yacc - "first use" error when change the code in lex file -

given .l file this:

%{ #include "y.tab.h" %}  %%  [ \t\n] "if" return if_token ; "while" return else_token ;  . yyerror("invalid character");  %%  int yywrap(void){     return 1; } 

and .y file this:

%{     #include <stdio.h>     void yyerror(char *); %}  %token if_token else_token minus_token digit_token  %% program :expr {printf("program accepted!!!");}; expr : if_token | digit_token ; %% void yyerror(char *s){     fprintf(stderr, "%s\n", s); }  int main(){     yyparse();     return 0; } 

i use these 3 commands compile these 2 files (my lex file named p.l , yacc file named p.y):

flex p.l yacc -d p.y gcc lex.yy.c y.tab.c 

it compiled no error. when changed "return else_token" "return while_token", got error , got no output file:

p.l: in function ‘yylex’: p.l:10:8: error: ‘while_token’ undeclared (first use in function) "while" return while_token ;         ^ p.l:10:8: note: each undeclared identifier reported once each function appears in 

also when change "while" "else" , add new rule like:

"for" return for_token ; 

i same error. how can correct code work correctly?

you didn't add:

%token while_token for_token 

to grammar, header didn't contain definition while_token or for_token, compilation of lexical analyzer failed.