Circular queue in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to define a circular queue.

Data Structures Theory

A circular queue connects the last position back to the [1] position.

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

Afirst

Bsecond

Cmiddle

Dlast

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'last' because it sounds like the end.

Confusing 'middle' with the start.

The circular queue connects the last position back to the first position to reuse empty spaces.

2fill in blank

medium

Complete the sentence about the main advantage of a circular queue.

Data Structures Theory

A circular queue efficiently uses memory by reusing [1] positions after they are freed.

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Aall

Bempty

Cfull

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'full' because it sounds like the queue is busy.

Choosing 'new' which is unrelated to reuse.

The circular queue reuses empty positions to avoid wasting memory space.

3fill in blank

hard

Fix the error in the statement about circular queue pointers.

Data Structures Theory

In a circular queue, the [1] pointer moves forward and wraps around to the start when it reaches the end.

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

Amiddle

Bfront

Crear

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'front' which is used for removing elements.

Choosing 'size' which is not a pointer.

The rear pointer moves forward to add new elements and wraps around in a circular queue.

4fill in blank

hard

Fill both blanks to complete the condition that checks if a circular queue is full.

Data Structures Theory

The queue is full if (rear + 1) [1] size and front [2] rear.

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Amod

B==

C!=

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of 'mod' for wrapping.

Using '!=' instead of '==' for comparison.

The queue is full when (rear + 1) mod size equals front, meaning the next position is the front.

5fill in blank

hard

Fill all three blanks to complete the formula for the number of elements in a circular queue.

Data Structures Theory

count = (rear - front + [1]) [2] size [3] 1

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Asize

B%

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' at the end.

Omitting the modulo operation.

The number of elements is calculated by adding size to (rear - front), then taking modulo % size, and finally subtracting - 1.

Practice

1. What is the main advantage of a circular queue compared to a regular linear queue?

easy

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

B. It efficiently reuses empty space by connecting the end to the start.

C. It automatically sorts elements while enqueuing.

D. It requires shifting elements after every dequeue operation.

Solution

1. Step 1: Understand the structure of a circular queue

   A circular queue connects the last position back to the first, forming a circle.

2. Step 2: Identify the advantage over a linear queue

   This connection allows the queue to reuse spaces freed by dequeued elements without shifting.

3. Final Answer:

   It efficiently reuses empty space by connecting the end to the start. -> Option B

4. Quick Check:

   Circular queue = space reuse [OK]

Hint: Circular queue wraps around to reuse space [OK]

Common Mistakes:

• Thinking it sorts elements automatically
• Believing it stores multiple data types
• Assuming it shifts elements after dequeue

2. Which of the following is the correct way to update the rear pointer in a circular queue after an enqueue operation?

easy

A. rear = (rear + 1) % size

B. rear = rear + 1

C. rear = rear - 1

D. rear = size - rear

Solution

1. Step 1: Recall how circular queue pointers move

   In a circular queue, pointers wrap around using modulo arithmetic to stay within bounds.

2. Step 2: Apply modulo to update rear pointer

   After enqueue, rear moves to (rear + 1) modulo the queue size to wrap if needed.

3. Final Answer:

   rear = (rear + 1) % size -> Option A

4. Quick Check:

   Rear update uses modulo [OK]

Hint: Use modulo (%) to wrap rear pointer [OK]

Common Mistakes:

• Incrementing rear without modulo
• Decrementing rear after enqueue
• Using subtraction instead of modulo

3. Consider a circular queue of size 5 with front = 3 and rear = 1. Which of the following statements is true about the queue state?

medium

A. The queue is empty.

B. The queue is full.

C. The queue has 3 elements.

D. The queue has 4 elements.

Solution

1. Step 1: Understand front and rear positions in circular queue

   Front at 3 and rear at 1 means rear has wrapped around past front.

2. Step 2: Calculate number of elements using formula

   Number of elements = (rear - front + size) % size = (1 - 3 + 5) % 5 = 3 % 5 = 3.

3. Final Answer:

   The queue has 3 elements. -> Option C

4. Quick Check:

   Elements count = 3 [OK]

Hint: Use (rear - front + size) % size to count elements [OK]

Common Mistakes:

• Confusing full and empty conditions
• Ignoring modulo in calculation
• Assuming rear < front means empty

4. Given this code snippet for dequeue in a circular queue, what is the error?

if front == -1:
    print("Queue is empty")
else:
    data = queue[front]
    if front == rear:
        front = rear = -1
    else:
        front = (front + 1) % size

medium

A. The code does not return or use the dequeued data.

B. The front pointer update should use rear instead of front.

C. The condition to check empty queue is incorrect.

D. The rear pointer should be incremented after dequeue.

Solution

1. Step 1: Analyze the dequeue operation steps

   The code checks if queue is empty, retrieves data, updates pointers correctly.

2. Step 2: Identify missing usage of dequeued data

   Though data is assigned, it is neither returned nor used, so the operation loses the dequeued value.

3. Final Answer:

   The code does not return or use the dequeued data. -> Option A

4. Quick Check:

   Dequeue must return data [OK]

Hint: Always return dequeued element [OK]

Common Mistakes:

• Not returning dequeued value
• Incorrect pointer updates
• Incrementing rear on dequeue

5. You have a circular queue of size 6. Initially, front = 0 and rear = 4. You perform two dequeue operations followed by three enqueue operations. What will be the final positions of front and rear?

hard

A. front = 1, rear = 2

B. front = 2, rear = 0

C. front = 0, rear = 5

D. front = 2, rear = 1

Solution

1. Step 1: Update front after two dequeues

   Each dequeue moves front by 1 modulo size: front = (0 + 2) % 6 = 2.

2. Step 2: Update rear after three enqueues

   Each enqueue moves rear by 1 modulo size: rear = (4 + 3) % 6 = 7 % 6 = 1.

3. Final Answer:

   front = 2, rear = 1 -> Option D

4. Quick Check:

   Front and rear updated with modulo [OK]

Hint: Add operations to pointers using modulo size [OK]

Common Mistakes:

• Not using modulo for pointer wrap-around
• Mixing front and rear updates
• Forgetting to update pointers after each operation