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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the front element in this queue?

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

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(10);
        q.Enqueue(20);
        q.Enqueue(30);
        Console.WriteLine(q.Peek());
    }
}

A 20

B 30

C Throws InvalidOperationException

D 10

💡 Hint

Peek returns the front element without removing it.

The queue has elements 10, 20, 30 in that order. Peek returns the first element, which is 10.

❓ Predict Output

intermediate

What happens when peeking an empty queue?

What will happen if you call Peek on an empty queue in C#?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        Console.WriteLine(q.Peek());
    }
}

A Throws InvalidOperationException

B Prints 0

C Prints null

D Prints -1

💡 Hint

Peek on empty queue throws an exception.

Calling Peek on an empty queue throws InvalidOperationException because there is no element to return.

🔧 Debug

advanced

Why does this code throw an exception when peeking?

This code throws an exception at runtime. What is the cause?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<string> q = new Queue<string>();
        q.Enqueue(null);
        Console.WriteLine(q.Peek().Length);
    }
}

A Peek throws InvalidOperationException because queue is empty

B Queue cannot enqueue null values, so Enqueue throws exception

C Peek returns null, so accessing Length causes NullReferenceException

D Code runs fine and prints 0

💡 Hint

Peek returns the front element, which is null here.

The queue contains one element which is null. Peek returns null, so accessing Length causes NullReferenceException.

❓ Predict Output

advanced

What is the queue state after peeking?

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

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(5);
        q.Enqueue(15);
        q.Enqueue(25);
        int front = q.Peek();
        Console.WriteLine(front);
        Console.WriteLine(string.Join(",", q));
    }
}

A Throws InvalidOperationException

B

5
5,15,25

C

5
25

D

5
15,25

💡 Hint

Peek does not remove the element.

Peek returns the front element without removing it, so queue remains unchanged.

🧠 Conceptual

expert

Which statement about Peek in a queue is true?

Select the correct statement about the Peek operation on a queue.

A Peek returns the front element without removing it and throws if queue is empty.

B Peek removes and returns the front element of the queue.

C Peek returns the last element added to the queue.

D Peek returns null if the queue is empty.

💡 Hint

Think about what Peek does and what happens if the queue is empty.

Peek returns the front element without removing it. If the queue is empty, it throws InvalidOperationException.

Practice

1. What does the Peek() method do in a queue?

easy

A. Checks if the queue is empty

B. Returns the front element without removing it

C. Adds an element to the back of the queue

D. Removes and returns the front element

Solution

1. Step 1: Understand the purpose of Peek()

   The Peek() method is designed to look at the front element of the queue without changing the queue itself.

2. Step 2: Differentiate from other queue methods

   Unlike Dequeue() which removes the front element, Peek() only returns it without removal.

3. Final Answer:

   Returns the front element without removing it -> Option B

4. Quick Check:

   Peek() returns front element without removal [OK]

Hint: Peek shows front without removing it [OK]

Common Mistakes:

• Confusing Peek() with Dequeue()
• Thinking Peek() removes the element
• Assuming Peek() adds elements

2. Which of the following is the correct syntax to peek the front element of a queue named myQueue in C#?

easy

A. myQueue.Front();

B. myQueue.GetFront();

C. myQueue.Peek();

D. myQueue.Top();

Solution

1. Step 1: Recall the Queue method for peeking

   In C#, the method to look at the front element without removing it is Peek().

2. Step 2: Verify method names

   Methods like Front(), GetFront(), or Top() do not exist in the standard Queue class.

3. Final Answer:

   myQueue.Peek(); -> Option C

4. Quick Check:

   Correct syntax is myQueue.Peek() [OK]

Hint: Use Peek() method to access front element [OK]

Common Mistakes:

• Using non-existent methods like Front()
• Confusing Peek() with Top() from stacks
• Missing parentheses after Peek

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

Queue<int> q = new Queue<int>();
q.Enqueue(10);
q.Enqueue(20);
q.Enqueue(30);
Console.WriteLine(q.Peek());

medium

A. 10

B. 20

C. 30

D. Throws an exception

Solution

1. Step 1: Understand queue order after enqueues

   Elements are added in order: 10 (front), then 20, then 30 (back).

2. Step 2: Peek returns front element

   Calling Peek() returns the front element without removing it, which is 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Peek returns first enqueued element 10 [OK]

Hint: Peek returns first enqueued element [OK]

Common Mistakes:

• Thinking Peek returns last element
• Confusing Peek with Dequeue
• Assuming exception on non-empty queue

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

Queue<string> q = new Queue<string>();
string front = q.Peek;
Console.WriteLine(front);

medium

A. Peek method does not exist for Queue

B. Queue is empty, so Peek throws exception

C. Queue should be initialized with elements first

D. Missing parentheses after Peek method

Solution

1. Step 1: Check method call syntax

   The code uses q.Peek without parentheses, but Peek is a method and requires ().

2. Step 2: Confirm other possible errors

   Though the queue is empty, the question focuses on syntax. The missing parentheses cause a compile-time error.

3. Final Answer:

   Missing parentheses after Peek method -> Option D

4. Quick Check:

   Peek() needs parentheses to call method [OK]

Hint: Always add () when calling methods [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Ignoring empty queue exceptions
• Assuming Peek is a property

5. Given a queue q with elements [5, 10, 15], which code snippet correctly peeks the front element and then adds 20 to the queue without removing any elements?

hard

A. int front = q.Peek(); q.Enqueue(20);

B. int front = q.Dequeue(); q.Enqueue(20);

C. int front = q.Peek; q.Enqueue(20);

D. int front = q.Pop(); q.Enqueue(20);

Solution

1. Step 1: Identify correct method to peek front element

   Use Peek() to get front element without removing it.

2. Step 2: Add element without removing any

   Enqueue(20) adds 20 to the back without removing elements.

3. Step 3: Check other options for errors

   Dequeue() removes front element, Peek without parentheses is invalid, and Pop() does not exist for Queue.

4. Final Answer:

   int front = q.Peek(); q.Enqueue(20); -> Option A

5. Quick Check:

   Peek() + Enqueue() keeps queue intact [OK]

Hint: Peek() to view front, Enqueue() to add [OK]

Common Mistakes:

• Using Dequeue() instead of Peek() when not removing
• Missing parentheses on Peek
• Using Pop() which is for stacks