This document discusses stacks, which are data structures that only allow adding or removing items from the top. The key operations are push to add an item and pop to remove an item, making stacks follow LIFO (last in, first out) order. Stacks have many uses like evaluating expressions, where a postfix expression uses a stack to evaluate terms without regard for precedence rules. The document also covers converting infix expressions to postfix and prefix forms using a stack.

1. STACK
TUTORIAL 2

2. WHAT IS A STACK?
IT’S A DATA STRUCTURE WHERE YOU CAN ADD OR
REMOVE ITEMS ONLY FROM THE TOP!
OPERATIONS:
• PUSH : When you ADD an Item
to stack
• POP : when you REMOVE an
item from the stack
STACK is a LIFO Data Structure

3. PUSH AND POP
I n i t i a l i z e
S t a c k s
S
SSSS
SSS
bat
bat
ball
bat
cat
ball
bat
cat
ball
bat
rat
ball
bat
rat
ball
bat
ball
S . p o p ( )S . p u s h ( r a t )X = S . p o pX = S . p o p ( )
S . p u s h ( c a t )S . p u s h ( b a l l )S . p u s h ( b a t )

4. HOW ARE STACKS IMPLEMENTED ?
ARRAY Implementation
56 56
s.push(56) s.pop()s.push(90)top=-1
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
056
90
top = top =
top = 90

5. EXPRESSIONS
( A + [ D - { B * C } )
) A + B (
Parenthesis Plays an important role in expressions

6. TYPES OF EXPRESSIONS
INFIX PREFIX POSTFIX
How do we determine the expressions?
The relative position of
the operator to operands
+AB AB+A+B
PREFIX POSTFIXINFIX

7. WHY DO WE NEED PREFIX, POSTFIX?
 NO need of precedence rules
 NO need of parenthesis
 Easier for Computer to parse

8. HOW TO CONVERT INFIX TO POSTFIX ?
INFIX POSTFIX
3 – ( 4 * 2 + 1 )
3 – ( 4 2 * 1 + )
3 – ( 4 2 * + 1 )
3 4 2 * 1 + -

9. HOW TO CONVERT INFIX TO PREFIX ?
INFIX PREFIX
3 – ( 4 * 2 + 1 )
3 – ( + * 4 2 1 )
3 – ( * 4 2 + 1 )
- 3 + * 4 2 1

10. I M P L E M E N T A T I O N