Queue Concept and FIFO Principle in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the queue after these operations?

Consider a queue initially empty. We perform these operations in order:

1. Enqueue 5
2. Enqueue 10
3. Dequeue
4. Enqueue 15

What is the state of the queue after these operations?

DSA C

Queue: front -> 5 -> 10 -> null
Operations:
1. Enqueue(5)
2. Enqueue(10)
3. Dequeue()
4. Enqueue(15)

A 15 -> 10 -> null

B 5 -> 10 -> 15 -> null

C 10 -> 15 -> null

D 5 -> 15 -> null

💡 Hint

Remember, dequeue removes the front element.

After enqueue 5 and 10, queue is 5 -> 10 -> null.
Dequeue removes 5.
Enqueue 15 adds at the end.
Final queue: 10 -> 15 -> null.

🧠 Conceptual

intermediate

Which statement correctly describes the FIFO principle in queues?

Choose the statement that best explains the FIFO (First In First Out) principle as it applies to queues.

A Elements are removed in the same order they were added.

B Elements are removed randomly regardless of insertion order.

C The last element added is the first to be removed.

D The middle element is always removed first.

💡 Hint

Think about a line of people waiting for service.

FIFO means the first element added to the queue is the first one removed, like people standing in line.

🔧 Debug

advanced

What error occurs when dequeuing from an empty queue?

Given a queue implemented with an array and front and rear pointers, what error or behavior occurs if you try to dequeue when the queue is empty?

DSA C

int dequeue() {
    if (front == -1 || front > rear) {
        // Queue is empty
        // What happens here?
    }
    int value = queue[front];
    front++;
    return value;
}

A Causes underflow error or invalid access

B Returns garbage value without error

C Automatically resets queue to empty state

D Throws a syntax error at compile time

💡 Hint

Think about accessing elements when none exist.

Dequeuing from empty queue causes underflow or invalid memory access because no elements exist to remove.

❓ Predict Output

advanced

What is the queue state after these operations with wrap-around?

A circular queue of size 3 is empty initially. We perform:

1. Enqueue 1
2. Enqueue 2
3. Enqueue 3
4. Dequeue
5. Enqueue 4

What is the queue content from front to rear?

DSA C

Circular queue size = 3
Operations:
1. Enqueue(1)
2. Enqueue(2)
3. Enqueue(3)
4. Dequeue()
5. Enqueue(4)

A 4 -> 2 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 3 -> 4 -> null

D 2 -> 3 -> 4 -> null

💡 Hint

Remember circular queue wraps rear to start after reaching end.

After enqueue 1,2,3 queue is full: 1->2->3.
Dequeue removes 1.
Enqueue 4 wraps rear to start.
Queue is 2->3->4.

🚀 Application

expert

How many elements remain after these queue operations?

Starting with an empty queue, perform these operations:

1. Enqueue 7
2. Enqueue 14
3. Dequeue
4. Enqueue 21
5. Dequeue
6. Dequeue

How many elements remain in the queue at the end?

A 3

B 0

C 2

D 1

💡 Hint

Count carefully how many enqueue and dequeue operations happen.

Enqueue 7,14 → queue has 2 elements.
Dequeue removes 7 → 1 element left.
Enqueue 21 → 2 elements.
Dequeue removes 14 → 1 element.
Dequeue removes 21 → 0 elements left.