front.

Peek checks if queue is empty using IsEmpty(), then returns the item at front index.

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