Peek Top Element of Stack in DSA Python - Execution Trace

Concept Flow - Peek Top Element of Stack

Start

↓

Check if stack is empty?

Yes→Return None or Error

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 Python

stack = [10, 20, 30]
top = stack[-1]
print(top)

Peek the top element of the stack and print it without removing it.

Execution Table

Step	Operation	Stack State	Pointer Changes	Visual State
1	Start with stack = [10, 20, 30]	[10, 20, 30]	top pointer not set	10 -> 20 -> 30 (top)
2	Check if stack is empty	[10, 20, 30]	No change	10 -> 20 -> 30 (top)
3	Access top element stack[-1]	[10, 20, 30]	top pointer points to 30	10 -> 20 -> 30 (top)
4	Return top element 30	[10, 20, 30]	top pointer unchanged	10 -> 20 -> 30 (top)
5	Print top element	[10, 20, 30]	No change	10 -> 20 -> 30 (top)
6	End	[10, 20, 30]	No change	10 -> 20 -> 30 (top)

💡 Peek operation ends after returning the top element without modifying the stack.

Variable Tracker

Variable	Start	After Step 3	Final
stack	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
top	undefined	30	30

Key Moments - 3 Insights

Why do we check if the stack is empty before peeking?

Because peeking an empty stack has no top element to return, so we avoid errors by checking first (see execution_table step 2).

Does peeking remove the top element from the stack?

No, peeking only reads the top element without removing it, so the stack stays the same (see execution_table steps 3 and 4).

What happens if we try to peek an empty stack?

The operation returns None or an error, depending on implementation, because there is no element to return (refer to concept_flow step 'Return None or Error').

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'top' after step 3?

A 30

B 20

C 10

D undefined

💡 Hint

Check the 'Pointer Changes' column at step 3 in the execution_table.

At which step does the code confirm the stack is not empty?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Operation' column for the check condition in the execution_table.

If the stack was empty, what would the execution_table show at step 2?

A Stack state with elements

B Return None or Error

C Pointer changes to top element

D Access top element

💡 Hint

Refer to the concept_flow where empty stack leads to returning None or Error.

Concept Snapshot

Peek Top Element of Stack:
- Check if stack is empty first
- Access top element without removing
- Return the top element
- Stack remains unchanged
- Commonly use stack[-1] in Python

Full Transcript

This visual trace shows how to peek the top element of a stack. First, we check if the stack is empty to avoid errors. If not empty, we access the last element (top) without removing it. The stack stays the same throughout. The top element is then returned and printed. This operation is safe and does not modify the stack.