Peek Top Element of Stack in DSA C# - Build from Scratch

Peek Top Element of Stack

📖 Scenario: You are managing a stack of books in a library. You want to check which book is currently on the top without removing it.

🎯 Goal: Build a simple program that creates a stack of book titles, then shows the top book using the peek operation.

📋 What You'll Learn

Create a stack of strings called books with these exact titles in this order: "Math", "Science", "History"

Create an integer variable called stackSize to hold the number of books in the stack

Use the Peek() method on the books stack to get the top book without removing it

Print the top book title exactly as shown

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and managing tasks in order.

💼 Career

Understanding stack operations like push, pop, and peek is fundamental for software development roles, especially in algorithm design and system programming.

1

Create the stack of books

Create a stack of strings called books and add these titles in this order: "Math", "Science", "History"

DSA C#

// Create the stack called books and add the three book titles
// Your code here

Hint

Use Stack<string> and the Push() method to add items.

2

Add a variable for stack size

Create an integer variable called stackSize and set it to the number of books in the books stack using the Count property

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack books = new Stack();
        books.Push("Math");
        books.Push("Science");
        books.Push("History");
        // Create stackSize and set it to books.Count
        // Your code here
    }
}

Hint

Use books.Count to get the number of items in the stack.

3

Peek the top book

Create a string variable called topBook and set it to the top book title using the Peek() method on the books stack

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack books = new Stack();
        books.Push("Math");
        books.Push("Science");
        books.Push("History");
        int stackSize = books.Count;
        // Create topBook and set it to books.Peek()
        // Your code here
    }
}

Hint

Use books.Peek() to see the top item without removing it.

4

Print the top book

Print the value of topBook using Console.WriteLine() exactly as shown

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack books = new Stack();
        books.Push("Math");
        books.Push("Science");
        books.Push("History");
        int stackSize = books.Count;
        string topBook = books.Peek();
        // Print topBook
        // Your code here
    }
}

Hint

Use Console.WriteLine(topBook); to print the top book.

Practice

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

easy

A. Returns the top element without removing it

B. Removes and returns the top element

C. Adds a new element to the top

D. Checks if the stack is empty

Solution

1. Step 1: Understand the Peek method purpose

   The Peek method is designed to look at the top element of the stack without changing the stack itself.

2. Step 2: Differentiate from Pop method

   Unlike Pop, Peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek() returns top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing Peek with Pop
• Thinking Peek adds elements
• Assuming Peek empties the stack

2. Which of the following is the correct syntax to peek the top element of a stack named myStack in C#?

easy

A. var top = myStack.Pop();

B. var top = myStack.Top();

C. var top = myStack.Peek();

D. var top = myStack.GetTop();

Solution

1. Step 1: Recall Stack method names in C#

   The Stack class in C# uses Peek() to look at the top element without removing it.

2. Step 2: Verify method correctness

   Top(), GetTop() are not valid Stack methods; Pop() removes the element.

3. Final Answer:

   var top = myStack.Peek(); -> Option C

4. Quick Check:

   Peek() is correct syntax for top element [OK]

Hint: Use Peek() method to get top element [OK]

Common Mistakes:

• Using Pop() instead of Peek()
• Using non-existent methods like Top()
• Missing parentheses after Peek

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

Stack<int> numbers = new Stack<int>();
numbers.Push(10);
numbers.Push(20);
numbers.Push(30);
Console.WriteLine(numbers.Peek());
Console.WriteLine(numbers.Count);

medium

A. 10
3

B. 30
3

C. 20
2

D. 30
2

Solution

1. Step 1: Trace the Push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Evaluate Peek and Count

   Peek returns top element 30 without removing it. Count remains 3 as no Pop occurred.

3. Final Answer:

   30 3 -> Option B

4. Quick Check:

   Peek = 30, Count = 3 [OK]

Hint: Peek shows last pushed; Count unchanged by Peek [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing Count after Peek
• Mixing order of pushed elements

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

Stack<string> stack = new Stack<string>();
string top = stack.Peek;
Console.WriteLine(top);

medium

A. Peek removes the element, so top is null

B. Stack is not initialized properly

C. Console.WriteLine cannot print stack elements

D. Peek is used without parentheses, causing a syntax error

Solution

1. Step 1: Check method call syntax

   Peek is a method and must be called with parentheses: Peek()

2. Step 2: Identify error type

   Using Peek without parentheses causes a compile-time syntax error.

3. Final Answer:

   Peek is used without parentheses, causing a syntax error -> Option D

4. Quick Check:

   Method calls need () [OK]

Hint: Always use parentheses when calling methods [OK]

Common Mistakes:

• Forgetting parentheses on Peek
• Assuming Peek removes element
• Thinking Console.WriteLine can't print strings

5. Given a stack Stack<char> letters with elements 'a', 'b', 'c' pushed in that order, write the output after this code:

char top1 = letters.Peek();
letters.Pop();
char top2 = letters.Peek();
Console.WriteLine(top1);
Console.WriteLine(top2);
Console.WriteLine(letters.Count);

hard

A. c
b
2

B. a
b
3

C. c
c
3

D. b
a
1

Solution

1. Step 1: Trace initial stack and first Peek

   Stack after pushes: bottom 'a', then 'b', top 'c'. Peek() returns 'c' as top1.

2. Step 2: Pop removes top element 'c'

   After Pop(), top 'c' is removed. Stack top is now 'b'.

3. Step 3: Second Peek returns 'b' as top2

   Peek() now returns 'b'. Count is 2 because one element was removed.

4. Final Answer:

   c b 2 -> Option A

5. Quick Check:

   Peek then Pop then Peek updates top correctly [OK]

Hint: Peek shows current top; Pop removes it; next Peek shows new top [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing order of elements after Pop
• Miscounting stack size after Pop