Return reference to hash keys

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

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Re: Return reference to hash keys by kennethk (Abbot) on Nov 30, 2010 at 14:13 UTC
First, you are reinventing the wheel: see `uniq` in List::MoreUtils and How do I compute the difference of two arrays? How do I compute the intersection of two arrays? in perlfaq4. But this, of course, does not answer your question. keys returns a list, not an array, and so if you want to return an array reference you necessarily need to create a new array. You could reduce your character and line count by simply returning an anonymous array reference: `sub unique { my @refs = @_; my %files = (); foreach my $ref (@refs) { $files{$_}++ foreach (@$ref); } return [keys %files]; }` [download] For more info, see Making References in perlref.	[reply] [d/l] [select]
Re^2: Return reference to hash keys by miketosh (Acolyte) on Nov 30, 2010 at 20:24 UTC
Thanks! That is exactly what I needed. I didn't know the difference between a LIST and an ARRAY. I guess I still need to learn about it, but your solution for an anonymous array reference is exactly what I was looking for. Thanks a million. Here is my final code, for what it is worth. Yes, I needed to reinvent the wheel for portability, since getting List::MoreUtils is not an option... `sub compare { my @refs = @_; my ($all, $inter, $diff); my %files; foreach my $ref (@refs) { foreach (@$ref) { $files{$_}++; } } $all = [ keys %files ]; foreach my $e (@$all) { push @{ $files{$e} > 1 ? $inter : $diff }, $e; } return ($all, $inter, $diff); }` [download]	[reply] [d/l]
Re^3: Return reference to hash keys by kennethk (Abbot) on Nov 30, 2010 at 21:14 UTC
The difference between a list and an array is discussed a bit in perldata. The short, oversimplified version of it is that lists are values and arrays are variables. Regarding not using List::MoreUtils, Yes, even you can use CPAN should have an on-point discussion for consideration.	[reply]
Re: Return reference to hash keys by Limbic~Region (Chancellor) on Nov 30, 2010 at 14:14 UTC
miketosh, There is a module that does this for you (List::MoreUtils). It is also a FAQ. The fact that you want a reference back is an implementation detail. If I were doing it and since order doesn't seem to matter, I would probably do something like: `sub uniq { my %uniq; @uniq{map @$_, @_} = (); return [ keys %uniq ]; }` [download] Update: This code is untested. I modified it to account for passing in array refs instead of arrays. Cheers - L~R	[reply] [d/l]
OT Re: Return reference to hash keys by ww (Archbishop) on Nov 30, 2010 at 14:15 UTC
"I'm using Perl 5.005_03" I didn't know 5.005 had been ported to the steam-powered, gear-driven Univacs. :-) Seriously, that was released in the last century (1999, I think). You might be well served by updating to the latest version available for your architecture.	[reply]
Re: OT Re: Return reference to hash keys by miketosh (Acolyte) on Nov 30, 2010 at 20:30 UTC
That would require administrative privileges on a processor used by hundreds of people for software development on many different programs. Upgrading is out of the question. It would require an Engineering Analysis, and a likely upgrade of many many scripts, costing our customers in excess of a few hundred thousand dollars. Something tells me that they wouldn't go for that. The good news is that we are upgrading to the newfangled AIX 5, which comes with a fancy-dancy Perl 5.8.3. It should take no more than 3 more years.	[reply]