Thread: How to get only a certain portion of the string

Hello there all. I have a small program I wrote (with help of some here). If takes a number from the command line and then parses a file based on it. For example:

If you gave the number 3 and had a file:

1231
23412 112312
aaa23231 234234

it would result in:

1
12 112312
23231 234234

cutting off the first 3 characters. I do that with the following code:

(b is the command line number)

Code:

       while(fgets(buf, sizeof(buf), infile))
             fprintf(outfile, "%s", buf + b);

Now, i want to hardcode in a number to say, only grab that many characters. So with the same example as above if i hardcoded "6" i would get:

1231
23412 112312
aaa23231 234234

would be:

1
12 112
23231


only taking the first 6 characters from the string.


i would imagine i'd need something similiar to this, but i can't figure out what to do:

Code:

       while(fgets(buf, sizeof(buf), infile))
             fprintf(outfile, "%s" + 6, buf + b);

Any help would be appreciated. Thanks!

Originally Posted by Dave_Sinkula

Code:

fprintf(outfile, "%.6s\n", buf + b);

Dave,
Thank you, but that turned input like this (using 8 as the command line number, and 6 as the hardcode number):

Code:

1234 ZZ 123456    XXXX.XXXX.XXAAAAAAXXXXXX.XXXXX.XXXXXBBB

into output like this:

Code:

123456
AAAAAA
BBB

instead of:

Code:

123456

Originally Posted by Dave_Sinkula

And what is the size that buf is declared?

25

Code:

void main(int arcg, char *argv[])
{

    char buf[25];
    char file1[20] = " ";
	 char file2[20] = " ";
    char a[20] = " ";
    int b;
	 FILE *infile, *outfile;


    if (arcg != 4) {printf("\cut [column] [file to scan] [file to write to]");
   return;
   }

 strcpy(a, argv[1]);
 strcpy(file1, argv[2]);
 strcpy(file2, argv[3]);

 b = atoi (a);

 if ((infile = fopen(file1,"r")) == NULL)
     { printf("The file that you want\n");
       printf("scanned is not found.  Try again.");

        return;
        }


  if ((outfile  = fopen(file2,"w")) == NULL){ fclose(infile); return;}

		   while(fgets(buf, sizeof(buf), infile))
             fprintf(outfile, "%.6s\n", buf + b);
}

Originally Posted by dwks

main returns int.

Code:

printf("\cut [column] ...

Invalid escape sequence . . . did you mean \n?

Do you include <stdio.h>?

Yes stdio.h is included.

\cut - cut is the name of the program, just prompting the user on what the sequence on the command line should be.

on error prints:

cut [column] [file to scan] [file to write to]

Originally Posted by itsme86

That's because you're reading 24 chars at a time (fgets() reads 1 less than the size you supply to leave room for the terminating '\0'). And every time you read 24 chars you're printing the buffer starting at index b and printing 6 characters.

Understood, didn't realize it. I'll bump the buffer up to 100.

Thank you!

Originally Posted by dwks

Just make the buffer bigger (like 80 or 200) and see what happens.

That's exactly what I did.


Thank you all for taking the time to explain.