Stack Implementation Using Array in DSA Python - Execution Trace

Concept Flow - Stack Implementation Using Array

Start

↓

Check if stack is full?

Yes→Cannot push, overflow

No↓

Add element at top index

↓

Increment top pointer

↓

Check if stack is empty?

Yes→Cannot pop, underflow

No↓

Remove element at top-1 index

↓

Decrement top pointer

↓

Peek element at top-1 index

↓

End

The stack uses an array and a top pointer. Push adds element at top and increments top. Pop removes element at top-1 and decrements top. Peek reads element at top-1.

Execution Sample

DSA Python

stack = []
top = 0
stack.append(10); top += 1
stack.append(20); top += 1
popped = stack.pop(); top -= 1
peek = stack[top-1]

This code pushes 10 and 20 onto the stack, pops the top element, then peeks at the current top.

Execution Table

Step	Operation	Top Pointer	Stack Array	Action Description	Visual State
1	Initialize stack	0	[]	Start with empty stack	[]
2	Push 10	1	[10]	Add 10 at index 0, increment top to 1	[10] (top=1)
3	Push 20	2	[10, 20]	Add 20 at index 1, increment top to 2	[10, 20] (top=2)
4	Pop	1	[10]	Remove element at top-1 (index 1), decrement top to 1	[10] (top=1)
5	Peek	1	[10]	Read element at top-1 (index 0), value=10	[10] (top=1)
6	Pop	0	[]	Remove element at top-1 (index 0), decrement top to 0	[] (top=0)
7	Pop	0	[]	Cannot pop, stack empty (underflow)	[] (top=0)

💡 Execution stops after underflow on pop when stack is empty (top=0).

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
top	0	1	2	1	1	0	0
stack	[]	[10]	[10, 20]	[10]	[10]	[]	[]

Key Moments - 3 Insights

Why does the top pointer increment after push but decrement after pop?

Because top points to the next free position in the array. After push, top moves up to next free slot (see Step 2 and 3). After pop, top moves down to reflect removal (see Step 4 and 6).

Why do we access stack[top-1] when peeking or popping?

Because top points to the next free position, so the last element is at top-1 index (see Step 4 and 5 in execution_table).

What happens if we try to pop when the stack is empty?

The pop operation cannot proceed and results in underflow (see Step 7). The stack remains empty and top stays at 0.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of top after pushing 20?

A 1

B 2

C 0

D 3

💡 Hint

Check Step 3 in the execution_table under 'Top Pointer'

At which step does the stack become empty again after popping all elements?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at Step 6 where top becomes 0 and stack is []

If we try to pop when top is 0, what happens according to the execution table?

A Pop removes an element

B Stack overflows

C Underflow error, cannot pop

D Top increments

💡 Hint

See Step 7 where pop is attempted on empty stack

Concept Snapshot

Stack using array:
- Use array and top pointer
- Push: add element at top, top++
- Pop: remove element at top-1, top--
- Peek: read element at top-1
- top points to next free slot
- Underflow if pop when empty

Full Transcript

This visualization shows how a stack is implemented using an array and a top pointer. The top pointer always points to the next free position in the array. When pushing, the element is added at the current top index and then top is incremented. When popping, the element at top-1 is removed and top is decremented. Peeking reads the element at top-1 without changing top. The execution table traces each operation step-by-step, showing the stack array and top pointer changes. Key moments clarify why top increments and decrements, why we access top-1 index, and what happens on underflow. The visual quiz tests understanding of top pointer values and stack empty conditions.