System Software Lab 6 | Read Now

System Software VTU Lab

Program 6

a) Write a LEX program to eliminate comment lines in a C program and copy the resulting program into a separate file.

b) Write the YACC program to recognize valid identifiers, operators, and keywords in the given text (C program) file.

Steps in writing LEX Program:

1. Step – Using gedit create a file with extension .l For example: prg1.l
2. Step – lex prg1.l
3. Step – cc lex.yy.c –ll
4. Step – ./a.out

Steps in writing YACC Program:

1. Step: Using gedit editor create a file with extension y. For example: gedit prg1.y
2. Step: YACC –d prg1.y
3. Step: lex prg1.l
4. Step: cc y.tab.c lex.yy.c -ll
5. Step: /a.out

System Software Lab 6a Code [lab6.l]

%{
#include<stdio.h>
int sl=0;
int ml=0;
%}
%%
"/*"[a-zA-Z0-9' '\t\n]+"*/" ml++;
"//".* sl++;
%%

main()
{
 yyin=fopen("f1.c","r");
 yyout=fopen("f2.c","w");
 yylex();
 fclose(yyin);
 fclose(yyout);
 printf("\n Number of single line comments are = %d\n",sl);
 printf("\nNumber of multiline comments are =%d\n",ml);
}

f1.c

#include<stido.h>

int main()
{
 // this is a comment
 printf("hello");
 /* this is another comment */
}

System Software lab 6 – Output

System Software Lab6.b Code

lab6.l

%{
#include <stdio.h>
#include "y.tab.h"
extern yylval;
%}

%%
[ \t];
[+|-|*|/|=|<|>] {printf("operator is %s\n",yytext);return OP;}
[0-9]+ {yylval = atoi(yytext); printf("numbers is %d\n",yylval); return DIGIT;}
int|char|bool|float|void|for|do|while|if|else|return|void {printf("keyword is %s\n",yytext);return KEY;}
[a-zA-Z0-9]+ {printf("identifier is %s\n",yytext);return ID;}
. ;
%%

lab6.y

%{
#include <stdio.h>
#include <stdlib.h>
int id=0, dig=0, key=0, op=0;
%}
%token DIGIT ID KEY OP

%%
input:
DIGIT input { dig++; }
| ID input { id++; }
| KEY input { key++; }
| OP input {op++;}
| DIGIT { dig++; }
| ID { id++; }
| KEY { key++; }
| OP { op++;}
;
%%

#include <stdio.h>
extern int yylex();
extern int yyparse();
extern FILE *yyin;
main()
{
 FILE *myfile = fopen("inputfile.c", "r");
 if (!myfile)
 {
  printf("I can't open inputfile.c!");
  return -1;
 }
 yyin = myfile;
 do{
  yyparse();
 }while (!feof(yyin));
 printf("numbers = %d\nKeywords = %d\nIdentifiers = %d\noperators = %d\n",dig, key,id, op);
}

void yyerror()
{
 printf("EEK, parse error! Message: ");
 exit(-1);
}

inputfile.txt

#include<stdio.h>

int main()
{
 int a ;
 int b ;
 a = 1 ;
 b = 2 ;
 a = a+b;
 return 0 ;
}

System Software lab6b – Output