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YACC Example

YACC Example. Taken from LEX & YACC Simple calculator a = 4 + 6 a a=10 b = 7 c = a + b c c = 17 pressure = (78 + 34) * 16.4 $. Grammar. expression ::= expression '+' term | expression '-' term | term term ::= term '*' factor |

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YACC Example

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  1. YACC Example • Taken from LEX & YACC • Simple calculator a = 4 + 6 a a=10 b = 7 c = a + b c c = 17 pressure = (78 + 34) * 16.4 $

  2. Grammar expression ::= expression '+' term | expression '-' term | term term ::= term '*' factor | term '/' factor | factor factor ::= '(' expression ')' | '-' factor | NUMBER | NAME

  3. parser.h

  4. 0 name value 1 name value 2 name value /* * Header for calculator program */ #define NSYMS 20 /* maximum number of symbols */ struct symtab { char *name; double value; } symtab[NSYMS]; struct symtab *symlook(); 3 name value 4 name value 5 name value 6 name value 7 name value 8 name value 9 name value 10 name value 11 name value 12 name value 13 name value 14 name value parser.h

  5. parser.y

  6. %{ #include "parser.h" #include <string.h> %} %union { double dval; struct symtab *symp; } %token <symp> NAME %token <dval> NUMBER %type <dval> expression %type <dval> term %type <dval> factor %% parser.y

  7. statement_list: statement '\n' | statement_list statement '\n‘ ; statement: NAME '=' expression { $1->value = $3; } | expression { printf("= %g\n", $1); } ; expression: expression '+' term { $$ = $1 + $3; } | expression '-' term { $$ = $1 - $3; } term ; parser.y

  8. term: term '*' factor { $$ = $1 * $3; } | term '/' factor { if($3 == 0.0) yyerror("divide by zero"); else $$ = $1 / $3; } | factor ; factor: '(' expression ')' { $$ = $2; } | '-' factor { $$ = -$2; } | NUMBER | NAME { $$ = $1->value; } ; %% parser.y

  9. /* look up a symbol table entry, add if not present */ struct symtab *symlook(char *s) { char *p; struct symtab *sp; for(sp = symtab; sp < &symtab[NSYMS]; sp++) { /* is it already here? */ if(sp->name && !strcmp(sp->name, s)) return sp; if(!sp->name) { /* is it free */ sp->name = strdup(s); return sp; } /* otherwise continue to next */ } yyerror("Too many symbols"); exit(1); /* cannot continue */ } /* symlook */ parser.y

  10. yyerror(char *s) { printf( "yyerror: %s\n", s); } parser.y

  11. typedef union { double dval; struct symtab *symp; } YYSTYPE; extern YYSTYPE yylval; # define NAME 257 # define NUMBER 258 y.tab.h

  12. calclexer.l

  13. %{ #include "y.tab.h" #include "parser.h" #include <math.h> %} %% calclexer.l

  14. %% ([0-9]+|([0-9]*\.[0-9]+)([eE][-+]?[0-9]+)?) { yylval.dval = atof(yytext); return NUMBER; } [ \t] ; /* ignore white space */ [A-Za-z][A-Za-z0-9]* { /* return symbol pointer */ yylval.symp = symlook(yytext); return NAME; } "$" { return 0; /* end of input */ } \n |. return yytext[0]; %% calclexer.l

  15. Makefile

  16. Makefile LEX = lex YACC = yacc CC = gcc calcu: y.tab.olex.yy.o $(CC) -ocalcuy.tab.olex.yy.o -ly -ll y.tab.cy.tab.h: parser.y $(YACC) -dparser.y y.tab.o: y.tab.cparser.h $(CC) -cy.tab.c lex.yy.o: y.tab.hlex.yy.c $(CC) -clex.yy.c lex.yy.c: calclexer.lparser.h $(LEX) calclexer.l clean: rm *.o rm *.c rm calcu

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