Parsing

TERM PAPER ON
PARSING
COURSE CODE: CAP632
SECTION: D1809
COURSES NAME:
FORMAL LANGUAGES AND AUTOMATION THEORY
SUBMITED BY: SUBMITED TO:
11607193 Shrikant Sharma Miss Pallvi

What is Parser?
 A Parser is a compiler or interpreter component that breaks the data into
smaller elements for easy translation into another language.
 It takes input form of sequence of tokens or program instructions and build
the data structure in the form of parse tree.
What is the Role of Parser?
source token parse intermediate
Programmed getnext token tree representation
 In the compiler model, the parser obtains a string of token from the lexical
analyser, and verifies the string can be generated by the grammar for the
sourcelanguage.
 The parser returns any syntax error for the source language.
 It collects sufficient number of tokens and builds a parse tree.
LEXICAL
ANALYSER
REST OF FRONT
END
SYMBOL TABLE
PARSER

What is Parsing?
o Parsing is used to derived a string using the production rules of grammar.
o It is used to check the acceptability of string.
Example of Parsing
Problem: Consider the grammar
S=S+S | S*S | a | b
Constructthe derivation tree or parse tree for the string (w = a + b * b)
Solution:
S
S + S
a S * S
b b

Types of Parsing
Types of Parsing
Top-Down Parsing Bottom-Up Parsing
Backtracking
Predictive Parsing
Left to Right (LR)
Parsing
Shift Reduce Parsing
Simple Left to Right
(SLR)
Left to Right (LR)
Look-Ahead Left to Right
(LALR)

Types of Parsing
There are two types of parsing:
(i) Top-Down Parsing
(ii) Bottom-Up Parsing
Top-DownParsing
 Top-Down Parsing starts from the top with the start symboland derives a
string using parse tree.
 It may Backtracking.
Designof Top-DownParsing
For top-down parsing, a PDA has the following four types of transitions −
 Pop the non-terminal on the left-hand side of the production at the top of the
stack and push its right-hand side string.
 If the top symbol of the stack matches with the input symbol being read,
pop it.
 Push the start symbol ‘S’ into the stack.
 If the input string is fully read and the stack is empty, go to the final state ‘F
Example of Top-DownParsing
Example 1: Let us consider the following grammar.
S -> cAd
A -> ab | a
Let input string w = cad

S S
c A d c A d
a b a
(a) (b)
Let written in programming language:
Procedure S
ProcedureS (),
Begin
If input symbol= ‘c’ then
Begin
ADVANCE ();
If A () then
If input symbol= ‘d’ then
Begin ADVANCE (),
Return true
End
End
Return false
End

Procedure A ()
ProcedureA (),
Begin
isave=input-pointer;
If input symbol= ‘a’ then
Begin
ADVANCE (),
If input symbol= ‘b’ then
Begin ADVANCE (),
Return true
End
End
** if return true will not be terminated.
input-pointer=isave;
If input symbol= ‘a’ then
Begin
ADVANCE (),
Return true
End
Else
Return false
End

Example 2: let consider these equation
E -> E+E
E -> E*E
E -> a
E -> b
Input strings: (a*b) + (a*b)
** This parsing technique uses Left Most Derivation.
Solution:
E -> E+E
E -> E*E + E
E -> a*E+E
E -> a*b + E
E -> a*b + E+E
E -> a*b + E*E
E -> a*b + a* E
E -> a*b + a*b
Problem of Top-DownParsing
(i) Backtracking
 Backtracking is a technique in which for expansion of non-terminal
symbol we chooseone alternative and if some mismatch occurs then
we try to other alternative if any.
(ii) Left Recursion
 Left Recursion is removed if the parser performs top-downparsing.
E
E + E
E * E E * E
a b a b

(iii) Left Factoring
 Left Factoring is removing the common left factor that appears in two
productions of the same non-terminal. It is done to avoid the
backtracking by the parser.
Bottom-Up Parsing
Bottom-Up Parsing starts from the bottom with the string and comes to the
start symbolusing parse tree.
It uses stack to store both state and sentential forms.
Designof a Bottom-Up Parser
For bottom-up parsing, a PDA has the following four types of transitions −
 Push the current input symbol into the stack.
 Replace the right-hand side of a production at the top of the stack with its
left-hand side.
 If the top of the stack element matches with the current input symbol, pop it.
 If the input string is fully read and only if the start symbol ‘S’ remains in the
stack, pop it and go to the final state ‘F’.
Example of Bottom-Up Parsing
let consider these equation
E -> E+E
E -> E*E
E -> a
E -> b

Input strings: (a*b) + (a*b)
** This parsing technique uses Right MostDerivation.
Solution:
E -> E+E
E -> E + E*E
E -> E + E * b
E -> E + a * b
E -> E*E + a*b
E -> E * b + a*b
E -> a*b + a*b
References
https://www.slideshare.net/khush_boo31/parsing-67077365
https://www.youtube.com/watch?v=WYb-Iblk7J0
https://www.youtube.com/watch?v=42nqWoHacxg
https://www.youtube.com/watch?v=7LwxK2B_H3k
E
E E
E E E E
a * b + a * b

Parsing

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