YACC Rules not reduced

问题:

This is my code calc.y. I keep getting the error: yacc: 1 rule never reduced yacc: 3 reduce/reduce conflicts

not really sure what this means

Ive done some research in other places but I am now lost. Im guessing the rules being referred to is program and statement but even so... what does the reduce rule mean?

%{   #include <stdio.h>   FILE *outfile;   int yyline = 1;   int yycolumn = 1; %}  %union{   int nw;   struct{     int v;     char s[1000];   }attr; }  %token   SEMInumber %token   LPARENnumber %token   <nw> ICONSTnumber %token   BEGINnumber %token   PROGRAMnumber %token   MINUSnumber %token   TIMESnumber %token   <nw> VARnumber %token   INTnumber %token   EOFnumber %token   COMMAnumber %token   RPARENnumber %token   <nw>IDnumber %token   ENDnumber %token   ISnumber %token   PLUSnumber %token   DIVnumber %token   PRINTnumber %token   EQnumber  %type <attr> exp %type <attr> term %type <attr> factor %%   program: PROGRAMnumber IDnumber ISnumber compstate          ;  compstate: BEGINnumber {print_header();} statement ENDnumber{print_end();}            | BEGINnumber {print_header();} statement SEMInumber statement ENDnumber{print_end();}            ;  statement: IDnumber EQnumber exp            | PRINTnumber exp  | declaration            ;  declaration: VARnumber IDnumber              | VARnumber IDnumber COMMAnumber IDnumber              ;  exp: term {$$.v = $1.v; strcpy($$.s, $1.s);}      | exp PLUSnumber term {$$.v = $1.v + $3.v; sprintf($$.s, "(%s) + (%s)", $1. s, $3.s);}      | exp MINUSnumber term {$$.v = $1.v - $3.v; sprintf($$.s, "(%s) - (%s)", $1. s, $3.s);}      ;  term: factor {$$.v = $1.v; strcpy($$.s, $1.s);}       | term TIMESnumber factor {$$.v = $1.v * $3.v; sprintf($$.s, "(%s) * (%s)", $1.s, $3.s);}       | term DIVnumber factor {$$.v = $1.v / $3.v; sprintf($$.s, "(%s) / (%s)", $1.s, $3.s);}       ;   factor: ICONSTnumber {$$.v = $1; sprintf($$.s, "%d", $1);}           | IDnumber {$$.v = $1.v; strcpy($$.s,  $1.s);}           | LPARENnumber exp RPARENnumber {$$.v = $2.v; strcpy($$.s,  $2.s);} ;   %%   int main() {   if(!yyparse())     {       printf("accept\n");     }   else     printf("reject\n"); }  void print_header() {} void print_end(){}  void yyerror(const char *str) {   printf("yyerror: %s at line %d\n", str, yyline); } 

回答1:

When compstate shifts the BEGINnumber token, two inner rules for the mid rule action {print_header();} can be both reduced resulting in a R/R conflict. Youe can replace

compstate: BEGINnumber {print_header();} statement ENDnumber{print_end();}            | BEGINnumber {print_header();} statement SEMInumber statement  ENDnumber{print_end();}            ; 

with, for example

begin_number:     BEGINnumber { print_header(); }  compstate: begin_number statement ENDnumber{print_end();}            | begin_number statement SEMInumber statement  ENDnumber{print_end();}            ; 

to solve the conflict.

回答2:

Informative Messages %s: %d rules never reduced
Some rules are never used, either because they weren't used in the grammar or because they were on the losing end of shift/reduce or reduce/reduce conflicts. Either change the grammar to use the rules or remove them.

文章来源: YACC Rules not reduced