Syntactic Analysis Tools

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Syntactic Analysis Tools

• Natawut Nupairoj, Ph.D.
• Department of Computer Engineering
• Chulalongkorn University

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Outline

• Overview.
• Yacc Specification Format.
• Examples.

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Parser Generator
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Yacc Specification

• A Yacc source program has three parts
• declarations
• translation rules
• supporting C-routines

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Example Calculator Program

• include ltstdio.hgt
• token DIGIT
• line expr '\n' printf("d\n", 1)
• expr expr '' term 1 3
• term
• term term '' factor 1 3
• factor
• factor '(' expr ')' 2
• DIGIT

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Example Calculator Program

• yylex()
• int c
• c getchar()
• if(c gt '0' c lt '9')
• yylval c - '0'
• return DIGIT
• return c
• yyerror(char errmsg)
• fprintf(stderr, "s\n", errmsg)
• main(int argc, char argv)

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How to use Yacc with Lex

• yyparse calls yylex to get the next token
  automatically.
• yylex returns
• token type or 0 (EOF).
• yylval - token attribute.
• Tokens are defined in yacc definition
• Lex definition can get them through y.tab.h.

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Example Yacc with Lex (Yacc)

• include ltstdio.hgt
• token EOL NUMBER
• line expr EOL printf("d\n", 1)
• expr expr '' term 1 3
• term
• term term '' factor 1 3
• factor
• factor '(' expr ')' 2
• NUMBER

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Example Yacc with Lex (Yacc)

• yyerror(char errmsg)
• fprintf(stderr, "s\n", errmsg)
• main(int argc, char argv)
• yyparse()
• return 0

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Example Yacc with Lex (Lex)

• / define constants for C program here /
• include ltstdlib.hgt
• include "y.tab.h"
• extern int yylval
• / regular definitions /
• delim \t
• ws delim
• eol \n
• number 0-9
• symbol \\\(\)
• ws / no action and no return /
• eol return EOL
• number yylval atoi(yytext)
  return(NUMBER)

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Compile Yacc and Lex

• byacc d calc.y
• flex calc.l
• gcc o calc y.tab.c lex.yy.c -lfl