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.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. First In, First Out - the first element added is removed first

B. Last In, First Out - the last element added is removed first

C. Last In, First Out - the first element added is removed first

D. First In, Last Out - the first element added is removed last

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with options

   Last In, First Out - the last element added is removed first correctly describes this behavior as "Last In, First Out".

3. Final Answer:

   Last In, First Out - the last element added is removed first -> Option B

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up order of insertion and removal

2. Which of the following is the correct way to push an element 'x' onto a stack named 's' in C?

easy

A. s.push = x;

B. s.push(x);

C. push(x, s);

D. push(s, x);

Solution

1. Step 1: Identify correct function call syntax in C

   In C, stack operations are usually functions like push(stack, element).

2. Step 2: Check options

   push(s, x); uses push(s, x); which is the correct syntax for pushing x onto stack s.

3. Final Answer:

   push(s, x); -> Option D

4. Quick Check:

   Function call push(stack, element) = push(s, x) [OK]

Hint: In C, function calls have parameters in parentheses [OK]

Common Mistakes:

• Using object.method() syntax (not C style)
• Swapping parameter order
• Assigning instead of calling function

3. Consider the following C code using a stack with push and pop operations:

push(s, 10);
push(s, 20);
pop(s);
push(s, 30);
pop(s);

What is the value returned by the last pop operation?

medium

A. 30

B. 20

C. 10

D. Error - stack empty

Solution

1. Step 1: Trace stack after each operation

   After push 10: stack = [10]
   After push 20: stack = [10, 20]
   pop(): removes 20, stack = [10]
   push 30: stack = [10, 30]
   pop(): removes 30

2. Step 2: Identify last pop value

   The last pop removes 30, so it returns 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pop returns last pushed element 30 [OK]

Hint: Last pushed is first popped, track stack top [OK]

Common Mistakes:

• Assuming FIFO order
• Forgetting previous pop removed top
• Mixing values pushed and popped

4. The following C code snippet is intended to pop an element from a stack 's'. What is the error?

int pop(Stack *s) {
    if (s->top == -1) {
        return -1;
    }
    return s->items[s->top++];
}

medium

A. The function returns -1 on empty stack

B. The stack pointer is not checked for NULL

C. The top index is incremented instead of decremented

D. The return type should be void

Solution

1. Step 1: Analyze pop operation logic

   Pop should return the top element and then decrease the top index.

2. Step 2: Identify error in index update

   Code uses s->top++ which increases top, wrong for pop. It should be s->top-- to remove top element.

3. Final Answer:

   The top index is incremented instead of decremented -> Option C

4. Quick Check:

   Pop decreases top index, not increases [OK]

Hint: Pop must decrease top index after returning element [OK]

Common Mistakes:

• Using post-increment instead of decrement
• Ignoring empty stack check
• Wrong return type for pop

5. You have a stack initially empty. You perform these operations:

push(s, 5);
push(s, 10);
pop(s);
push(s, 15);
push(s, 20);
pop(s);
pop(s);

What is the final state of the stack (from top to bottom)?

hard

A. 5

B. 10 -> 5

C. 15 -> 5

D. Empty stack

Solution

1. Step 1: Trace stack after each operation

   push 5: stack = [5]
   push 10: stack = [5, 10]
   pop: removes 10, stack = [5]
   push 15: stack = [5, 15]
   push 20: stack = [5, 15, 20]
   pop: removes 20, stack = [5, 15]
   pop: removes 15, stack = [5]

2. Step 2: Determine final stack state

   After all operations, stack has only 5 left at bottom and top.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Only 5 remains after pops [OK]

Hint: Track pushes and pops carefully to find final stack [OK]

Common Mistakes:

• Forgetting to remove top on pop
• Mixing order of elements
• Assuming stack is empty after pops