Pop Operation on Stack in DSA C# - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty?

No↓

Remove top element

↓

Update top pointer

↓

Return removed element

↓

Done

Yes↓

Error: Stack Underflow

The pop operation first checks if the stack is empty. If not, it removes the top element, updates the top pointer, and returns the removed element. If empty, it signals an error.

Execution Sample

DSA C#

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
int popped = stack.Pop();
Console.WriteLine(popped);

This code pushes 10 and 20 onto the stack, then pops the top element (20) and prints it.

Execution Table

Step	Operation	Nodes in Stack	Top Pointer	Visual State
1	Push 10	[10]	Points to 10	10 -> null
2	Push 20	[20, 10]	Points to 20	20 -> 10 -> null
3	Pop top element	[10]	Points to 10	10 -> null
4	Return popped element	N/A	N/A	Popped value: 20
5	Pop last element	[]	null	Popped value: 10

💡 Execution stops after popping top element and returning it; error occurs if pop attempted on empty stack.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
stack contents	[]	[10]	[20, 10]	[10]	[10]	[]
top pointer	null	points to 10	points to 20	points to 10	N/A	null
popped element	N/A	N/A	N/A	20	20	10

Key Moments - 3 Insights

Why does the top pointer move after popping?

After popping (see Step 3 in execution_table), the top pointer moves from the removed element to the next element below it to reflect the new top.

What happens if we pop when the stack is empty?

popping an empty stack causes an error called 'Stack Underflow' because there is no element to remove.

Why is the popped element returned after removal?

Step 4 shows the popped element is returned so the program knows which value was removed from the stack.

Visual Quiz - 3 Questions

Look at the execution_table, what is the stack content after Step 2?

A []

B [20, 10]

C [10]

D [10, 20]

💡 Hint

Check the 'Nodes in Stack' column for Step 2 in execution_table.

At which step does the top pointer move from 20 to 10?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Top Pointer' column in execution_table and see when it changes from pointing to 20 to 10.

If we try to pop again after Step 3, what will happen?

A Popped element is 10

B Top pointer points to 20

C Stack contains [20, 10]

D Stack Underflow error

💡 Hint

After Step 3, the stack has [10], so popping again removes 10 as shown in the variable_tracker.

Concept Snapshot

Pop Operation on Stack:
- Removes the top element from the stack
- Updates the top pointer to the next element
- Returns the removed element
- If stack is empty, pop causes 'Stack Underflow' error
- Stack follows Last-In-First-Out (LIFO) order

Full Transcript

The pop operation on a stack first checks if the stack is empty. If not empty, it removes the top element, updates the top pointer to the next element below, and returns the removed element. If the stack is empty, it signals an error called 'Stack Underflow'. For example, pushing 10 then 20 results in a stack with 20 on top. Popping removes 20 and moves the top pointer to 10. Popping again removes 10. Trying to pop when empty causes an error.

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