Peek Front Element of Queue in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a queue?

It shows the front element of the queue without removing it, so you can see which item will be removed next.

beginner

In a queue, which end is the 'front'?

The front is the end where elements are removed (dequeued) from the queue.

beginner

Why is peeking useful in a queue?

Peeking lets you check the next element to be removed without changing the queue, helping you decide what to do next.

intermediate

What happens if you peek an empty queue?

Usually, it returns a special value or error because there is no front element to show.

beginner

How do you peek the front element in a C++ STL queue?

Use the 'front()' method, like queue.front(), to get the front element without removing it.

What does the peek operation in a queue do?

A Shows the front element without removing it

B Removes the front element

C Adds an element to the back

D Checks if the queue is empty

Peek shows the front element without removing it, unlike dequeue which removes it.

Which method is used to peek the front element in a C++ STL queue?

A queue.pop()

B queue.peek()

C queue.back()

D queue.front()

queue.front() returns the front element without removing it.

What will happen if you peek an empty queue?

A Returns the last element

B Adds a new element

C Returns a special value or error

D Removes an element

Peeking an empty queue usually returns an error or special value because no front element exists.

In a queue, where are elements removed from?

A Front

B Middle

C Back

D Random position

Elements are removed from the front of the queue.

Why might you want to peek at the front element of a queue?

A To remove it immediately

B To see what will be removed next without changing the queue

C To add a new element

D To clear the queue

Peeking lets you see the next element to be removed without changing the queue.

Explain how the peek operation works in a queue and why it is useful.

Think about checking the next item without changing the queue.

Describe what happens when you try to peek at an empty queue and how to handle it.

Consider what the queue contains before peeking.

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