Lex Program to check for comment lines - lex

Question

Could anyone please help me in writing a LEX program to check and count comment lines in a C program. I have searched for it everywhere and failed to get the complete code.

Answer 1

comment lines are of the form
%%
{START} {whatever symbols,alphanumerics and letters} {END}
%%
START symbol can be // or /* in C.
END symbol is */
If it is "//" any other symbol can follow it in the LINE including a second occurances of // (which is ignored).
IF it is " /* " it will find the next immediate match for " */ " and all the rest of the patterns that appear in between is matched(This matching may include blank spaces,tabs ,"//" and " /* " in it)
Here is a sample code to do it
`
%{
#include<stdio.h>
int c=0;
%}
START "/*"
END "*/"
SIMPLE [^*]
SPACE [ \t\n]
COMPLEX "*"[^/]
%s newstate
%%
"//"(.*[ \t]*.*)*[\n]+ {c++; fprintf(yyout," ");}
{START} {yymore();BEGIN newstate;}
<newstate>{SIMPLE} {yymore();BEGIN newstate;}
<newstate>{COMPLEX} {yymore();BEGIN newstate;}
<newstate>{SPACE} {yymore();BEGIN newstate;}
<newstate>{END} {c++;fprintf(yyout," ");BEGIN 0;}
%%
main()
{//program to remove comment lines
yyin=fopen("file4","r");
yyout=fopen("fileout4","w");system("cat file4");
yylex();system("cat fileout4");
printf("no.of comments=%d",c);
fclose(yyin);
fclose(yyout);
}
`
The input file "file4" is below
/* #include <stdlib.h>
{}.#All nonsense symbols */
//another comment
int main{}
{/**/
printf("hello");
/* comment inside// comment is ignored */
//how about//this?
/* now we /* try this */ */
printf("COOL!!");
return (0);
}
It gives the following output which is stored in "fileout4" `
int main{}
{
printf("hello");
*/
printf("COOL!!");
return (0);
}
`
The number of comment lines in the above program is 6.

Answer 2

You can maintain a boolean variable which keeps track of whether the line you're parsing is inside a multi-line comment or not.
%{
#include <stdio.h>
#include <stdbool.h>
int comment_lines = 0;
bool in_comment = false;
%}
%%
"/*".*\n { in_comment = true; ++comment_lines; }
.*"*/"\n { in_comment = false; ++comment_lines; }
"//".*\n { ++comment_lines; }
.*\n {
if (in_comment) {
++comment_lines;
} else {
fprintf(yyout, yytext);
}
}
%%
int yywrap() {
return 1;
}
int main(void) {
printf("Enter input file: ");
char filename[64];
scanf("%s", filename);
yyin = fopen(filename, "r");
printf("Enter output file: ");
scanf("%s", filename);
yyout = fopen(filename, "w");
yylex();
printf("Number of comments: %d\n", comment_lines);
}
In the above code, in_comment is used to indicate the state of the line being analyzed. If it is in a comment, the comment_lines value is incremented, otherwise the text is written to an output file given by yyout.
input file
#include <stdio.h>
/*
*
* This is a multi-line comment
*
*/
void dummy();
// main function is the entry point.
int main(void) {
/*
* Here we print a message on user's
* console.
*/
printf("Hello world\n");
// That is it.
}
output file generated after lexical analysis
#include <stdio.h>
void dummy();
int main(void) {
printf("Hello world\n");
}
Number of comments: 11

Answer 3

%{
#undef yywrap
#define yywrap() 1
int single=0, multiline =0;
%}
%%
"//".*"\n" {printf("%s\n", yytext); single++;}
"/*".*[\n]*.*"*/" {printf("%s\n", yytext); multiline++;}
.;
%%
int main()
{
yyin = fopen("yourfile","r");
yylex(); //calling the rules section
printf(" Single line comments = %d, multiline comments = %d", single, multiline);
fclose(yyin);
}