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Information about Lexical

Published on October 12, 2014

Author: ShubhamShah001

Source: slideshare.net


Lexical Analysis

1. Seminar on LEXICAL PROGRAM Presented by- Sonali Sulgadle Shrushti Swami Vijaylaxmi Yemul KajalWaghmare

2. An overview of Lex Lex is a program generator designed for lexical processing of character input streams. +-------+ Source -> | Lex | -> yylex +-------+ +-------+ Input -> | yylex | -> Output +-------+

3. OVERVIEW lexical grammar rules rules | | v v +---------+ +---------+ | Lex | | Yacc | +---------+ +---------+ | | v v +---------+ +---------+ Input -> | yylex | >| yyparse| -> Parsed input +---------+ +---------+

4. Parser generator: – Takes a specification for a context-free grammar. – Produces code for a parser. 4 Yacc: Overview Input: a set of grammar rules and actions Output: C code implementing a parser: function: yyparse() file [default]: y.tab.c yacc

5. Lex Source. The general format of Lex source is: {definitions} %% {rules} %% {user subroutines} where the definitions and the user subroutines are often omitted. The second %% is optional, but the first is required to mark the beginning of the rules. The absolute minimum Lex program is thus %%which translates into a program which copies the input to the output unchanged.

6. Lex Source. . Thus an individual rule might appear integer printf("found keyword INT"); As a slightly more useful example, suppose it is desired to change a number of words from British to American spelling. Lex rules such as colour printf("color"); mechanise printf("mechanize"); petrol printf("gas");

7. Special Functions 7 yytext where text matched mst recently is stored yyleng number of characters in text most recently matched yylval associated value of current token yymore() append next string matched to current contents of yytext yyless(n) remove from yytext all but the first n characters unput(c) return character c to input stream yywrap() may be replaced by user The yywrap method is called by the lexical analyser whenever it inputs an EOF as the first character when trying to match a regular expression

8. Program %{ #include<stdio.h> //#include"y.tab.h" int num_lines=0; %} label [a-z][a-z 0-9 _]* number[0-9]+ %%

9. n++num_lines; char printf("n Keyword=Char n"); double printf("n Keyword=Doublen"); int printf("n Keyword=Intn"); float printf("n Keyword=Floatn"); for printf("n Keyword=For n"); if printf("n Keyword=Ifn"); else printf("n Keyword=Elsen"); while printf("n Keyword=Whilen"); exit printf("n Keyword=Exitn"); auto printf("n Keyword auton"); switch printf("n Keyword switch");

10. continue printf("n keyword=continue n"); "{"printf("nOpen brace"); "+""|""-""|""*""|""/""|""%" printf("n Arithmetic Operators"); " " printf("n Blank spaces n"); "++" printf("Increment Operator"); "--" printf("Decrement Operator"); {label} printf("n Labeln"); {number} printf("n Numbern"); "==""|"">=""|""<=""|"">""|""<" printf("n Conditional operatorn");

11. "=" printf("nAssignment operatorn"); %% main() { yylex(); printf("# of lines is %d",num_lines); yywrap(); } int yywrap() { return 0 }

12. THANK YOU…….

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