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

int peek(Stack* s) {
    if (s->top == -1) return -1; // empty
    return s->arr[s->top];
}

This code returns the top element of the stack if it exists, otherwise returns -1 indicating empty stack.

Execution Table

Step	Operation	Stack Top Index	Stack Array	Pointer Changes	Visual State
1	Start peek operation	2	[10, 20, 30]	top points to index 2	30 (top) -> 20 -> 10 -> null
2	Check if stack is empty (top == -1?)	2	[10, 20, 30]	No change	30 (top) -> 20 -> 10 -> null
3	Access element at top index	2	[10, 20, 30]	No change	30 (top) -> 20 -> 10 -> null
4	Return top element (30)	2	[10, 20, 30]	No change	30 (top) -> 20 -> 10 -> null
5	End peek operation	2	[10, 20, 30]	No change	30 (top) -> 20 -> 10 -> null

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

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
top	2	2	2	2	2
stack array	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
peeked value	N/A	N/A	N/A	30	30

Key Moments - 3 Insights

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

Because if the stack is empty (top == -1), there is no element to peek. The execution_table row 2 shows this check prevents accessing invalid memory.

Does peeking remove the top element from the stack?

No, peeking only reads the top element without changing the stack. The pointer 'top' remains the same as shown in all steps in the execution_table.

What happens if we peek on an empty stack?

The function returns -1 or an error value as shown in the concept_flow and execution_table step 2, preventing invalid access.

Visual Quiz - 3 Questions

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

A 2

B -1

C 3

D 0

💡 Hint

Check the 'Stack Top Index' column in execution_table row 3.

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

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Operation' column in execution_table where the empty check happens.

If the stack was empty (top = -1), what would the peek function return?

A The bottom element

B The top element

C -1 or error value

D 0

💡 Hint

Refer to concept_flow and key_moments about empty stack handling.

Concept Snapshot

Peek Top Element of Stack:
- Check if stack is empty (top == -1)
- If empty, return error or special value
- Else, return element at stack[top]
- Does NOT remove element
- Stack remains unchanged after peek

Full Transcript

Peeking the top element of a stack means looking at the element on top without removing it. The process starts by checking if the stack is empty by verifying if the top index is -1. If empty, the function returns an error value like -1. If not empty, it accesses the element at the top index and returns it. The stack's content and top pointer do not change during this operation. This ensures safe access to the top element without modifying the stack.