How to skip a comment in c programming with using fopen - c

Question

I want to ignore/skip the comments in a text file when I use fgets.
The problem is that I only can skip a comment if the first character in a line starts is #. Comments starts with # in my text file. But there are some # in my file.txt that are not the first character of a line, like so;
#Paths
A B #Path between A and B.
D C #Path between C and D.
A is my first node, B is my second node and when # comes I want to ignore the rest of text until the next line. My new node should be D and C etc. I can only use "r" in fopen function.
I have tried fgets but it reads line by line and fgetc doesn't help either.
bool ignore_comments(const char *s)
{
int i = 0;
while (s[i] && isspace(s[i])) i++;
return (i >= 0 && s[i] == '#');
}
FILE *file;
char ch[BUFSIZE];
file = fopen("e.txt", "r");
if (file == NULL) {
printf("Error\n");
fprintf(stderr, "ERROR: No file input\n");
exit(EXIT_FAILURE);
}
while(fgets(ch, BUFSIZE, file) != NULL)
{
if (line_is_comment(ch)) {
// Ignore comment lines.
continue;
printf("%c",*ch);
}
fscanf(file, "%40[0-9a-zA-Z]s", ch);
....
}

Answer 1

the following proposed code:
performs the desired functionality
cleanly compiles
properly checks for errors
this answer uses a state machine, based on: 'InComment'
and now, the proposed code:
#include <stdio.h>
#include <stdlib.h>
int main( void )
{
int InComment = 0;
FILE *fp = fopen( "file.txt", "r" );
if( !fp )
{
perror( "fopen to read -file.txt- failed" );
exit( EXIT_FAILURE );
}
int ch;
while( (ch = fgetc(fp)) != EOF )
{
if( ch == '#' )
{
InComment = 1;
}
else if( ch == '\n' )
{
InComment = 0;
fputc( ch, stdout );
}
else if( !InComment )
{
fputc( ch, stdout );
}
}
fclose( fp );
}

Answer 2

Also method names are different, but am I right with this version ?
Ignore my dirty method line_is_comment - from first version unless you want to play with ;-)
Extended test input:
#Paths
A B #Path between A and B.
D C #Path between C and D.
E F
G H
Output:
rest of line read
AB rest of line read
DC rest of line read
EF rest of line read
GH rest of line read
#include <stdio.h>
bool line_is_comment(const char *s)
{
char *commentPos = const_cast<char*>(strchr(s, '#'));
if(commentPos != NULL) {
*commentPos = 0; // cut-off chars after comment
//return true; // or false then to accept the line
return commentPos == s;
}
return false;
}
#define BUFSIZE 50
int main()
{
FILE *file;
char ch[BUFSIZE];
file = fopen("e.txt", "r");
if (file == NULL) {
printf("Error\n");
fprintf(stderr, "ERROR: No file input\n");
exit(EXIT_FAILURE);
}
int x;
while(!feof(file)) {
x = fscanf(file, "%40[0-9a-zA-Z]s", ch);
if(x == 0) {
ch[0] = fgetc(file);
if(ch[0] == '#' || ch[0] == '\n') {
if(ch[0] != '\n') fgets(ch, BUFSIZE, file);
printf(" rest of line read\n");
}
} else if(x<0) break;
else {
printf("%c",*ch); // continue with ... undisclosed part here
}
}
return 0;
}

Answer 3

You can also make use of strcspn to trim all comments (and if not present, trim the line-endings from your buffer) in a single simple call. Where you would normally trim the line-ending from the buffer read by fgets() with:
ch[strcspn (ch, "\r\n")] = 0; /* trim line-ending */
You can simply add the "#" character to your reject list and nul-terminate there if a comment is present. That would reduce the complete task of removing comments beginning with '#' and outputting the newly formatted line to:
while (fgets (ch, BUFSIZE, fp)) { /* read every line */
ch[strcspn (ch, "#\r\n")] = 0; /* trim comment or line-ending */
puts (ch); /* output line w/o comment */
}
A short example taking the file to read as the first argument to the program (or reading from stdin by default if no argument is given), you could do:
#include <stdio.h>
#include <string.h>
#define BUFSIZE 1024 /* if you need a constant, #define one (or more) */
int main (int argc, char **argv) {
char ch[BUFSIZE];
/* use filename provided as 1st argument (stdin by default) */
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
perror ("file open failed");
return 1;
}
while (fgets (ch, BUFSIZE, fp)) { /* read every line */
ch[strcspn (ch, "#\r\n")] = 0; /* trim comment or line-ending */
puts (ch); /* output line w/o comment */
}
if (fp != stdin) fclose (fp); /* close file if not stdin */
return 0;
}
Example Input File
Borrowing Tom's example file :)
$ cat dat/comments_file.txt
#Paths
A B #Path between A and B.
D C #Path between C and D.
E F
G H
Example Use/Output
$ ./bin/comments_remove <dat/comments_file.txt
A B
D C
E F
G H
Look things over and let me know if you have further questions.