/* File: ex.y */

%{
  int vbltable[26];
%}


%token  NAME 
%token  NUMBER
%token  EQ
%token PLUS MINUS TIMES DIV
%left MINUS PLUS
%left TIMES DIV 
%nonassoc UMINUS

%%

statement_list
        : statement 
        | statement statement_list
        
statement
        : NAME EQ expression ';' {vbltable[$1] = $3; }
        
expression
        : expression PLUS expression {$$ = $1 + $3;}
        | expression MINUS expression {$$ = $1 - $3;}
        | expression TIMES expression {$$ = $1 * $3;}
        | expression DIV   expression {$$ = $1 / $3;}
        | MINUS expression %prec UMINUS {$$ = - $2;}    
        | '(' expression ')' { $$ = $2; } 
        | NUMBER
        | NAME   { $$ = vbltable[$1]; }
        
%%
main()
{
   int i;

 for (i = 0; i < 26; i++)
   vbltable[i] = 0;
 yyparse();
 for (i = 0; i < 26; i++)
  if (vbltable[i] != 0)
   printf("Table: %d :%d\n",  i, vbltable[i]);

}

Corresponding Lex file

/* File: ex.l */

%{
#include "y.tab.h"
extern int yylval;
%}

digit   [0-9]
letter  [a-z]
%%
"+"     {return PLUS;}
"="     {return EQ;}
"-"     {return MINUS;}
"*"     {return TIMES;}
"/"     {return DIV;}
{digit}+  {yylval = atoi(yytext);  return NUMBER; }
{letter}  {yylval = yytext[0] - 'a'; return NAME;}
[ \t]+ ;
\n    ;
.     {return yytext[0];}


%%