Stack and the LIFO Principle: Definition and Applications (Class 12 Computer Science)

Understand Stack: LIFO Principle and Applications

Stack: LIFO Principle and Applications: key facts

Stack in one line

• •Linear data structure
• •Insert and delete at one end only
• •That end is the top
• •Order is LIFO (Last In, First Out)

The first item pushed is the last one popped.

Core terms

Push	add at top
Pop	remove from top
Peek	read top without removing
Overflow	push into a full stack
Underflow	pop from an empty stack

Peek is also called top.

Uses of a stack

• •Undo
• •Browser back button
• •Reversing a string
• •Matching brackets
• •Function call return order

Learn at least three with the reason in a few words.

Reverse by stack

• •Push items in order
• •Pop until empty
• •Items come out in reverse order

A string reversed this way: CODE gives EDOC.

How to answer Stack: LIFO Principle and Applications questions

1. 1Write the stack as a list with the bottom on the left and the top on the right, starting empty unless the question gives items.
2. 2Process the operations one by one. A push adds at the right end; a pop removes the right end and gives that value.
3. 3Before every pop, check the stack is not empty; if it is empty the answer for that step is underflow.
4. 4After the last operation, write the final content and name the top item (the rightmost one).
5. 5For a "which pop sequence is possible" question, simulate: push until the wanted item is on top, pop it, and repeat; the unwanted order is the one where the needed item is buried.
6. 6For an application question, name the property of LIFO that matches (last action first, reverse order, nearest match first).

Stack: LIFO Principle and Applications in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Stack: LIFO Principle and Applications

Short memory aids. Say each one aloud once, then use it on a question the same day.

• LIFO = plates

  A pile of plates: the last plate placed is the first plate lifted.

  Use it when: Deciding the order in which items leave a stack.

• Top is the active end

  Everything happens at the top; the bottom is never touched until the stack has emptied.

  Use it when: Answering "where do insertion and deletion take place?"

• Push in, pop reversed

  Whatever order you push in, the pops come back in the opposite order.

  Use it when: Reverse a string, undo, trace questions.

• Underflow = under empty

  Underflow is popping when there is nothing under the top: the stack is empty.

  Use it when: Separating underflow from overflow.

Stack: LIFO Principle and Applications: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Where students lose marks in Stack: LIFO Principle and Applications

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying a stack is FIFO

  FIFO is a queue. In a stack the last item inserted leaves first.

  Wrong working

  A stack is First In First Out.

  Right working

  A stack is Last In First Out (LIFO).

  ✓Do this: Link LIFO to the pile of plates before writing.

  Usually costs: 1 mark

• ✗Mistake: Popping from the wrong end

  The top is the last element in the list used as a stack; the element at index 0 is the bottom.

  Wrong working

  After push 1, 2, 3 the pop returns 1.

  Right working

  The pop returns 3; the stack becomes [1, 2].

  ✓Do this: Always say "the most recently pushed item" for pop.

  Usually costs: 1 mark

• ✗Mistake: Confusing overflow and underflow

  Overflow is pushing into a full stack; underflow is popping from an empty one.

  Wrong working

  Underflow occurs when the stack is full.

  Right working

  Underflow occurs when the stack is empty.

  ✓Do this: Remember: underflow, under the top there is nothing left.

  Usually costs: 1 mark

Practice questions on Stack: LIFO Principle and Applications

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   A stack works on which principle of data handling?
   (a) First In First Out (b) Last In First Out (c) Sorted order (d) Random order

   Hint

   Think of the pile of plates.

   Answer

   (b) Last In First Out

2. Q2 · 2 marks · Short answerBoard level

   Name any two applications of a stack.

   Hint

   Think of editors and browsers.

   Answer

   Any two: undo in an editor, browser back button, reversing a string, matching brackets, order of function returns.

3. Q3 · 2 marks · Short answerHarder board level

   A stack contains [10, 20, 30] (top is 30). The operations push 40, pop, pop, push 50 are performed. Write the final stack and the top item.

   Hint

   Update the list after each step.

   Answer

   push 40: [10, 20, 30, 40]; pop gives 40: [10, 20, 30]; pop gives 30: [10, 20]; push 50: [10, 20, 50]. Final stack [10, 20, 50], top is 50.

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output:

   s = []
   for w in ["red", "green", "blue"]:
       s.append(w)
   s.pop()
   s.append("pink")
   s.append("red")
   print(s, len(s))

   Hint

   Which word does pop() remove?

   Answer

   ['red', 'green', 'pink', 'red'] 4. The pop removes "blue"; then "pink" and "red" are added at the top.

5. Q5 · 1 mark · MCQHOTS / competency

   Items A, B, C, D, E are pushed in this order, and pops may be done at any time between pushes. Which of these pop orders is impossible?
   (a) E D C B A (b) A B C D E (c) C B E D A (d) A C E B D

   Hint

   Simulate each order; the item to pop must be on top.

   Answer

   (d) A C E B D. After A, C and E are popped, the stack holds B (below) and D (on top), so the next pop gives D, not B. The other three orders are possible.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does LIFO stand for?Show answer

   Last In, First Out.

2. 2.Where do insertion and deletion happen in a stack?Show answer

   At one end only, the top.

3. 3.What is the term for removing from an empty stack?Show answer

   Underflow.

4. 4.Which end of a Python list is the top when the list is used as a stack?Show answer

   The last (right) end.

5. 5.Is a stack a linear or a non-linear data structure?Show answer

   Linear.

6. 6.Which real feature of a browser uses a stack?Show answer

   The Back button (the last page opened is shown first).

Stack: LIFO Principle and Applications: frequently asked questions