Circular Queue Implementation Using Array in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Python

class CircularQueue:
    def __init__(self, size):
        self.size = size
        self.queue = [None] * size
        self.front = [1]
        self.rear = -1

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

A-1

B0

CNone

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting front to 0 initially 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 Python

def is_full(self):
    return (self.rear + 1) % self.size == [1]

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

A-1

Bself.rear

C0

Dself.front

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with rear or 0 instead of front causes wrong full condition.

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

3fill in blank

hard

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

DSA Python

def enqueue(self, data):
    if self.is_full():
        print("Queue is full")
        return
    if self.front == -1:
        self.front = 0
    self.rear = [1]
    self.queue[self.rear] = data

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

Aself.rear + 1

B(self.rear + 1) % self.size

Cself.rear - 1

Dself.front + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing rear without modulo causes index out of range errors.

The rear pointer must move forward circularly using modulo to wrap around.

4fill in blank

hard

Fill both blanks to correctly update the front pointer in the dequeue method.

DSA Python

def dequeue(self):
    if self.front == -1:
        print("Queue is empty")
        return None
    data = self.queue[self.front]
    if self.front == self.rear:
        self.front = [1]
        self.rear = [2]
    else:
        self.front = (self.front + 1) % self.size
    return data

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

A-1

B0

Cself.rear

Dself.front

Attempts:

3 left

💡 Hint

Common Mistakes

Setting front or rear to 0 instead of -1 causes incorrect empty state.

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

5fill in blank

hard

Fill in the blanks to complete the display method that prints the queue elements in order.

DSA Python

def display(self):
    if self.front == -1:
        print("Queue is empty")
        return
    i = self.front
    while True:
        print(self.queue[i], end=" ")
        if i == self.rear:
            break
        i = (i [1] 1) [2] self.size
    print()

# Example usage:
cq = CircularQueue(5)
cq.enqueue(10)
cq.enqueue(20)
cq.enqueue(30)
cq.display()  # Output: 10 20 30

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

A+

B-

C%

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or floor division causes wrong index updates.

To move forward circularly, add 1 and take modulo with size.

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 available 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 eliminates the need for front and rear pointers.

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 is space at the front.

2. Step 2: Circular queue reuses space

   Circular queue connects the end of the array back to the front, allowing reuse of freed slots and efficient space usage.

3. Final Answer:

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

4. Quick Check:

   Space reuse = Advantage [OK]

Hint: Circular queue reuses space unlike linear queue [OK]

Common Mistakes:

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

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

easy

A. if rear == max_size - 1

B. if rear == front

C. if front == -1

D. if (rear + 1) % max_size == front

Solution

1. Step 1: Understand full condition in circular queue

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

2. Step 2: Use modulo for circular increment

   Using modulo operator, (rear + 1) % max_size gives the next position circularly.

3. Final Answer:

   if (rear + 1) % max_size == front -> Option D

4. Quick Check:

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

Hint: Full when next rear equals front using modulo [OK]

Common Mistakes:

• Checking rear == front means full (actually empty)
• Using rear == max_size - 1 ignores circular wrap
• Checking front == -1 means empty, not full

3. Consider this circular queue code snippet with max_size = 5:

front = 0
rear = 3
queue = [10, 20, 30, 40, None]

# Enqueue operation
rear = (rear + 1) % 5
queue[rear] = 50

# Dequeue operation
front = (front + 1) % 5

print(queue)

What will be the printed output?

medium

A. [10, 20, 30, 40, 50] with front=1 and rear=4

B. [10, 20, 30, 40, None]

C. [10, 20, 30, 40, 50]

D. [10, 20, 30, 40, 50] with front=0 and rear=4

Solution

1. Step 1: Perform enqueue operation

   rear moves from 3 to (3+1)%5 = 4; queue[4] = 50; queue becomes [10, 20, 30, 40, 50]

2. Step 2: Perform dequeue operation

   front moves from 0 to (0+1)%5 = 1; front=1, rear=4

3. Final Answer:

   [10, 20, 30, 40, 50] with front=1 and rear=4 -> Option A

4. Quick Check:

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

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

Common Mistakes:

• Ignoring front and rear pointer updates
• Assuming queue content changes on dequeue
• Confusing front and rear positions

4. The following code snippet for dequeue operation in a circular queue has a bug:

def dequeue(queue, front, rear, max_size):
    if front == -1:
        return None, front, rear  # Queue empty
    data = queue[front]
    if front == rear:
        front = rear = -1
    else:
        front = (front + 1) % max_size
    return data, front, rear

# Initial state
queue = [10, 20, 30, 40, 50]
front = 0
rear = 4

# Dequeue call
value, front, rear = dequeue(queue, front, rear, 5)
print(value, front, rear)

What is the bug in this code?

medium

A. The condition to check empty queue should be rear == -1, not front == -1.

B. The function does not update the queue after dequeue.

C. The function does not handle the case when queue is full.

D. The function incorrectly resets front and rear to -1 when queue becomes empty.

Solution

1. Step 1: Check queue update after dequeue

   The dequeue operation removes the element logically by moving front, but the queue array still holds the old value.

2. Step 2: Identify missing update

   To avoid stale data, the dequeued position should be cleared or updated (e.g., set to None) after dequeue.

3. Final Answer:

   The function does not update the queue after dequeue. -> Option B

4. Quick Check:

   Queue array update after dequeue [OK]

Hint: Dequeue should clear or update the dequeued slot [OK]

Common Mistakes:

• Checking only front for empty queue
• Not resetting pointers correctly after last dequeue
• Ignoring queue full condition in dequeue

5. You have a circular queue of size 6 (indices 0 to 5). Initially, front = 4 and rear = 2. The queue array is:
[70, 80, 90, None, 40, 50]

Which of the following sequences of operations will correctly enqueue 100 and dequeue two elements, updating front and rear properly?

hard

A. Enqueue: rear = (rear + 1) % 6; queue[rear] = 100; Dequeue: front = (front + 1) % 6

B. Enqueue: rear = (rear + 2) % 6; queue[rear] = 100; Dequeue: front = (front + 1) % 6; front = (front + 1) % 6

C. Enqueue: rear = (rear + 1) % 6; queue[rear] = 100; Dequeue: front = (front + 1) % 6; front = (front + 1) % 6

D. Enqueue: rear = (rear + 1) % 6; queue[rear] = 100; Dequeue: front = (front + 2) % 6

Solution

1. Step 1: Enqueue operation

   rear = (2 + 1) % 6 = 3; queue[3] = 100; queue becomes [70, 80, 90, 100, 40, 50]

2. Step 2: Dequeue two elements

   front moves twice: front = (4 + 1) % 6 = 5, then front = (5 + 1) % 6 = 0

3. Final Answer:

   Enqueue: rear = (rear + 1) % 6; queue[rear] = 100; Dequeue: front = (front + 1) % 6; front = (front + 1) % 6 -> Option C

4. Quick Check:

   One enqueue + two dequeues update pointers correctly [OK]

Hint: Increment pointers modulo size for each enqueue/dequeue [OK]

Common Mistakes:

• Incrementing rear or front by more than 1 at once
• Not using modulo for circular wrap
• Dequeueing fewer or more times than asked