in reply to Re: reading a line from file two times
in thread reading a line from file two times

hi jettro, 
       if ( $line =~ /NEWTABLE/) {
         print "b4 seek $line\n";
         print "len :$length:\n";
         seek(FH, (0 - length($line)), SEEK_CUR);
         chomp($line=<FH>);
         print "--> $line\n";
         exit;
       }

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The above coed still prints the next line after using seek. am I missing something there in the seek function ?

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Re^3: reading a line from file two times
by jettero (Monsignor) on Dec 21, 2006 at 03:35 UTC

    The above coed still prints the next line after using seek. am I missing something there in the seek function ?

    The negative seek may not work in windows... not sure. It definitely does work on my linux box though. I don't have any windows handy or I'd test it. I did notice you didn't use "or die $!. " Most likely it would return an error result on your platform.

    use strict;
use Fcntl qw(:seek);

open my $in, "filename" or die $!;

my $line = <$in>;
print "$line\n";

seek $in, (0 - length $line), SEEK_CUR or die $!;

$line = <$in>;
print "$line\n";

close $in;

    [download]

    It should either work or complain...

    -Paul

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Re^3: reading a line from file two times
by jesuashok (Curate) on Dec 21, 2006 at 03:22 UTC
    Hi 

    if ( $line =~ /NEWTABLE/) {
   seek(FH, (0 - length($line)+tell(FH)), SEEK_CUR);
   $line = <FH>;
   chomp($line);
   print "LINE AFTER SEEK :$line:\n"
}

    [download]
    Output :- 
    "LINE AFTER SEEK :EWTABLE:\n"

    [download]
    why I am missing 'N' after the seek ?

    "Keep pouring your ideas"
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      There's definitely someting screwy with using a negative, relative seek (on Win32). To back up over a line consisting of 8 chars + NL, requires seeking back -6.

      I'd expect to have to seek back more than the line length, not less to account for the newline? 

      #! perl -slw
use strict;

while( <DATA> ) {
    chomp;
    print;
    if( /the line/ ) {
        my $backup = -length() + 2;
        print "Backing up $backup places";
        seek DATA, $backup, 1;
        chomp( $_ = <DATA> );
        print "?$_?";
    }
}

=output
c:\test>junk
junk
junk
more junk
the line
Backing up -6 places
?the line?
other junk
yet more junk
=cut
__DATA__
junk
junk
more junk
the line
other junk
yet more junk

      [download]
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
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        Should be on systems where seek($fh, $amount, SEEK_CUR) works correctly, you'd have to backup over all the characters read. So if the length($entire_string) is 10 (including LF or CRLF), that'd be: seek($fh, -10, SEEK_CUR). I do seem to recall that it behaves unexpectedly (for a unix nerd) in windows, but my experience was in foxpro, not perl — that definitely shouldn't count.

        -Paul

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      seek(FH, (0 - length($line)+tell(FH)), SEEK_CUR);

      You are seeking too far back. You're seeking back the length of the string plus the byte-number of your current position...

      If you wish to use tell() to record the old position before your read, then you'd use: seek(FH, $old_position, SEEK_SET); instead.

      -Paul

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