ch3-04.y

lex 和yacc 的中文指南

%{
#include "ch3hdr.h"
#include <string.h>
%}

%union {
	double dval;
	struct symtab *symp;
}
%token <symp> NAME
%token <dval> NUMBER
%left '-' '+'
%left '*' '/'
%nonassoc UMINUS

%type <dval> expression
%%
statement_list:	statement '\n'
	|	statement_list statement '\n'
	;

statement:	NAME '=' expression	{ $1->value = $3; }
	|	expression		{ printf("= %g\n", $1); }
	;

expression:	expression '+' expression { $$ = $1 + $3; }
	|	expression '-' expression { $$ = $1 - $3; }
	|	expression '*' expression { $$ = $1 * $3; }
	|	expression '/' expression
				{	if($3 == 0.0)
						yyerror("divide by zero");
					else
						$$ = $1 / $3;
				}
	|	'-' expression %prec UMINUS	{ $$ = -$2; }
	|	'(' expression ')'	{ $$ = $2; }
	|	NUMBER
	|	NAME			{ $$ = $1->value; }
	;
%%
/* look up a symbol table entry, add if not present */
struct symtab *
symlook(s)
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;
		
		/* is it free */
		if(!sp->name) {
			sp->name = strdup(s);
			return sp;
		}
		/* otherwise continue to next */
	}
	yyerror("Too many symbols");
	exit(1);	/* cannot continue */
} /* symlook */