How do you move around a text file in C?

This is a discussion on How do you move around a text file in C? within the C Programming forums, part of the General Programming Boards category; Hello! I'm new to C and i'm trying to figure out how to move down a text file and print ...

  1. #1
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    How do you move around a text file in C?

    Hello! I'm new to C and i'm trying to figure out how to move down a text file and print out only certain lines.

    I know that fgets reads one line from a file but how do you skip 4 lines and then print out the 5th line?

    Do i use a while loop?

    eg.
    Code:
     
    FILE *ptr;
    int a = 0;
    char word[20];
    
    if ((ptr = fopen("input.txt","r") == NULL)
    {
    printf("Cannot open file");
    }
    else
    {
        while (a < 5)
        {
         ....don't know what to put here to skip these lines....
        }
        if (fgets(word,20,ptr) != NULL)   ......printing the 5th line
        {
        printf("&#37;s", word);
        }
    }
    fclose(ptr);
    i would just like to know what allows you to skip lines in a text file.
    Last edited by buggerboy_3; 04-29-2007 at 12:42 AM.

  2. #2
    CSharpener vart's Avatar
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    If you do not know the length of the line - you read it with fgets but do not print it - to skip.
    If you know the length - you can use fsetpos for example to skip known number of bytes
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    Registered User Noir's Avatar
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    If you know the length - you can use fsetpos for example to skip known number of bytes
    Nope, you can't do arbitrary seeking on a text file. It has to be opened as a binary file.

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    CSharpener vart's Avatar
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    Quote Originally Posted by Noir View Post
    Nope, you can't do arbitrary seeking on a text file. It has to be opened as a binary file.
    Yeah, right... And of course you have a quote from the reference guide to support your opinion?
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  5. #5
    Registered User Noir's Avatar
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    And of course you have a quote from the reference guide to support your opinion?
    Why yes, since you asked so nicely. Here's the quote that says fsetpos() won't work because you have to have been to the position first and called fgetpos():
    The fsetpos function sets the mbstate_t object (if any) and file position indicator
    for the stream pointed to by stream according to the value of the object pointed to by
    pos, which shall be a value obtained from an earlier successful call to the fgetpos
    function
    on a stream associated with the same file.
    And here's the quote that says you can't do arbitrary seeking on a text file with the only other option, fseek():
    For a text stream, either offset shall be zero, or offset shall be a value returned by
    an earlier successful call to the ftell function on a stream associated with the same file
    and whence shall be SEEK_SET.
    Oh, and those quotes are from the C standard, not just any old reference guide that happens to agree with me.

  6. #6
    CSharpener vart's Avatar
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    yeah, but this is about these 2 functions... but OMG we also have a fseek... What a suprise...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  7. #7
    Registered User Noir's Avatar
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    yeah, but this is about these 2 functions...
    They won't work though. You can't jump to somewhere you haven't been yet and fsetpos() requires a successful call to fgetpos(). The only way to jump somewhere you haven't been is with fseek() and fseek() doesn't work for arbitrary jumps on a text file.

  8. #8
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Noir View Post
    fseek() doesn't work for arbitrary jumps on a text file.
    No, it doesn't work on stream which are opened in text mode. Not the same thing. You can open a text file in binary mode, or a binary file in text mode, if you want.

  9. #9
    Registered User Noir's Avatar
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    That was bad wording on my part. I meant "text stream". Sorry for the confusion.

  10. #10
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    Quote Originally Posted by buggerboy_3 View Post
    Hello! I'm new to C and i'm trying to figure out how to move down a text file and print out only certain lines.

    I know that fgets reads one line from a file but how do you skip 4 lines and then print out the 5th line?
    You don't "skip" lines in a text file, you read them in, BUT YOU ONLY MAY PRINT EVERY 4th OR 5TH LINE (whatever you want).

    You can use a while loop, like this:
    Code:
    while (there is another line to be read)  
       increment line_count;
       if ((line_count &#37; 4 == 0) && (line_count > 0))  /* selects every 4th line */
          print that line;
    loop
    So you're not "jumping over", your program is just selecting every 4th line to do something with. The other lines are just ignored.

    Adak

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