Re: Perl Idioms Explained - keys %{{map{$_=>1}@list}}

[Aristotle]

Just as memory hungry, still not order preserving, and still broken with regards to undefined values, but faster approach:

my %seen;
@seen{@list}=();
my @uniq = keys %seen;

And if you want to wrap it up without "leaking" lexicals:

my @uniq = do {
    my %seen;
    @seen{@list}=();
    keys %seen;
};

This approach does not (explicitly) iterate - more of the data structure setup work is done implicitly by perl.

Makeshifts last the longest.

[Juerd]

@seen{@list}=();

A bit faster:

undef @seen{@list};

Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }

[Roy Johnson]

undef is not documented to work on slices. That command should probably generate at least a warning.

[pilcrow]

perlfunc's entry for undef suggests this behavior on slices, if one squints:

Undefines the value of EXPR, which must be an lvalue... Saying undef $hash{$key} will probably not do what you expect...

That is, taking "undef EXPR" as (usually) equivalent to "EXPR = undef", the behavior is sensible, even expected. At any rate, a warning seems inappropriate, as this feature is long-standing:

$ perl -le 'undef @h{qw|a b c|}; print "$]: ", join " ", keys %h'
5.00404: a b c

[Jenda]

This actually seems to be slightly quicker than the grep based solution:

Benchmark: timing 100000 iterations of broquaint, grep_seen, aristotle
+, juerd...
 broquaint: 25 wallclock secs (21.62 usr +  0.01 sys = 21.63 CPU) @ 46
+22.99/s (n=100000)
 grep_seen: 12 wallclock secs (11.55 usr +  0.00 sys = 11.55 CPU) @ 86
+60.26/s (n=100000)
 aristotle: 10 wallclock secs ( 9.05 usr +  0.00 sys =  9.05 CPU) @ 11
+044.84/s (n=100000)
     juerd:  9 wallclock secs ( 8.66 usr +  0.00 sys =  8.66 CPU) @ 11
+543.35/s (n=100000)

on

my @list = qw/a b c d d a e b a b d e f a erti wen udfgn
 wei sdej usdjdh iu t k a b b b b a a a a c d  f e f s r s
 gkl h u s y t d s w s d log u log ds/;

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

[Aristotle]

Yep. As I said, it does not iterate on the Perl side. The pertinent nodes in optree are only ever seen once, and the loop is implicit in the execution of that single op. With all other approaches, the pertinent ops have to be dispatched and executed as many times as there are items in the list.

In fact, the keys %{{map{$_=>1}@list}} approach needs to first iterate over all elements of @list for the map, building a list as it goes, then build another list for keys.

In contrast, the @uniq = grep ! $seen{$_}++, @list; approach only builds a list once, while iterating over the elements of @list.

And of course as already explained, undef @seen{@list}; @uniq = keys %seen; just builds a single list - without iterating at all.

Makeshifts last the longest.