YACC program to check whether given string is Palindrome or not

Problem: Write a YACC program to check whether given string is Palindrome or not.

Explanation:
Yacc (for “yet another compiler compiler.”) is the standard parser generator for the Unix operating system. An open source program, yacc generates code for the parser in the C programming language. The acronym is usually rendered in lowercase but is occasionally seen as YACC or Yacc.

Examples:

Input: naman
Output: palindrome

Input: geeksforgeeks
Output: not palindrome 

Lexical Analyzer Source Code :

%{
    /* Definition section */
    #include <stdio.h>
    #include <stdlib.h>
    #include "y.tab.h"
%}

/* %option noyywrap */

/* Rule Section */
%%

[a-zA-Z]+   {yylval.f = yytext; return STR;}
[-+()*/]    {return yytext[0];}
[ \t\n]      {;}

%%

 int yywrap()
 {
  return -1;
 }

Parser Source Code:

%{
    /* Definition section */
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    extern int yylex();

    void yyerror(char *msg);
    int flag;

    int i;
    int k =0;
%}

%union {
    char* f;
 }

%token <f> STR
%type <f> E

/* Rule Section */
%%

S : E    {
         flag = 0;
         k = strlen($1) - 1;
         if(k%2==0){

         for (i = 0; i <= k/2; i++) {
           if ($1[i] == $1[k-i]) {
            } else {
               flag = 1;
              }
          }
         if (flag == 1) printf("Not palindrome\n");
         else printf("palindrome\n");
         printf("%s\n", $1);

        }else{

        for (i = 0; i < k/2; i++) {
          if ($1[i] == $1[k-i]) {
          } else {
              flag = 1;
             }
            }
        if (flag == 1) printf("Not palindrome\n");
        else printf("palindrome\n");
        printf("%s\n", $1);


          }
       }
  ;

E :  STR    {$$ = $1;}
  ;

%%

void yyerror(char *msg)
 {
    fprintf(stderr, "%s\n", msg);
    exit(1);
 }

//driver code
int main()
 {
    yyparse();
    return 0;
 }

Output: