Re: Reversing A File

Probably it is not the fastest way, but Tie::File should work

use Tie::File;

my $file = shift @ARGV;

tie my @data, 'Tie::File', $file or die $!;
tie my @reversed, 'Tie::File', $file."rev" or die $!;

@reversed = reverse(@data);
untie @data;
untie @reversed;
[download]

Tie::File doesn't load the file into memory, it indexes it, so this should work even for very big files

citromatik

Comment on Re: Reversing A File Download Code

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Re^2: Reversing A File by ikegami (Patriarch) on Dec 12, 2007 at 13:09 UTC
Your `reverse` puts all 1 million lines in memory. That's on top of the 2 million indexes you place in memory when only 1 million are needed. Fix: `use Tie::File; tie(my @data, 'Tie::File', $fname_in) or die("Unable to open \"$fname_in\": $!\n"); open(my $fh_out, '>', $fname_out) or die("Unable to create \"$fname_out\": $!\n"); for (my $i=@data; $i--; ) { print $fh_out $data[$i]; } untie @data;` [download] Although I recommend File::ReadBackwards. `use File::ReadBackwards qw( ); my $fh_in = File::ReadBackwards->new($fname_in) or die("Unable to open \"$fname_in\": $!\n"); open(my $fh_out, '>', $fname_out) or die("Unable to create \"$fname_out\": $!\n"); while (defined(my $line = $fh_in->readline())) { print $fh_out $line; }` [download]	[reply] [d/l] [select]
Re^3: Reversing A File by camelcom (Sexton) on Dec 12, 2007 at 13:40 UTC
Unfortunately, this firm doesn't have File::ReadBackwards and it's a real pain getting approval for non-core modules! I tried the Tie::File idea (thanks for that), but killed the job after 10mins of max'd out cpu (it was about half way through) - the file is only 180MB There must be a faster way??	[reply]
Re^4: Reversing A File by marto (Cardinal) on Dec 12, 2007 at 13:50 UTC
Ten minutes to get half way through processing a 180MB file? What spec of machine are you running this on? Have you tried something like `tail -r filename.log >> backwards.log` (if your OS supports it) and compared the time that it takes to finish? Martin	[reply] [d/l]
Re^4: Reversing A File by runrig (Abbot) on Dec 12, 2007 at 15:42 UTC
File::ReadBackwards is pure perl, all you have to do is copy the source to a file...what's the difference between that and writing your own module that does the same thing (besides the amount of work)?	[reply]