Stack Concept and LIFO Principle in DSA C - Execution Trace

Concept Flow - Stack Concept and LIFO Principle

Start

↓

Push element

→Add element on top

↓

Pop element

→Remove element from top

↓

Peek element

→View element on top

↓

Is stack empty?

Yes→Stop

No↓

Repeat operations

Stack works by adding and removing elements only at the top, following Last In First Out (LIFO).

Execution Sample

DSA C

push(10);
push(20);
push(30);
pop();
peek();

Push three elements onto the stack, pop the top, then peek at the current top.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	push(10)	10	top -> 10	10 -> null
2	push(20)	20, 10	top -> 20	20 -> 10 -> null
3	push(30)	30, 20, 10	top -> 30	30 -> 20 -> 10 -> null
4	pop()	20, 10	top -> 20	20 -> 10 -> null
5	peek()	20, 10	top unchanged	20 -> 10 -> null
6	End	20, 10	No change	20 -> 10 -> null

💡 No more operations; stack top is 20, stack not empty.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
top	null	10	20	30	20	20	20
stack_size	0	1	2	3	2	2	2

Key Moments - 3 Insights

Why does pop() remove the last pushed element and not the first?

Because stack follows LIFO, the last element pushed (step 3) is on top and pop() removes it (step 4), as shown in the execution_table.

What happens to the top pointer after pop()?

After pop() at step 4, the top pointer moves from 30 to 20, the next element below, as shown in the Pointer Changes column.

Does peek() change the stack?

No, peek() only views the top element without removing it, so the stack and top pointer remain unchanged at step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the top element after step 3?

A 10

B 20

C 30

D null

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3.

At which step does the stack size decrease?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'stack_size' variable in variable_tracker after each step.

If we add push(40) after step 5, what will be the new top element?

A 40

B 30

C 20

D 10

💡 Hint

Pushing adds element on top; see how push changed top in earlier steps.

Concept Snapshot

Stack stores elements in a LIFO order.
push() adds element on top.
pop() removes element from top.
peek() views top without removing.
Only top element is accessible.
Used for undo, backtracking, expression evaluation.

Full Transcript

A stack is a data structure where elements are added and removed only from the top, following the Last In First Out (LIFO) principle. When we push elements, they go on top of the stack. When we pop, we remove the top element, which is the last one added. Peeking lets us see the top element without removing it. The top pointer always points to the current top element. This behavior is shown step-by-step in the execution table and variable tracker, helping beginners understand how the stack changes with each operation.