How to make character parsing using lex and yacc?

Question

I want to make a parser with characters rather than numbers.

Like (a+b)*c. I want to make a rule according to the precedence of each operator. For example, expr + term | It is such a construct that follows the same precedence as term .. and inserts the value of identify at the end.

lex(flex) file:

%{
    #include <stdio.h> 
    #include <stdlib.h>
    #include "y.tab.h"
    #include <ctype.h>
    #include <string>
%}
alpha [a-zA-Z]
%%
{alpha}*        return *yytext;
"+"             return *yytext;
"*"             return *yytext;
[()]            return *yytext;
[ \b\t\n]       ;
%%
int yywrap() { return 1; }
int main() { yylex(); return 0; }

yacc(bison) file:

%{
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <ctype.h>
    int yyerror();
    int yylex();

%}
%token id
%%

expr : expr '+' term {$$ = $1 + $3;}
    |term            {$$ = $1;}
    ;
term : term '*' factor  {$$ = $1 + $3;}
    |factor             {$$ = $1;}
    ;
factor : '(' expr ')'  {$$ = $2;}
    |id                 {$$ = $1;}
    |id id              {$$ = $1+$2;}
    ;

%%

void yyerror() {
    printf("yyerror\n");

}

int main() { yyparse(); return 0; }

I honestly don't know what went wrong. I've been studying for three days, but I haven't been able to solve it.

what should I do?