Stack operations (push, pop, peek) in Data Structures Theory - Step-by-Step Execution

Concept Flow - Stack operations (push, pop, peek)

Start

↓

Check Operation

↓

Push?

Yes→Add item on top

↓

Update top pointer

↓

Done

↓

Pop?

Yes→Remove item from top

↓

Update top pointer

↓

Return removed item

↓

Peek?

Yes→Look at top item

↓

Return top item

The stack operation flow checks which operation is requested: push adds an item on top, pop removes the top item, and peek returns the top item without removing it.

Execution Sample

Data Structures Theory

stack = []
stack.append(10)
stack.append(20)
top = stack[-1]
item = stack.pop()

This code adds two items to the stack, looks at the top item, then removes the top item.

Analysis Table

Step	Operation	Stack State	Top Pointer	Output/Return
1	Initialize stack	[]	None	None
2	Push 10	[10]	10	None
3	Push 20	[10, 20]	20	None
4	Peek	[10, 20]	20	20
5	Pop	[10]	10	20
6	End	[10]	10	No more operations

💡 No more operations to perform, stack remains with one item [10]

State Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
stack	[]	[10]	[10, 20]	[10, 20]	[10]	[10]
top pointer	None	10	20	20	10	10
output	None	None	None	20	20	None

Key Insights - 3 Insights

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

Peek only looks at the top item without changing the stack, as shown in step 4 of the execution_table where the stack remains [10, 20] after peek.

What happens to the top pointer after a pop operation?

After pop (step 5), the top pointer updates to the next item below the removed one, changing from 20 to 10, reflecting the new top.

Can we pop from an empty stack?

No, popping from an empty stack is invalid and would cause an error; the execution_table starts with an empty stack and only pops after items are pushed.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the top pointer value?

A None

B 10

C 20

D 30

💡 Hint

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

At which step does the stack change from [10, 20] to [10]?

A Step 4

B Step 5

C Step 2

D Step 6

💡 Hint

Look at the 'Stack State' column in the execution_table to see when the stack size decreases.

If we perform another pop after step 5, what would be the new top pointer?

A None

B 20

C 10

D Error

💡 Hint

After popping the last item, the stack becomes empty, so the top pointer is None.

Concept Snapshot

Stack operations:
- push(item): add item on top
- pop(): remove and return top item
- peek(): return top item without removing
Top pointer always points to the last added item
Cannot pop from empty stack

Full Transcript

A stack is a data structure where items are added and removed from the top only. The main operations are push, which adds an item on top; pop, which removes the top item; and peek, which looks at the top item without removing it. The top pointer always points to the current top item. Popping from an empty stack is not allowed. This visual trace shows how the stack changes step-by-step with these operations.