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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enqueue and dequeue operations on a queue

What is the printed state of the queue after the following operations?

DSA C++

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
q.pop();
q.push(40);

// Print queue elements from front to back
std::queue<int> temp = q;
while (!temp.empty()) {
    std::cout << temp.front() << " -> ";
    temp.pop();
}
std::cout << "null";

A 10 -> 20 -> 30 -> 40 -> null

B 20 -> 30 -> 40 -> null

C 30 -> 40 -> null

D 10 -> 20 -> 40 -> null

💡 Hint

Remember that pop() removes the front element in a queue.

The queue starts with 10, 20, 30. After pop(), 10 is removed. Then 40 is added at the back. So the queue is 20, 30, 40.

🧠 Conceptual

intermediate

Understanding FIFO principle in queue

Which statement best describes the FIFO principle in a queue?

A Elements are removed randomly.

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

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

D Elements are removed based on their value.

💡 Hint

Think about a line of people waiting for service.

FIFO means First In, First Out. The first element added to the queue is the first one to be removed.

🔧 Debug

advanced

Identify the error in queue usage

What error will occur when running this code snippet?

DSA C++

std::queue<int> q;
q.push(1);
q.pop();
q.pop();  // Extra pop

A Runtime error: popping from an empty queue.

B No error, code runs fine.

C Compilation error: pop() not defined.

D Logical error: queue still contains elements.

💡 Hint

Check what happens if you remove more elements than present.

Calling pop() on an empty queue causes undefined behavior or runtime error.

🚀 Application

advanced

Queue usage in real-life scenario

Which real-life example best illustrates the use of a queue data structure?

A A list of tasks sorted by priority.

B Stack of plates where you take the top plate first.

C Random access to books on a shelf.

D People waiting in line at a ticket counter.

💡 Hint

Think about who gets served first in a line.

People waiting in line are served in the order they arrive, matching the FIFO queue behavior.

❓ Predict Output

expert

Final queue state after mixed operations

What is the final printed state of the queue after executing this code?

DSA C++

std::queue<int> q;
q.push(5);
q.push(10);
q.push(15);
q.pop();
q.push(20);
q.pop();
q.push(25);

std::queue<int> temp = q;
while (!temp.empty()) {
    std::cout << temp.front() << " -> ";
    temp.pop();
}
std::cout << "null";

A 15 -> 20 -> 25 -> null

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

C 5 -> 10 -> 15 -> 20 -> 25 -> null

D 20 -> 25 -> null

💡 Hint

Track each push and pop carefully in order.

After first pop, 5 is removed. Queue: 10,15. Then push 20: 10,15,20. Next pop removes 10: 15,20. Then push 25: 15,20,25.

Practice

1. What does the FIFO principle mean in the context of a queue?

easy

A. Last In, First Out - the last element added is the last removed

B. First In, First Out - the last element added is the first removed

C. Last In, First Out - the first element added is the last removed

D. First In, First Out - the first element added is the first removed

Solution

1. Step 1: Understand FIFO principle

   FIFO means the first element that enters the queue is the first one to leave.

2. Step 2: Match FIFO with queue behavior

   Queues follow FIFO, so the first added element is removed first.

3. Final Answer:

   First In, First Out - the first element added is the first removed -> Option D

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue = line; first come, first served [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking last added is removed first
• Mixing queue with stack behavior

2. Which of the following is the correct way to declare a queue of integers in C++ using STL?

easy

A. queue<int> q();

B. queue q<int>;

C. queue<int> q;

D. queue<int> q[];

Solution

1. Step 1: Recall STL queue syntax

   The correct syntax to declare a queue of integers is queue<int> q; without parentheses or brackets.

2. Step 2: Analyze options

   queue<int> q; matches correct syntax. queue q<int>; has wrong order. queue<int> q(); looks like a function declaration. queue<int> q[]; uses array syntax incorrectly.

3. Final Answer:

   queue<int> q; -> Option C

4. Quick Check:

   Correct STL queue declaration = queue<int> q; [OK]

Hint: Use template syntax: queue<Type> name; [OK]

Common Mistakes:

• Adding parentheses like a function
• Using brackets as if declaring array
• Swapping template and variable names

3. What will be the output of the following C++ code?

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
int x = q.front();
q.pop();
int y = q.front();
std::cout << x << " " << y << std::endl;

medium

A. 10 20

B. 20 30

C. 30 20

D. 10 30

Solution

1. Step 1: Trace queue operations

   Elements pushed in order: 10, 20, 30. Front is 10 initially.

2. Step 2: After pop and front calls

   After q.pop(), 10 is removed. New front is 20. So x=10, y=20.

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   First out is 10, next front is 20 [OK]

Hint: front() shows first element; pop() removes it [OK]

Common Mistakes:

• Assuming pop() returns value
• Mixing front() and back() values
• Confusing order of elements

4. Identify the error in the following C++ code snippet using queue:

std::queue<int> q;
q.push(5);
q.pop();
int val = q.front();
std::cout << val << std::endl;

medium

A. Accessing front() on an empty queue causes undefined behavior

B. pop() does not remove element from queue

C. push() is used incorrectly

D. Missing include for queue header

Solution

1. Step 1: Analyze queue state after operations

   After push(5), queue has one element. pop() removes it, queue becomes empty.

2. Step 2: Check front() call on empty queue

   Calling front() on empty queue is undefined and causes runtime error.

3. Final Answer:

   Accessing front() on an empty queue causes undefined behavior -> Option A

4. Quick Check:

   front() on empty queue = error [OK]

Hint: Never call front() if queue might be empty [OK]

Common Mistakes:

• Ignoring empty queue before front()
• Thinking pop() returns value
• Assuming push() syntax is wrong

5. You have a queue with elements 1, 2, 3, 4 (front to back). You perform these operations:
q.pop(); q.push(5); q.pop(); q.push(6);
What is the final order of elements in the queue from front to back?

hard

A. 2 -> 3 -> 4 -> 5 -> 6

B. 3 -> 4 -> 5 -> 6

C. 1 -> 2 -> 3 -> 4 -> 5 -> 6

D. 4 -> 5 -> 6

Solution

1. Step 1: Initial queue and first operations

   Queue: 1(front),2,3,4(back). q.pop() removes 1. Queue: 2,3,4. q.push(5) adds 5 at back: 2,3,4,5.

2. Step 2: Remaining operations and final queue

   q.pop() removes 2. Queue: 3,4,5. q.push(6) adds 6 at back: 3,4,5,6.

3. Final Answer:

   3 -> 4 -> 5 -> 6 -> Option B

4. Quick Check:

   After pops and pushes, queue front to back = 3,4,5,6 [OK]

Hint: Pop removes front; push adds to back, track stepwise [OK]

Common Mistakes:

• Forgetting pop removes front element
• Adding push elements at front instead of back
• Not updating queue after each operation