Queue Implementation Using Array in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue array with the given size.

DSA Java

int[] queue = new int[[1]];

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

Asize

BmaxSize

Ccapacity

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' which is a property of arrays, not a variable.

Using 'capacity' or 'maxSize' without defining them.

The queue array is initialized with the variable size which represents the maximum number of elements the queue can hold.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA Java

return front == [1];

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Arear

B0

Csize

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if front is 0, which can be a valid index.

Comparing front with rear instead of -1.

The queue is empty when the front index is -1, indicating no elements are present.

3fill in blank

hard

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

DSA Java

rear = (rear + [1]) % size;

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A1

Bsize

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 0 which does not move the rear index.

Adding -1 which moves backward incorrectly.

Adding size which results in no change after modulo.

To move the rear index forward by one position in a circular queue, add 1 and then take modulo with size.

4fill in blank

hard

Fill both blanks to correctly check if the queue is full.

DSA Java

return (rear + [1]) % size == [2];

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

A1

Bfront

C0

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 for increment.

Comparing with 0 or size instead of front.

The queue is full when the next position of rear (calculated by adding 1 modulo size) equals front.

5fill in blank

hard

Fill all three blanks to correctly dequeue an element and update front index.

DSA Java

if (front == [1]) {
    front = [2];
    rear = [3];
} else {
    front = (front + 1) % size;
}

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

A-1

Brear

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Resetting front and rear to 0 instead of -1.

Not resetting rear when queue becomes empty.

If front equals rear, the queue has one element. After dequeue, both front and rear reset to -1 to show empty queue.

Practice

1. What is the main purpose of using an array in a queue implementation?

easy

A. To store elements in a fixed-size linear order

B. To allow random access to elements anywhere in the queue

C. To sort elements automatically when inserted

D. To store elements in a tree structure

Solution

1. Step 1: Understand queue storage

   A queue stores elements in order, so an array can hold them in a fixed linear sequence.

2. Step 2: Compare other options

   Random access and sorting are not queue properties; tree structure is unrelated.

3. Final Answer:

   To store elements in a fixed-size linear order -> Option A

4. Quick Check:

   Queue uses array for linear storage [OK]

Hint: Queue uses array for fixed linear storage [OK]

Common Mistakes:

• Thinking queue allows random access
• Assuming array sorts elements automatically
• Confusing queue with tree structures

2. Which of the following is the correct way to check if a queue implemented using an array is empty?

easy

A. if (front == rear) return true;

B. if (rear == size) return true;

C. if (front == -1) return true;

D. if (front > rear) return true;

Solution

1. Step 1: Understand empty queue condition

   In array queue, front = -1 means no elements have been enqueued yet.

2. Step 2: Analyze other conditions

   front == rear means queue is full or empty depending on implementation; rear == size means full; front > rear is invalid.

3. Final Answer:

   if (front == -1) return true; -> Option C

4. Quick Check:

   Empty queue check uses front == -1 [OK]

Hint: Empty queue usually has front at -1 [OK]

Common Mistakes:

• Using front == rear to check empty (can be full)
• Checking rear == size for empty instead of full
• Using front > rear which is invalid

3. What will be the output after the following operations on a queue of size 5 implemented using an array?

enqueue(10); enqueue(20); dequeue(); enqueue(30); enqueue(40); dequeue(); enqueue(50); display();

medium

A. 30 -> 40 -> 50 -> null

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

C. 10 -> 20 -> 30 -> 40 -> 50 -> null

D. 40 -> 50 -> null

Solution

1. Step 1: Trace enqueue and dequeue operations

   enqueue(10), enqueue(20) adds 10,20; dequeue removes 10; enqueue(30), enqueue(40) adds 30,40; dequeue removes 20; enqueue(50) adds 50.

2. Step 2: Determine queue content after operations

   Remaining elements are 30, 40, 50 in order.

3. Final Answer:

   30 -> 40 -> 50 -> null -> Option A

4. Quick Check:

   Queue after operations = 30,40,50 [OK]

Hint: Track enqueue/dequeue stepwise to find queue content [OK]

Common Mistakes:

• Not removing correct elements on dequeue
• Assuming queue resets after full
• Confusing order of elements after operations

4. Identify the error in this queue implementation snippet using an array:

if (rear == size) {
  System.out.println("Queue is full");
} else {
  rear++;
  queue[rear] = value;
}

medium

A. rear should be incremented after assigning value

B. queue array size is not checked before insertion

C. front pointer is not updated when queue is empty

D. Condition should be rear == size - 1 to check full

Solution

1. Step 1: Analyze full queue condition

   Array index goes from 0 to size-1, so rear == size means out of bounds; full condition is rear == size - 1.

2. Step 2: Check rear increment and assignment

   Incrementing rear before assignment is correct if rear starts at -1.

3. Final Answer:

   Condition should be rear == size - 1 to check full -> Option D

4. Quick Check:

   Full queue check uses rear == size - 1 [OK]

Hint: Array index max is size - 1, not size [OK]

Common Mistakes:

• Checking rear == size instead of size - 1
• Incrementing rear after assignment causing overwrite
• Ignoring front pointer update on empty queue

5. Given a queue implemented using an array of size 4, initially empty, what will be the state of the queue after these operations?

enqueue(5); enqueue(10); dequeue(); enqueue(15); enqueue(20); enqueue(25); dequeue(); enqueue(30); display();

Assume the queue does not support circular wrapping and front and rear start at -1.

hard

A. 10 -> 15 -> 20 -> 25 -> null

B. Queue Overflow error occurs

C. 20 -> 25 -> 30 -> null

D. 15 -> 20 -> 25 -> null

Solution

1. Step 1: Trace each operation stepwise

   enqueue(5) adds 5; enqueue(10) adds 10; dequeue removes 5; enqueue(15) adds 15; enqueue(20) adds 20; enqueue(25) attempts to add but queue size is 4 and rear is at last index.

2. Step 2: Check for overflow due to no circular wrap

   Since queue is full at rear index 3, enqueue(25) causes overflow; subsequent operations cannot proceed.

3. Final Answer:

   Queue Overflow error occurs -> Option B

4. Quick Check:

   No circular wrap causes overflow at full array [OK]

Hint: Without circular queue, overflow occurs when rear reaches end [OK]

Common Mistakes:

• Assuming queue wraps around without circular logic
• Ignoring overflow condition on enqueue
• Miscounting elements after dequeue