Yacc Programs

1. Program to test validity of a sample expression involving operators +, -,* and /.

Calcval.y

%{

   #include<stdio.h>

   #include<stdlib.h>

%}

%token NUM

%%

start : S | S start ;

S : E'\n' {printf("Valid Expression");}

  | error '\n' {yyerror("Enter valid expression");}

  ;

E : E '+' T

  | E '-' T

  | T

  ;

T : T '*' F

  | T '/' F

  | F

  ;

F : '(' E ')'

  | '-' F

  | NUM

  ;

%%

yyerror(char *msg)

{

printf("Invalid Expression : %s\n",msg);

}

void main()

{

printf("Enter Expression...> \n");

yyparse();

}

Call.l

%{

#include<stdio.h>

#include<stdlib.h>

#include "y.tab.h"

%}

%%

[0-9]+(\.[0-9]+)?([eE][0-9]+)? {yylval=atof(yytext);return NUM;}

[-+()*/]|\n {return yytext[0];}

[ \t] {;}

%%

 2. Program to evaluate an arithmetic expression involving operators +, -, * and /.

Calc.y

%{

   #include<stdio.h>

   #include<stdlib.h>

%}

%union {

  float f;

 

}

%token <f> NUM

%type <f> E T F

%%

start : S | S start ;

S : E'\n' {printf("%f\n",$1);}

  | error '\n' {yyerror("Enter valid expression");}

  ;

E : E '+' T {$$ = $1 + $3;}

  | E '-' T {$$ = $1 - $3;}

  | T {$$ = $1;}

  ;

T : T '*' F {$$ = $1 * $3;}

  | T '/' F {$$ = $1 / $3;}

  | F {$$ = $1;}

  ;

F : '(' E ')' {$$ = $2;}

  | '-' F {$$ = -$2;}

  | NUM {$$ = $1;}

  ;

%%

yyerror(char *msg)

{

printf("Invalid Expression : %s\n",msg);

}

void main()

{

printf("Enter Expression...> \n");

yyparse();

}

Caal.l

%{

#include<stdio.h>

#include<stdlib.h>

#include "y.tab.h"

%}

%%

[0-9]+(\.[0-9]+)?([eE][0-9]+)? {yylval.f=atof(yytext);return NUM;}

[-+()*/]|\n {return yytext[0];}

[ \t] {;}

%%

3. Program to recognize the grammar (An B, n >=10 ) .

Anb.y

%{

#include<stdio.h>

%}

%token A B

%%

start        :      variable | variable start ;

variable     :      A A A A A A A A A A var B '\n' {printf("\nValid variable \n");}

              |      error '\n' {yyerror("\nInvalid variable \n");}

              ;

var           :      A var

              |      {;}

              ;

%%

main()

{

printf("Enter the variable:\n");

yyparse();

}

yyerror(char *s)

{

 printf("%s",s);

}

Anb.l

%{

#include<stdio.h>

#include "y.tab.h"

%}

%%

[aA]         {return A;}

[bB]         {return B;}

.|\n           {return yytext[0];}

%%

 

4. Program to recognize a valid variable, which starts with a letter, followed by any number of letters or digits.

Vari.y

%{

   #include<stdio.h>

   #include<ctype.h>

   #include<stdlib.h>

%}

%token let dig

%%

start : TERM | TERM start ;

TERM:  XTERM '\n'       {printf("\nAccepted\n");}

    |  error  '\n'         {yyerror ("\nRejected\n");}

    ;

XTERM: XTERM let

     | XTERM dig

     | let

     ;

%%

main()

{

  printf("Enter a variable:");

  yyparse();

}

yyerror(char *s)

{

  printf("%s",s);

}

Vari.l

%{

#include<stdio.h>

#include "y.tab.h"

%}

%%

[a-zA-Z_]         {return let;}

[0-9]         {return dig;}

.|\n           {return yytext[0];}

%%

 5. Program to recognize nested IF control statements and display the number of levels of

nesting.

Nestif.y

%{

#include "stdio.h"

int level=0;

%}

%token IF SIMP OP

%%

stmt: if_stmt    {printf("\nValid levels=%d\n",level);}

    | error      { printf("---ERROR---\n");}

    ;

if_stmt: IFCOND SIMP

       | IFCOND if_stmt

       | IFCOND '{' EXP '}'

       | IFCOND '{' if_stmt '}'

IFCOND:  IF '(' EXP ')' {level++;}

      ;

EXP: | SIMP OP SIMP

     | SIMP

     ;

%%

yyerror(char *s)

{

  printf("Invalid");

}  

Nestif.l

%{

#include "y.tab.h"

%}

%%

"if"       return IF;

"=="       return OP;

[<>+]      return OP;

[{()}]     return *yytext;

[a-zA-Z]*  return SIMP;

" "        {;}

.          { }

%%

void main()

{

  yyin=fopen("a.c","r");

  yyparse();

}

6. Program to recognize strings ' aaab', ' abbb', 'ab' and 'a' using the grammar.

%{

   #include<stdio.h>

   #include<stdlib.h>

%}

%token a b

%%

start : S start | S ;

S      :      A '\n' {printf("\nString belongs to grammar..\n");}

       |      error '\n'            {yyerror("\nstring does not belong to grammar..\n");}

       ;

A      :      a a a b

       |      a b b b

       |      a b

       |      a

       ;

%%

main()

{

 printf("Enter String for GRAMMAR:\n");

 yyparse();

}

yylex()

{

 char ch;

 while((ch=getchar())==' ');

 if(ch=='a'||ch=='A')

   return a;

 if(ch=='b'||ch=='B')

   return b;

 return ch;

}

yyerror(char *s)

{

 printf("%s",s);

}