How do I seek to a certain position in a file?

[Anonymous Monk]

I would like to know how to go to a specific line in a file, followed by the specific char in that line. Could you explain more on the function fseek, and how to utilize it?

Originally posted as a Categorized Question.

[btrott]

Perl doesn't have fseek--it has seek, which does the same thing. As far as I know.

There really isn't a good way of jumping to a specific line in a file, unless your "lines" are fixed-length. This is because Perl (and your computer in general) thinks of files as a sequence of bytes, not as a sequence of lines.

There are exceptions (one being files with fixed-length records). Take a look at perlfaq5 and the DB_File manpage (in particular, you'll want to look at the DB_RECNO section).

One suggestion in perlfaq5 is to build an index of line numbers mapping to byte positions in the file. Then, when you need to jump to a particular line X, you read the byte position for line X from your index, then seek to that byte position.

This may work for you.

[turnstep]

If you just want to find a certain line/character, try something like this:

$line=0;
$matchline=5;
$char = 1;
open(MYFILE, "$myfile") || die;
while(<MYFILE>) {
  ++$line >= $matchline || next;
  $l=length($_);
  print "Found line $line (length: $l)\n$_\n";
  if ($l>$char) {
    print "Character $char is ", substr($_,$char-1,1), "\n";
  }
  last;
}

[DamnDirtyApe]

To get to a known row in a text file, take a look at the Tie::File module on CPAN.

[simonflk]

GhodMode said:

my @array = <FILE>;
close FILE;

# Print line 10
print $array[10];

Not quite. That prints line 11. I would do something like this:

my $findline = 10;

while (<FILE>) {
    next unless $. == $findline;
    # .. do something
    last; #don't read any more
}

For the original question "do something" would probably be a seek backwards to the desired char, or perhaps more sensibly, stop the line before and seek forwards.

for more info on $. and friends, see perldoc perlvar

-- simonflk

[GhodMode]

open (FILE, "filename.txt")
    or die "Could not open filename.txt";

# Store the contents of the file in an array, one line per element
my @array = <FILE>;
close FILE;

# Print line 10
print $array[10];

[Anonymous Monk]

If there is a specific pattern you are looking for then you might want to use a if statement and match for the pattern using the ~m/pattern/ Shashidhar Iddamsetty