Queue Implementation Using Array in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the array for the queue.

DSA C#

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

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

Asize

Bqueue

C10

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names instead of a number for array size.

Leaving the array size empty.

The queue array needs a fixed size, here 10 is used to initialize the array length.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA C#

public bool IsEmpty() { return [1] == -1; }

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

Afront

Brear

Csize

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Checking rear instead of front for emptiness.

Using size or count variables incorrectly.

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

3fill in blank

hard

Fix the error in the Enqueue method to add an element correctly.

DSA C#

public void Enqueue(int item) {
    if (rear == queue.Length - 1) {
        Console.WriteLine("Queue is full");
        return;
    }
    if (front == -1) front = 0;
    rear = [1];
    queue[rear] = item;
}

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

Afront - 1

Brear - 1

Cfront + 1

Drear + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Decrementing rear instead of incrementing.

Using front index instead of rear.

To add an element, rear index must move forward by 1 before inserting the item.

4fill in blank

hard

Fill both blanks to correctly remove an element from the queue.

DSA C#

public int Dequeue() {
    if (IsEmpty()) {
        Console.WriteLine("Queue is empty");
        return -1;
    }
    int item = queue[[1]];
    if (front == rear) {
        front = [2];
        rear = -1;
    } else {
        front++;
    }
    return item;
}

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

Afront

Brear

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using rear index to remove item.

Resetting front to 0 instead of -1.

The item to remove is at the front index. When the last element is removed, front resets to -1.

5fill in blank

hard

Fill both blanks to implement the Peek method that shows the front element without removing it.

DSA C#

public int Peek() {
    if (IsEmpty()) {
        Console.WriteLine("Queue is empty");
        return [1];
    }
    return queue[[2]];
}

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

A-1

Bfront

Crear

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 instead of -1 when empty.

Returning element at rear instead of front.

Peek returns -1 if empty, otherwise returns the element at front index without removing it.

Practice

1. What is the main characteristic of a queue implemented using an array?

easy

A. It removes elements from the end only.

B. It allows random access to elements.

C. It stores elements in sorted order automatically.

D. It follows First-In-First-Out (FIFO) order.

Solution

1. Step 1: Understand queue behavior

   A queue adds elements at the rear and removes from the front, following FIFO order.

2. Step 2: Compare options with queue properties

   Only It follows First-In-First-Out (FIFO) order. correctly describes FIFO behavior; others describe different data structures or incorrect behavior.

3. Final Answer:

   It follows First-In-First-Out (FIFO) order. -> Option D

4. Quick Check:

   Queue = FIFO order [OK]

Hint: Remember queue = first in, first out [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue allows random access
• Assuming queue sorts elements automatically

2. Which of the following is the correct way to declare a queue array of size 5 in C#?

easy

A. int queue = new int[5];

B. int queue[] = new int(5);

C. int[] queue = new int[5];

D. int[] queue = int[5];

Solution

1. Step 1: Recall C# array declaration syntax

   In C#, arrays are declared as type[] name = new type[size];

2. Step 2: Check each option

   int[] queue = new int[5]; matches correct syntax; others have syntax errors or wrong usage.

3. Final Answer:

   int[] queue = new int[5]; -> Option C

4. Quick Check:

   C# array declaration = int[] name = new int[size] [OK]

Hint: Use type[] name = new type[size] for arrays [OK]

Common Mistakes:

• Missing [] after type
• Using parentheses instead of brackets
• Assigning array without new keyword

3. Consider this C# code for a queue using an array of size 3:

int[] queue = new int[3];
int front = -1, rear = -1;

// Enqueue 10
rear++;
queue[rear] = 10;
if(front == -1) front = 0;

// Enqueue 20
rear++;
queue[rear] = 20;

// Dequeue
int val = queue[front];
front++;

// Enqueue 30
rear++;
queue[rear] = 30;

// Current queue elements from front to rear?

medium

A. [10, 20, 30]

B. [20, 30]

C. [30]

D. [10, 30]

Solution

1. Step 1: Trace enqueue and dequeue operations

   Enqueue 10: rear=0, front=0, queue=[10,_,_] Enqueue 20: rear=1, queue=[10,20,_] Dequeue: val=queue[front]=10, front=1 Enqueue 30: rear=2, queue=[10,20,30]

2. Step 2: Identify current queue elements

   Front=1, rear=2, so elements are queue[1]=20 and queue[2]=30

3. Final Answer:

   [20, 30] -> Option B

4. Quick Check:

   After dequeue front=1, queue elements = [20, 30] [OK]

Hint: Track front and rear indexes carefully [OK]

Common Mistakes:

• Not updating front after dequeue
• Including dequeued elements in output
• Confusing front and rear positions

4. What is the error in this queue enqueue method using array?

void Enqueue(int[] queue, ref int rear, int value, int size) {
  if (rear == size) {
    Console.WriteLine("Queue is full");
    return;
  }
  rear++;
  queue[rear] = value;
}

medium

A. rear should be checked against size - 1, not size.

B. rear should be incremented after assigning value.

C. The method should check if rear is less than zero.

D. The queue array should be resized inside the method.

Solution

1. Step 1: Analyze boundary check for full queue

   Array indices go from 0 to size-1, so rear == size means out of bounds.

2. Step 2: Correct condition for full queue

   Check if rear == size - 1 before enqueue to avoid overflow.

3. Final Answer:

   rear should be checked against size - 1, not size. -> Option A

4. Quick Check:

   Array max index = size - 1 [OK]

Hint: Array max index = size - 1, check accordingly [OK]

Common Mistakes:

• Checking rear == size causes index out of range
• Incrementing rear before check
• Ignoring array bounds in enqueue

5. You want to implement a circular queue using an array of size 4. Initially, front = -1 and rear = -1. After these operations:

Enqueue(5)
Enqueue(10)
Enqueue(15)
Dequeue()
Enqueue(20)
Enqueue(25)

What will be the state of the queue array and the values of front and rear?

hard

A. Queue: [25, 10, 15, 20], front=1, rear=0

B. Queue: [5, 10, 15, 20], front=0, rear=3

C. Queue: [20, 25, 15, 10], front=2, rear=1

D. Queue: [10, 15, 20, 25], front=0, rear=3

Solution

1. Step 1: Perform each operation with circular logic

   Enqueue 5: front=0, rear=0, queue=[5,_,_,_] Enqueue 10: rear=1, queue=[5,10,_,_] Enqueue 15: rear=2, queue=[5,10,15,_] Dequeue: front=1 (removed 5) Enqueue 20: rear=3, queue=[5,10,15,20] Enqueue 25: rear wraps to 0, queue=[25,10,15,20]

2. Step 2: Final front and rear positions

   front=1, rear=0, queue array = [25,10,15,20]

3. Final Answer:

   Queue: [25, 10, 15, 20], front=1, rear=0 -> Option A

4. Quick Check:

   Circular queue wraps rear index [OK]

Hint: Remember rear wraps to 0 in circular queue [OK]

Common Mistakes:

• Not wrapping rear index after reaching end
• Not updating front after dequeue
• Assuming linear queue behavior