Question

当我在以下文件上运行yacc -d parser.y时，我收到以下错误：

parser.y:23.3-24.4: warning: unused value: $4
15 rules never reduced
parser.y: warning: 7 useless nonterminals and 15 useless rules
parser.y:16.1-14: fatal error: start symbol statement_list does not derive any sentence
make: *** [y.tab.c] Error 1

我特别关注如何摆脱致命错误。

%{
#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
%%

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
;

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; }
;
%%
/* 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 */
yyerror(char *s)
{
printf( "yyerror: %s\n", s);
}

Answer 1

所有这些警告和错误都是由|规则中term之前遗失expression引起的。提示是一个片段中未使用的$4，显然应该只有3个参数。这个问题落到了其他所有问题上。

变化：

expression: expression '+' term { $$ = $1 + $3; }
| expression '-' term { $$ = $1 - $3; }
term
;

成：

expression: expression '+' term { $$ = $1 + $3; }
| expression '-' term { $$ = $1 - $3; }
| term
;

再试一次。

Answer 2

你忘了或|这里

expression: expression '+' term { $$ = $1 + $3; }
| expression '-' term { $$ = $1 - $3; }
term
;

最后一条规则应为|term {};

这个yacc文件有什么问题？

2 个答案: