Peek Front Element of Queue in DSA C++ - Build from Scratch

Peek Front Element of Queue

📖 Scenario: You are managing a line of customers waiting to buy tickets at a counter. You want to check who is the first customer in line without removing them.

🎯 Goal: Build a simple program that creates a queue of customers, then shows the name of the first customer waiting in line.

📋 What You'll Learn

Create a queue with exact customer names in order

Use a variable to store the front element of the queue

Print the front element without removing it

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting, print jobs, or tasks waiting to be processed.

💼 Career

Understanding queues and how to peek at elements is important for software developers working with task scheduling, event handling, and data streaming.

1

Create a queue with customers

Create a std::queue<std::string> called customers and add these names in order: "Alice", "Bob", "Charlie".

DSA C++

#include 
#include 
#include 

int main() {
    // Create queue and add customers
    // Your code here
    return 0;
}

Hint

Use customers.push("Name") to add each customer in order.

2

Store the front customer

Create a std::string variable called frontCustomer and set it to the front element of the customers queue using customers.front().

DSA C++

#include 
#include 
#include 

int main() {
    std::queue customers;
    customers.push("Alice");
    customers.push("Bob");
    customers.push("Charlie");

    // Store front customer
    // Your code here
    return 0;
}

Hint

Use customers.front() to get the first element without removing it.

3

Print the front customer

Use std::cout to print the text "First customer in line: " followed by the value of frontCustomer.

DSA C++

#include 
#include 
#include 

int main() {
    std::queue customers;
    customers.push("Alice");
    customers.push("Bob");
    customers.push("Charlie");

    std::string frontCustomer = customers.front();

    // Print front customer
    // Your code here
    return 0;
}

Hint

Use std::cout with << to print the message and variable.

4

Run and check output

Run the program and ensure it prints exactly: First customer in line: Alice

DSA C++

#include 
#include 
#include 

int main() {
    std::queue customers;
    customers.push("Alice");
    customers.push("Bob");
    customers.push("Charlie");

    std::string frontCustomer = customers.front();

    std::cout << "First customer in line: " << frontCustomer << std::endl;
    // Check output
    // Your code here
    return 0;
}

Hint

Make sure the output matches exactly, including capitalization and spacing.

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