Queue Implementation Using Array in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the array size for the queue.

DSA C++

const int MAX_SIZE = [1];

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

A100

B0

C-1

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or negative numbers for array size.

The queue size is set to 100 to allocate enough space for elements.

2fill in blank

medium

Complete the code to initialize the front index of the queue.

DSA C++

int front = [1];

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

A1

BMAX_SIZE

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting front to 0 which may cause confusion with valid index.

Initializing front to -1 indicates the queue is empty initially.

3fill in blank

hard

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

DSA C++

void enqueue(int value) {
    if (rear == MAX_SIZE - 1) {
        cout << "Queue is full" << endl;
        return;
    }
    rear = rear [1] 1;
    arr[rear] = value;
    if (front == -1) {
        front = 0;
    }
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' which moves rear backward causing errors.

We add 1 to rear to move to the next position before inserting the new value.

4fill in blank

hard

Fill both blanks to correctly check if the queue is empty in the dequeue function.

DSA C++

bool isEmpty() {
    return (front == [1] || front > [2]);
}

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

A-1

Brear

Cfront

DMAX_SIZE

Attempts:

3 left

💡 Hint

Common Mistakes

Using front instead of rear in the second condition.

The queue is empty if front is -1 or front has passed rear.

5fill in blank

hard

Fill all three blanks to correctly update front and rear in dequeue and return the dequeued value.

DSA C++

int dequeue() {
    if (isEmpty()) {
        cout << "Queue is empty" << endl;
        return -1;
    }
    int value = arr[[1]];
    if (front == rear) {
        front = [2];
        rear = [3];
    } else {
        front = front + 1;
    }
    return value;
}

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

Afront

Brear

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Resetting front and rear to 0 instead of -1.

We get the value at front, then if front equals rear, reset both to -1 to mark empty queue.

Practice

1. What is the main purpose of using an array to implement a queue?

easy

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

B. To store elements randomly for quick access

C. To sort elements automatically

D. To store elements in a LIFO order

Solution

1. Step 1: Understand queue behavior

   A queue follows FIFO (First In First Out), meaning elements are processed in the order they arrive.

2. Step 2: Role of array in queue

   An array stores elements in a fixed linear order, which helps maintain the FIFO order by using front and rear indices.

3. Final Answer:

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

4. Quick Check:

   Queue uses array for FIFO storage = C [OK]

Hint: Queue means FIFO; array stores elements linearly [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking array sorts elements automatically
• Assuming random access is main goal

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

easy

A. array queue(5);

B. int queue[5];

C. int[5] queue;

D. queue int[5];

Solution

1. Step 1: Recall C++ array declaration syntax

   In C++, arrays are declared as: type name[size]; so int queue[5]; is correct.

2. Step 2: Check other options

   Options A, B, and C do not follow valid C++ syntax for array declaration.

3. Final Answer:

   int queue[5]; -> Option B

4. Quick Check:

   C++ array syntax = type name[size] [OK]

Hint: Remember C++ arrays: type name[size]; [OK]

Common Mistakes:

• Placing size before name
• Using wrong keywords like 'array'
• Incorrect order of type and name

3. What will be the output after performing these operations on an empty queue of size 3 implemented using array?
enqueue(10); enqueue(20); dequeue(); enqueue(30); display();
Assume enqueue adds at rear and dequeue removes from front.

medium

A. 10 -> 30 -> null

B. 10 -> 20 -> 30 -> null

C. 30 -> null

D. 20 -> 30 -> null

Solution

1. Step 1: Perform enqueue(10) and enqueue(20)

   Queue becomes: 10 -> 20 -> null

2. Step 2: Perform dequeue()

   Removes 10 from front, queue becomes: 20 -> null

3. Step 3: Perform enqueue(30)

   Add 30 at rear, queue becomes: 20 -> 30 -> null

4. Step 4: Display queue

   Output is: 20 -> 30 -> null

5. Final Answer:

   20 -> 30 -> null -> Option D

6. Quick Check:

   After dequeue and enqueue, queue = 20, 30 [OK]

Hint: Track front and rear after each operation [OK]

Common Mistakes:

• Not removing element on dequeue
• Adding new element at front instead of rear
• Confusing order of elements

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

int queue[3], front = -1, rear = -1;
void enqueue(int x) {
  if (rear == 3) {
    cout << "Queue Full";
    return;
  }
  rear++;
  queue[rear] = x;
  if (front == -1) front = 0;
}

medium

A. front should be incremented in enqueue

B. rear should be initialized to 0

C. Condition should be rear == 2 to check full queue

D. No error, code is correct

Solution

1. Step 1: Understand array indexing for size 3

   Array indices are 0, 1, 2. So rear should not exceed 2.

2. Step 2: Check condition for full queue

   Condition rear == 3 is wrong because index 3 is out of bounds. It should be rear == 2.

3. Final Answer:

   Condition should be rear == 2 to check full queue -> Option C

4. Quick Check:

   Array max index = size-1 = 2 [OK]

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

Common Mistakes:

• Using size instead of size-1 for index check
• Incrementing front in enqueue
• Initializing rear incorrectly

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); dequeue(); enqueue(25); display();
Assume no circular queue and enqueue fails if rear reaches size-1.

hard

A. 15 -> 20 -> null

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

C. 15 -> 20 -> 25 -> null

D. 5 -> 10 -> 15 -> 20 -> null

Solution

1. Step 1: Perform enqueue(5), enqueue(10)

   Queue: 5 -> 10 -> null; front=0, rear=1

2. Step 2: Perform dequeue()

   Remove 5; Queue: 10 -> null; front=1, rear=1

3. Step 3: Perform enqueue(15), enqueue(20)

   Queue: 10 -> 15 -> 20 -> null; front=1, rear=3

4. Step 4: Perform dequeue()

   Remove 10; Queue: 15 -> 20 -> null; front=2, rear=3

5. Step 5: Perform enqueue(25)

   rear=3 (max index), cannot enqueue 25; operation fails, queue unchanged

6. Step 6: Display queue

   Queue elements from front=2 to rear=3: 15 -> 20 -> null

7. Final Answer:

   15 -> 20 -> null -> Option A

8. Quick Check:

   Check enqueue failure at full rear index [OK]

Hint: No circular queue means no wrap-around; watch rear limit [OK]

Common Mistakes:

• Assuming enqueue works after rear reaches max
• Not updating front after dequeue
• Confusing queue order after operations