http://www.perlmonks.org?node_id=342467


in reply to Re: Find unique elements in an array
in thread Find unique elements in an array

saying = () instead of = 1 is slightly faster, but the hash slice approach may not scale well with large @a's, since the whole array needs to be placed on the stack at once.

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Re: Re: Re: Find unique elements in an array
by kvale (Monsignor) on Apr 04, 2004 at 15:52 UTC
    If there is a stack penalty, it is not terrible, as the routine get faster with large arrays: 
    use Benchmark qw(:all) ;

my @a;
push @a, int (rand(100)) foreach 1..2_000_000;

my %unique;
my (@awd1, @awd2, @awd3);
cmpthese(5, {
                  'jc' =>
                  sub { foreach my $thingy (@a) { $unique{$thingy} = 1
+; }
                        @awd1 = keys %unique;
                     },
                  'mk' =>
                  sub { @unique{ @a} = 1;
                        @awd2 = keys %unique;
                     },
                  'ys' =>
                  sub { @unique{ @a} = ();
                        @awd3 = keys %unique;
                     },
                 });

    [download]
    yields 
    Benchmark: timing 5 iterations of jc, mk, ys...
        jc: 19 wallclock secs (16.75 usr +  0.35 sys = 17.10 CPU) @  0
+.29/s (n=5)
        mk:  6 wallclock secs ( 6.00 usr +  0.01 sys =  6.01 CPU) @  0
+.83/s (n=5)
        ys:  7 wallclock secs ( 6.00 usr +  0.00 sys =  6.00 CPU) @  0
+.83/s (n=5)
   s/iter   jc   mk   ys
jc   3.42   -- -65% -65%
mk   1.20 185%   --  -0%
ys   1.20 185%   0%   --

    [download]
    The = () optimization does not seem to make much difference.

    -Mark

[reply]
[d/l]
[select]
      With your original benchmark, I saw a consistent 4-5% increase for (). Obviously this is a constant difference that disappears into the woodwork with larger slices.
[reply]
      Anyway @unique{@a} = 1; looks a bit curious. Why do we set to 1 the only value in a huge hash?
[reply]
[d/l]

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