open(PREF,"+< preffile") or die;

while (<PREF>) {
    # if we see pref3 set to 0
    if (/^pref3\t0$/) {
        # you need to rewind two chars to get 
        # before the 0 and the newline , which 
        # you've already gone past
        seek(PREF,-2,1) or die;
        # print a "9" in this spot
        print PREF "9";
        # move forward one char past the newline
        # to continue reading.
        seek(PREF,1,1) or die;
    }
}
close PREF;
[download]

Update: I should note that this only works when the replacement string is the same length as the original. See "perldoc -q 'how do I change one line'" for a complete answer.

Update2:Replaced the read() with while(<PREF>), since it's not likely that all the lines in the file are 8 chars long.

Thanks for your guidance, the seek command worked like a charm. Here's what I ended up using:

open THIS_FILE, "+<$listOption" or die "ERROR: SearchFile: $!";
    while (<THIS_FILE>) {
        if (/^$genre\t\d$/) {
            seek(THIS_FILE, -2, 1) or die;
            print THIS_FILE $option;
            seek(THIS_FILE, 1, 1) or die;
        }
      }
close THIS_FILE;
[download]

Pretty close to what you updated to. It took me a while to realize I shouldn't be using the 8 char read.

Thanx
-Zach (SFSC)

This is a much better solution than mine. :)

-----------------------------------
Frank Wiles <frank@wiles.org>
http://frank.wiles.org

pref1\t0\n

and you want to be able to change it to:

pref1\t1\n

without rewriting the entire file? If that is correct you can do this which is basically straight out of the Perl Cookbook. Note it uses quite a bit of memory if the file is going to be large.

my $pref = 'pref1'; 

open(FH, "+< $FILE") or die "Error opening $FILE: $!\n"
my @lines = <FH>; 
seek(FH,0,0); 

my $count = 0; 
foreach my $l ( @lines ) { 
    $count++; 
    # Skip lines not matching $pref
    if( $line !~ /^$pref/ ) { next; }

    my ($pref, $num) =~ /^(.*?)\t(.*?)$/; 
    $num = 1; 

    $lines[$count] = "$pref\t$num\n"; 
}

print FH @lines; 
truncate(FH, tell(FH)); 
close(FH);
[download]

Edit kudra, 2001-07-30 Changed pre tags to code tags