Pop Operation on Stack in DSA C# - Build from Scratch

PMC: Pop Operation on Stack

📖 Scenario: Imagine you are managing a stack of books. You can only take the top book off the stack at a time. This is called the pop operation in a stack data structure.

🎯 Goal: You will create a stack of books, set up a variable to hold the popped book, perform the pop operation, and then print the stack after popping.

📋 What You'll Learn

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

Create a variable called poppedBook to hold the popped book

Use the Pop() method on books and assign the result to poppedBook

Print the contents of books after popping

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world situations like undo features in text editors, browser history, and managing function calls.

💼 Career

Understanding stack operations like pop is essential for software developers, especially when working with algorithms, parsing, and memory management.

1

Create the stack of books

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

DSA C#

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

Hint

Use Stack<string> to create the stack and Push() to add each book.

2

Create a variable to hold the popped book

Create a variable called poppedBook of type string to hold the popped book

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 a string variable called poppedBook
        // Your code here
    }
}

Hint

Declare poppedBook as a string variable.

3

Pop the top book from the stack

Use the Pop() method on books and assign the result to poppedBook

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");

        string poppedBook;
        // Pop the top book from books and assign to poppedBook
        // Your code here
    }
}

Hint

Use poppedBook = books.Pop(); to remove and store the top book.

4

Print the stack after popping

Print the contents of books after popping using a foreach loop with variable book

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");

        string poppedBook;
        poppedBook = books.Pop();

        // Print each book in books using foreach with variable book
        // Your code here
    }
}

Hint

Use foreach (string book in books) and Console.WriteLine(book); to print each book.

Practice

1. What does the Pop() operation do in a stack?

easy

A. Adds an element to the bottom of the stack

B. Clears all elements from the stack

C. Returns the bottom element without removing it

D. Removes and returns the top element of the stack

Solution

1. Step 1: Understand the stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is on top.

2. Step 2: Define the Pop operation

   Pop removes and returns the top element, which is the last inserted item.

3. Final Answer:

   Removes and returns the top element of the stack -> Option D

4. Quick Check:

   Pop = remove top element [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking Pop adds elements
• Confusing Pop with Peek
• Assuming Pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack named myStack in C#?

easy

A. myStack.Pop();

B. myStack.pop();

C. Pop(myStack);

D. myStack.PopElement();

Solution

1. Step 1: Recall C# method naming conventions

   In C#, method names are case-sensitive and usually PascalCase.

2. Step 2: Identify the correct method call

   The Stack class has a method named Pop() with uppercase P, called on the stack instance.

3. Final Answer:

   myStack.Pop(); -> Option A

4. Quick Check:

   Correct method name and syntax = myStack.Pop(); [OK]

Hint: C# methods are case-sensitive and PascalCase [OK]

Common Mistakes:

• Using lowercase 'pop()' instead of 'Pop()'
• Trying to call Pop as a standalone function
• Using incorrect method names like PopElement

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

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
stack.Push(30);
int popped = stack.Pop();
Console.WriteLine(popped);
Console.WriteLine(string.Join(", ", stack));

medium

A. 10
20, 30

B. 30
20, 10

C. 20
10, 30

D. 30
10, 20

Solution

1. Step 1: Trace the Push operations

   Elements pushed in order: 10 (bottom), 20, 30 (top).

2. Step 2: Pop removes top element

   Pop removes 30, so popped = 30. Remaining stack: 20 (top), 10 (bottom).

3. Step 3: Print statements

   First line prints popped = 30. Second line prints stack elements from top to bottom: 20, 10.

4. Final Answer:

   30 20, 10 -> Option B

5. Quick Check:

   Pop returns top (30), stack now 20, 10 [OK]

Hint: Pop returns last pushed; stack order reverses on print [OK]

Common Mistakes:

• Confusing order of elements after pop
• Printing stack in wrong order
• Assuming Pop returns bottom element

4. Identify the error in the following C# code snippet that uses a stack:

Stack<string> stack = new Stack<string>();
stack.Push("apple");
stack.Push("banana");
string fruit = stack.Pop();
Console.WriteLine(fruit);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Pop());

medium

A. Pop method is called incorrectly

B. No error, code runs fine

C. Third Pop causes InvalidOperationException (stack empty)

D. Stack should be initialized with capacity

Solution

1. Step 1: Count Push and Pop calls

   Two Push calls add "apple" and "banana". Three Pop calls remove elements.

2. Step 2: Analyze stack state after Pops

   First Pop removes "banana", second Pop removes "apple", third Pop tries to remove from empty stack causing error.

3. Final Answer:

   Third Pop causes InvalidOperationException (stack empty) -> Option C

4. Quick Check:

   Pop on empty stack throws exception [OK]

Hint: Pop more times than Push causes error [OK]

Common Mistakes:

• Ignoring stack empty error
• Assuming Pop returns null if empty
• Thinking stack needs capacity on init

5. Given a stack with elements [5, 10, 15, 20] (5 at bottom, 20 at top), what will be the stack content after performing two Pop() operations followed by one Push(25) operation?

hard

A. [5, 10, 25]

B. [5, 10, 15, 25]

C. [10, 15, 25]

D. [5, 15, 25]

Solution

1. Step 1: Understand initial stack order

   Stack bottom to top: 5, 10, 15, 20.

2. Step 2: Perform two Pop operations

   First Pop removes 20, second Pop removes 15. Remaining stack: 5, 10.

3. Step 3: Perform Push(25)

   Push adds 25 on top. Final stack: 5, 10, 25.

4. Final Answer:

   [5, 10, 25] -> Option A

5. Quick Check:

   Pop twice removes top two; Push adds new top [OK]

Hint: Pop removes top; Push adds new top [OK]

Common Mistakes:

• Removing wrong elements on Pop
• Adding Push element at bottom
• Confusing stack order after operations