Peek Top Element of Stack in DSA C++ - Execution Trace

Concept Flow - Peek Top Element of Stack

Start

↓

Check if stack is empty?

Yes→Return error or null

No↓

Access top element

↓

Return top element

↓

End

The flow checks if the stack is empty. If not, it accesses and returns the top element without removing it.

Execution Sample

DSA C++

#include <iostream>
#include <stack>
using namespace std;

int main() {
    stack<int> s;
    s.push(10);
    s.push(20);
    int top = s.top();
    cout << top;
    return 0;
}

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

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Initialize empty stack	[]	head = null	null
2	Push 10	[10]	head -> 10	10 -> null
3	Push 20	[20, 10]	head -> 20	20 -> 10 -> null
4	Peek top element	[20, 10]	head -> 20	20 -> 10 -> null
5	Return top element value	20	head unchanged	20 -> 10 -> null
6	End	[20, 10]	head -> 20	20 -> 10 -> null

💡 Peek stops after returning the top element without modifying the stack.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
stack contents	[]	[10]	[20, 10]	[20, 10]	[20, 10]
head pointer	null	points to 10	points to 20	points to 20	points to 20
top element value	undefined	undefined	undefined	20	20

Key Moments - 3 Insights

Why does peek not remove the top element from the stack?

Peek only accesses the top element without changing the stack, as shown in execution_table step 5 where the stack remains [20, 10].

What happens if we peek when the stack is empty?

The flow in concept_flow shows that if the stack is empty, peek returns an error or null, preventing access to a non-existent element.

Why does the head pointer not change during peek?

Because peek only reads the top element, the head pointer remains pointing to the same node, as seen in variable_tracker after step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the visual state of the stack?

A 10 -> 20 -> null

B 20 -> 10 -> null

C null

D 20 -> null

💡 Hint

Check the 'Visual State' column at step 3 in the execution_table.

At which step does the top element value become defined?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at variable_tracker row 'top element value' to see when it first gets a value.

If the stack was empty, what would the peek operation do according to concept_flow?

A Return the last pushed element

B Add a new element

C Return error or null

D Remove the top element

💡 Hint

Refer to the decision branch in concept_flow for empty stack check.

Concept Snapshot

Peek Top Element of Stack:
- Check if stack is empty
- If not empty, access top element
- Return top element without removing
- Stack remains unchanged
- Useful to see current top value

Full Transcript

Peeking the top element of a stack means looking at the last item added without removing it. The process starts by checking if the stack is empty. If it is, peek returns an error or null to avoid invalid access. If not empty, peek accesses the top element and returns its value. The stack's structure and pointers do not change during peek. This is shown in the execution where after pushing 10 and 20, peeking returns 20 while the stack remains 20 -> 10 -> null.

Practice

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

easy

A. Returns the top element without removing it

B. Removes the top element from the stack

C. Adds a new element to the top of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack operations

   A stack is a collection where elements are added or removed from the top only.

2. Step 2: Define peek operation

   The peek operation returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   peek() = top element [OK]

Hint: Peek shows top without removal [OK]

Common Mistakes:

• Confusing peek with pop
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the top element of a stack named stk in C++ STL?

easy

A. stk.pop();

B. stk.peek();

C. stk.top();

D. stk.getTop();

Solution

1. Step 1: Recall C++ STL stack methods

   The standard method to access the top element is top().

2. Step 2: Identify correct syntax

   stk.top(); returns the top element without removing it.

3. Final Answer:

   stk.top(); -> Option C

4. Quick Check:

   stk.top() = peek top element [OK]

Hint: Use .top() to peek in C++ STL stack [OK]

Common Mistakes:

• Using .peek() which is not in C++ STL
• Using .pop() which removes element
• Using non-existent .getTop() method

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

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.push(30);
std::cout << stk.top() << std::endl;

medium

A. 30

B. 10

C. 20

D. Compilation error

Solution

1. Step 1: Trace push operations

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

2. Step 2: Check top() output

   stk.top() returns 30 without removing it.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed = top = 30 [OK]

Hint: Top is last pushed element [OK]

Common Mistakes:

• Confusing top with bottom element
• Assuming top removes element
• Thinking output is 20 or 10

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

int topElement = stk.peek();

medium

A. stk should be declared as a queue

B. peek() is not a valid method in C++ STL stack

C. topElement should be a pointer

D. stk.pop() should be used instead

Solution

1. Step 1: Check method availability in C++ STL stack

   C++ STL stack does not have a method named peek().

2. Step 2: Correct method to peek top element

   The correct method is top(), so stk.top() should be used.

3. Final Answer:

   peek() is not a valid method in C++ STL stack -> Option B

4. Quick Check:

   Use stk.top() not stk.peek() [OK]

Hint: Use .top(), not .peek() in C++ STL [OK]

Common Mistakes:

• Using peek() method which doesn't exist
• Confusing pop() with peek()
• Thinking topElement must be pointer

5. Given a stack stk with elements [5, 10, 15] pushed in that order, which code snippet correctly prints the top element without removing it and then prints the stack size?

hard

A. std::cout << stk.pop() << std::endl; std::cout << stk.size() << std::endl;

B. std::cout << stk.top() << std::endl; stk.pop(); std::cout << stk.size() << std::endl;

C. std::cout << stk.peek() << std::endl; std::cout << stk.size() << std::endl;

D. std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl;

Solution

1. Step 1: Understand peek without removal

   To peek, use stk.top() which returns top element without removing it.

2. Step 2: Check stack size after peek

   Since peek does not remove, stk.size() remains 3.

3. Final Answer:

   std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl; -> Option D

4. Quick Check:

   top() no removal + size() correct [OK]

Hint: Use top() to peek, size() unchanged [OK]

Common Mistakes:

• Using pop() which removes element
• Using peek() which is invalid in C++ STL
• Printing size after pop() expecting no change