Llama ./tac stumper

bluethundr has asked for the wisdom of the Perl Monks concerning the following question:

Oh wise oh, powerful monks! On p.97 of Llama, merlyn states thusly:

Write a program that acts like cat, but reverses the order of the output lines. (Some systems have a utility like this called tac.) If you run yours as ./tac fred barney betty, the output should be all of file betty from last line to first, then barney then fred, also from last line to first.

I seem to be having trouble reversing the input, but not the files.

#!/usr/bin/perl -w
use strict;

@ARGV = reverse @ARGV;
while (<>) {
 chomp;
 print "\n@_\n";
}
[download]

I've tried a couple of different approaches that didn't work. Here's an example:

#!/usr/bin/perl -w
my $in;

@ARGV = ( reverse @ARGV);
foreach my $x (@ARGV){
 @in = <$x>;
 @in = reverse @in;
}
while (@in) {
  chomp;
  print "\n$_\n";
}
[download]

I get perl complaining of "readline() on unopened file at ./tac.pl line 7."when I tried that solution. This seems simple, but in practice I am stumped. I'd appreciate the help of a more powerful monk.

Many THANKS!

Comment on Llama ./tac stumper Select or Download Code

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Re: Llama ./tac stumper by davido (Cardinal) on Jun 07, 2004 at 03:39 UTC
You're pretty close: `use strict; use warnings; my @lines; foreach my $file ( reverse @ARGV ) { open my $fh, '<', $file or die "Bleah: $!"; my( @slurp ) = <$fh>; push @lines, reverse @slurp; close $fh; } print foreach @lines;` [download] This could be done with a lot more memory efficiency (without slurping the files) if you take steps to read the file in backwards in the first place (using File::ReadBackwards or Tie::File). But I kept it to stuff that you would probably find explained in the Llama book. You usually automagically open the contents of @ARGV with the bare <> operator. Since I wanted to treat the files in reverse order, I didn't use <>'s magic, and instead opened the files explicitly. Actually that step could have been avoided if I had reversed the contents of @ARGV first, and then still relied upon <> to do its magic. UPDATE: I wanted to go ahead and show how it is simplified by letting <> do its magic. Here goes: `use strict; use warnings; my @lines; @ARGV = reverse @ARGV; foreach ( @ARGV ) { my( @slurp ) = <>; push @lines, reverse @slurp; } print foreach @lines;` [download] Enjoy! Dave	[reply] [d/l] [select]
Re^2: Llama ./tac stumper by BUU (Prior) on Jun 07, 2004 at 05:21 UTC
Unless I'm badly confused, I'm pretty sure that `foreach( @ARGV ){ <> }` isn't doing what you want. `<>` in list context should open every file in @ARGV, read all the lines, and return them. `@ARGV=reverse @ARGV; print reverse <>;` [download]	[reply] [d/l] [select]
Re^3: Llama ./tac stumper by davido (Cardinal) on Jun 07, 2004 at 06:23 UTC
My apologies on botching the 2nd solution. Here's an update: `use strict; use warnings; my( @slurp ) = <>; @slurp = reverse @slurp; print @slurp;` [download] Dave	[reply] [d/l]
Re^4: Llama ./tac stumper by BUU (Prior) on Jun 07, 2004 at 06:37 UTC
Re^5: Llama ./tac stumper by davido (Cardinal) on Jun 07, 2004 at 06:59 UTC
Re: Llama ./tac stumper (GOLF) by BrowserUk (Patriarch) on Jun 07, 2004 at 05:18 UTC
42. Read more... (276 Bytes) Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail	[reply] [d/l]
Re^2: Llama ./tac stumper (GOLF) by tilly (Archbishop) on Jun 07, 2004 at 05:41 UTC
That is more complex than you need. Read more... (302 Bytes)	[reply] [d/l]
Re^3: Llama ./tac stumper (GOLF) by BrowserUk (Patriarch) on Jun 07, 2004 at 06:58 UTC
Agreed. The only downside is that all the files to be reversed have to be in memory simultaneuosly, but that probably ok for golf :) Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail	[reply] [d/l]
Re: Llama ./tac stumper by ggg (Scribe) on Jun 07, 2004 at 14:15 UTC
On p.256, Appendix A of Llama, merlyn states thusly: `print reverse <>;` [download] ggg	[reply] [d/l]