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.

Practice

1. What does FIFO stand for in the context of a queue?

easy

A. First Input, First Operation

B. First In, Final Out

C. Fast Input, Fast Output

D. First In, First Out

Solution

1. Step 1: Understand FIFO meaning

   FIFO means the first element added to the queue is the first one to be removed.

2. Step 2: Match the acronym

   First In, First Out exactly describes this behavior.

3. Final Answer:

   First In, First Out -> Option D

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue removes oldest item first [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking FIFO means last added first removed
• Mixing up acronym letters

2. Which of the following is the correct syntax to enqueue an element `x` into a queue named `q` in C?

easy

A. push(q, x);

B. enqueue(q, x);

C. q.enqueue(x);

D. q.add(x);

Solution

1. Step 1: Identify common C queue operation

   In C, queue operations are usually functions like enqueue(queue, element).

2. Step 2: Check options for C style

   enqueue(q, x); uses function call syntax typical in C. Others use object-oriented syntax not common in C.

3. Final Answer:

   enqueue(q, x); -> Option B

4. Quick Check:

   C queue enqueue = enqueue(q, x); [OK]

Hint: C uses function calls, not dot notation [OK]

Common Mistakes:

• Using object-oriented syntax in C
• Confusing push with enqueue
• Using wrong function names

3. Consider the following C code snippet using a queue:

enqueue(q, 10);
enqueue(q, 20);
dequeue(q);
enqueue(q, 30);
dequeue(q);

What is the next element to be dequeued?

medium

A. 30

B. 10

C. 20

D. Queue is empty

Solution

1. Step 1: Trace enqueue and dequeue operations

   Queue after enqueue 10 and 20: [10, 20]. Dequeue removes 10, queue: [20]. Enqueue 30: [20, 30]. Dequeue removes 20, queue: [30].

2. Step 2: Identify next element to dequeue

   Next element is the front of queue, which is 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Queue front after operations = 30 [OK]

Hint: Follow FIFO order step-by-step [OK]

Common Mistakes:

• Assuming last enqueued is dequeued next
• Forgetting previous dequeues
• Mixing enqueue and dequeue order

4. Identify the error in the following C code snippet for queue operations:

enqueue(q, 5);
dequeue(q);
dequeue(q);

Assuming the queue was initially empty.

medium

A. Second dequeue causes underflow error

B. No error, code runs fine

C. First enqueue is incorrect syntax

D. Queue overflow error occurs

Solution

1. Step 1: Analyze queue state after operations

   Initially empty queue. Enqueue 5 adds one element. First dequeue removes it, queue empty again.

2. Step 2: Check second dequeue

   Second dequeue tries to remove from empty queue, causing underflow error.

3. Final Answer:

   Second dequeue causes underflow error -> Option A

4. Quick Check:

   Dequeue empty queue = underflow error [OK]

Hint: Check if queue is empty before dequeue [OK]

Common Mistakes:

• Ignoring empty queue before dequeue
• Confusing overflow with underflow
• Assuming enqueue syntax error

5. You have a queue that can hold a maximum of 3 elements. The following operations are performed:

enqueue(q, 1);
enqueue(q, 2);
enqueue(q, 3);
dequeue(q);
enqueue(q, 4);
dequeue(q);
dequeue(q);

What is the state of the queue after these operations?

hard

A. Queue is empty

B. 3 -> 4 -> null

C. 4 -> null

D. 2 -> 3 -> 4 -> null

Solution

1. Step 1: Track queue after each operation

   Enqueue 1,2,3: queue = [1,2,3]. Dequeue removes 1: queue = [2,3]. Enqueue 4: queue = [2,3,4]. Dequeue removes 2: queue = [3,4]. Dequeue removes 3: queue = [4].

2. Step 2: Final queue state

   Only element left is 4, so queue is 4 -> null.

3. Final Answer:

   4 -> null -> Option C

4. Quick Check:

   Final queue front = 4 [OK]

Hint: Follow enqueue and dequeue stepwise, watch max size [OK]

Common Mistakes:

• Forgetting max size limit
• Miscounting dequeues
• Assuming queue empty incorrectly