Peek Front Element of Queue in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after peeking the front element of the queue?

Consider the following C++ code that uses a queue. What will be printed after peeking the front element?

DSA C++

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
std::cout << q.front() << std::endl;

A 30

B 10

C 20

D Compilation error

💡 Hint

The front() function returns the first element inserted in the queue.

The queue stores elements in FIFO order. The first element pushed is 10, so front() returns 10.

❓ Predict Output

intermediate

What happens if we peek front of an empty queue?

What will happen if we call front() on an empty std::queue in C++?

DSA C++

std::queue<int> q;
std::cout << q.front() << std::endl;

A Undefined behavior (likely crash or garbage value)

B Prints 0

C Throws std::out_of_range exception

D Compilation error

💡 Hint

Calling front() on an empty queue is not safe.

Accessing front on an empty queue causes undefined behavior because no element exists to return.

🧠 Conceptual

advanced

Which operation does NOT affect the front element of a queue?

Given a queue, which of the following operations does NOT change the front element?

A Calling <code>front()</code> to peek the element

B Calling <code>pop()</code> to remove the front element

C Calling <code>push()</code> to add a new element

D Calling <code>emplace()</code> to add a new element

💡 Hint

Think about which operations modify the queue contents.

Calling front() only reads the front element without removing or adding anything, so the front element stays the same.

🔧 Debug

advanced

Why does this code crash when peeking front element?

Identify the reason for the crash in the following code snippet:

DSA C++

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

A The queue was never initialized

B front() is called incorrectly without parentheses

C pop() does not remove the element, so front() is invalid

D The queue is empty after pop(), so front() causes undefined behavior

💡 Hint

Check the state of the queue after pop() before calling front().

After pop(), the queue becomes empty. Calling front() on an empty queue causes undefined behavior and likely crash.

🚀 Application

expert

What is the output after multiple peek and pop operations?

Given the following code, what is the printed output sequence?

DSA C++

std::queue<int> q;
q.push(1);
q.push(2);
q.push(3);

std::cout << q.front() << " ";
q.pop();
std::cout << q.front() << " ";
q.pop();
q.push(4);
std::cout << q.front() << " ";

A 1 2 3

B 1 3 4

C 1 2 4

D 2 3 4

💡 Hint

Track the queue contents after each operation carefully.

Initially queue: [1,2,3]. After first front(): prints 1. After pop(): queue is [2,3]. Second front(): prints 2. After pop(): queue is [3]. After push(4): queue is [3,4]. Third front(): prints 3.

Practice

1. What does the peek() operation do in a queue?

easy

A. Removes the front element from the queue

B. Returns the front element without removing it

C. Adds an element to the back of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the front element of the queue without changing the queue.

2. Step 2: Compare with other operations

   Unlike dequeue, peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the front element without removing it -> Option B

4. Quick Check:

   peek() = front element [OK]

Hint: Peek shows front without removal [OK]

Common Mistakes:

• Confusing peek with dequeue
• Thinking peek adds elements
• Assuming peek empties queue

2. Which of the following is the correct syntax to peek the front element of a std::queue<int> named q in C++?

easy

A. q.front();

B. q.peek();

C. q.top();

D. q.getFront();

Solution

1. Step 1: Recall C++ queue methods

   In C++, std::queue uses front() to access the front element.

2. Step 2: Check other options

   peek() and getFront() are not valid methods; top() is for stacks, not queues.

3. Final Answer:

   q.front(); -> Option A

4. Quick Check:

   std::queue front() method = q.front() [OK]

Hint: Use front() to peek in std::queue [OK]

Common Mistakes:

• Using peek() which doesn't exist
• Using top() which is for stacks
• Trying getFront() which is invalid

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

#include <iostream>
#include <queue>

int main() {
    std::queue<int> q;
    q.push(10);
    q.push(20);
    q.push(30);
    std::cout << q.front() << std::endl;
    q.pop();
    std::cout << q.front() << std::endl;
    return 0;
}

medium

A. 10\n20

B. 10\n10

C. 20\n30

D. 30\n20

Solution

1. Step 1: Trace initial queue state

   After pushes, queue is [10 (front), 20, 30].

2. Step 2: Output front and pop

   First output is 10 (front). Then pop removes 10, queue becomes [20 (front), 30]. Next output is 20.

3. Final Answer:

   10\n20 -> Option A

4. Quick Check:

   First front=10, after pop front=20 [OK]

Hint: Pop removes front; peek shows new front [OK]

Common Mistakes:

• Not updating front after pop
• Printing popped element
• Confusing push order

4. Identify the error in this code snippet that tries to peek the front element of a queue:

#include <queue>
#include <iostream>

int main() {
    std::queue<int> q;
    q.push(5);
    std::cout << q.peek() << std::endl;
    return 0;
}

medium

A. q.push() requires two arguments

B. Queue is empty before peek

C. Using q.peek() which is not a valid method

D. Missing #include <vector>

Solution

1. Step 1: Check method used to peek

   The code uses q.peek(), but std::queue does not have a peek() method.

2. Step 2: Identify correct method

   The correct method to access the front element is q.front().

3. Final Answer:

   Using q.peek() which is not a valid method -> Option C

4. Quick Check:

   std::queue peek method = front() [OK]

Hint: Use front(), not peek(), to access front element [OK]

Common Mistakes:

• Using peek() method
• Assuming peek() exists
• Ignoring compiler errors

5. Given a queue q containing integers, write a C++ function int peekFrontOrDefault(std::queue<int>& q, int defaultVal) that returns the front element if the queue is not empty, or defaultVal if it is empty. What is the correct implementation?

hard

A. return q.front();

B. if (q.empty()) return q.front(); else return defaultVal;

C. if (q.size() > 1) return q.front(); else return defaultVal;

D. if (!q.empty()) return q.front(); else return defaultVal;

Solution

1. Step 1: Check if queue is empty

   Use q.empty() to verify if queue has elements.

2. Step 2: Return front or default value

   If not empty, return q.front(); otherwise, return defaultVal.

3. Final Answer:

   if (!q.empty()) return q.front(); else return defaultVal; -> Option D

4. Quick Check:

   Check empty before front() to avoid errors [OK]

Hint: Always check empty before front() to avoid errors [OK]

Common Mistakes:

• Not checking empty before front()
• Wrong condition on size
• Returning front when empty