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 what will be removed next.

beginner

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

The front is the end where elements are removed (dequeued). It's like the front of a line where the first person is served.

intermediate

What happens if you peek an empty queue in C#?

It throws an InvalidOperationException because there is no element to show.

beginner

How do you peek the front element of a queue in C#?

Use the Queue's Peek() method, like: queue.Peek().

beginner

Why is peek useful compared to dequeue?

Peek lets you see the next element without removing it, so you can decide what to do next without changing the queue.

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

A Returns the front element without removing it

B Removes and returns 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.

What happens if you call Peek() on an empty queue in C#?

A Returns null

B Removes last element

C Throws InvalidOperationException

D Returns default value

Peek() throws InvalidOperationException if the queue is empty.

Which end of the queue does Peek() look at?

A Front

B Middle

C Back

D Random

Peek() shows the front element of the queue.

Which method removes the front element from a queue?

A Remove()

B Peek()

C Enqueue()

D Dequeue()

Dequeue() removes and returns the front element.

Why use Peek() instead of Dequeue()?

A To add elements

B To see the front element without removing it

C To clear the queue

D To check queue size

Peek() lets you see the front element without removing it.

Explain how the Peek() method works in a queue and what happens if the queue is empty.

Think about looking at the first person in line without asking them to leave.

Describe the difference between Peek() and Dequeue() in a queue.

One is just looking, the other is taking out.

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