This document discusses predictive parsers, which are an efficient form of top-down parsing that does not require backtracking. It describes how to construct the transition diagram and parsing tables for a predictive parser from a given grammar. A predictive parser uses these tables to parse input strings in a non-recursive manner by tracking the stack contents and remaining input at each step.

1. PREDICTIVE PARSER
BY
JOTHI LAKSHMI

2. • Predictive parsers
• Transition diagrams for predictive parsers
• Non recursive predictive parser
• Construction of predictive parsing tables
CONTENT

3. • It is top – down parsing
• An efficient non-backtracking form of top-down parser
called a predictive parser.
• LL(1) grammars from which predictive parsers can be
constructed automatically.
PREDICTIVE PARSER

4. •To construct a predictive parser we must know
• Input symbol a
• Non terminal A to be expanded
• Alternatives of production A-->a1|a2|….|an
• That derives a string beginning with a
• Proper alternative must be detectable by looking at
only first symbol it derives.
• Flow of control constructs most programming
language with their keywords that are detected.
PREDICTIVE PARSERS

5. • Recursive descent parser that needs no
backtracking i.e (predictive parser).
• Eg:- stmt  if expr then stmt else stmt
while expr do stmt
begin stmt_list end
• Keywords tells us which alternative is the one
that could find the statement.
PREDICTIVE PARSERS

6. • To construct the transition diagram of a
predictive parser from a grammar.
 Eliminate LR from the grammar
Then left factor the grammar
Then for each non terminalA do the following
 Create an initial and final state
 For each production A  x1 ,x2….xn.
 Create a path from initial to final state
 With edges labeled x1,x2,….xn
TRANSITION DIAGRAMS FOR PREDICTIVE PARSERS

7. NON RECURSIVE PREDICTIVE PARSER

8. • it is possible to build a non recursive predictive parser
by maintaining stack-explicit
recursive calls – implicit
• key problem – determining production to be
applied for a non terminal
• predictive parser has :-
input - contain string to parser followed by $
stack - sequence of grammar symbols with $
parsing table - 2 dimension array M[A,a]
output stream - gives output
NON RECURSIVE PREDICTIVE PARSER

9. • $ - right endmarker to indicate end of string
• A – non terminal , a – terminal
• program considers X – top of the stack
a – current input symbol
• the behavior of the parser can be described in
terms of its configuration which gives the stack
contents and the remaining input
NON RECURSIVE PREDICTIVE PARSER

10. • two symbols determine the action of the parser.
• there are 3 possibilities
 if X=a=$ , parser halts and announces
successful completion of parsing.
 If X=a!=$ , parser deletes X off the stack and
the input pointer to next input symbol.
 If X is non terminal , program consults entry
M[X,a]Of the parsing table M
NON RECURSIVE PREDICTIVE PARSER

11. • The following rules are used to construct the
predictive parsing table:
1. for each terminal a in FIRST(α),
add A → α to matrix M[A,a]
2. if λ is in FIRST(α), then
for each terminal b in FOLLOW(A),
add A → α to matrix M[A,b]
Construction of predictive parsing tables