This document discusses predictive parsing and left recursion in compilers design. It provides examples of predictive parsing functions for expressions, statements, and matching. It also discusses how to handle left recursion by rewriting nonterminals. The document notes that lexical analysis removes whitespace and comments, and may need to read ahead characters to determine the proper token to return to the parser, such as determining >= versus > in C/Java.

1. Compilers Design
Course
Dr. Ramadan Babers
Compilers Design – Lec6
Faculty of Science Helwan University
1st Term – 2017/2018

2. 2
Chapter 2
A Simple Syntax-Directed
Translator

3. 2.4.2 Predictive Parsing
3
Recursive-descent parsing is a top-down method of syntax analysis in which
a set of recursive procedures is used to process the input. One procedure is
associated with each nonterminal of a grammar.

4. 2.4.2 Predictive Parsing
4
Now we need a predictive
parser for the following
code (page 62- Fig 2.16)
There are three functions:
1- optexpr
2- stmt
3- match

5. 2.4.2 Predictive Parsing
5
Example:
term xy + ; term () {
x();
y();
match(+);
match(;);
}

6. 2.4.5 Left Recursion
6
Example:
expr expr + term
term 0
1
2
expr() { expr();
match (“+”);
term (); }
term() { if (lookahead == ‘0’) match (“0”);
elseif (lookahead == ‘1’) match (“1”);
else error (); }
Left recursion problem

7. 2.4.5 Left Recursion
7
nonterminal terminal or nonterminal
anything

8. 2.4.5 Left Recursion
8
A
αA
A
β
α
Rewriting new nonterminal

9. 2.4.5 Left Recursion
9
A
αA
A
β
α
A
R
e
α
β
R
α R

10. 2.5.2 Adapting the Translation Scheme
10
In general

11. 2.5.2 Adapting the Translation Scheme
11

12. 2.5.2 Adapting the Translation Scheme
12
9-5+2 95-2+

13. 2.5.3 Procedures for the Nonterminals
13

14. 2.5.4 Simplifying the Translator
14
Tail recursion

15. 2.5.5 The Complete Program
15

16. 2.6 Lexical Analysis
16
a basic lexical unit of a
language, consisting of one
word or several words,
considered as an abstract
unit, and applied to a family
of words related by form or
meaning.

17. 2.6.1 Removal of White Space and Comments
17
White space is:
1- blank,
2- a tab, or
3- a newline
 Variable peek holds the next input character.

18. 2.6.2 Reading Ahead
18
A lexical analyzer may need to read ahead some characters
before it can decide on the token to be returned to the parser.
For example, a lexical analyzer for C or Java must read
ahead after it sees the character >.
If the next character is =, then > is part of the character
sequence >=, the lexeme for the token for the "greater than or
equal to" operator.
Otherwise > itself forms the "greater than" operator, and the
lexical analyzer has read one character too many.