in reply to Fast Way to Combine Two Hashes

You might want to compare how these Benchmark: 
@foo{keys %bar} = values %bar;
%foo = (%foo, %bar);
foreach (keys %bar) { $foo{$_} = $bar{$_} }

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You can speculate on what might be the fastest, but Benchmark can be one way to validate it. No benchmark is perfect, though, but it can at least give you an idea.

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Re: Re: Fast Way to Combine Two Hashes
by thelenm (Vicar) on Jun 18, 2002 at 22:22 UTC
    For this particular test I ran, hash assignment and slices seemed to be much faster than creating the union one item at a time: 
    use strict;
use warnings;
use Benchmark 'cmpthese';

my (%set_a, %set_b);
@set_a{1..100} = undef;
@set_b{50..150} = undef;

sub slices {
  my %set;
  @set{keys %set_a, keys %set_b} = undef;
}

sub items {
  my %set;
  $set{$_} = undef for keys %set_a, keys %set_b;
}

sub hasheq {
  my %set;
  %set = (%set_a, %set_b);
}

cmpthese(10000, {
  slices => \&slices,
  items  => \&items,
  hasheq => \&hasheq,
});

    [download]
    gives these results on my machine: 
    Benchmark: timing 10000 iterations of hasheq, items, slices...
    hasheq:  3 wallclock secs ( 2.97 usr +  0.00 sys =  2.97 CPU) @ 33
+67.00/s (n=10000)
     items:  4 wallclock secs ( 4.18 usr +  0.00 sys =  4.18 CPU) @ 23
+92.34/s (n=10000)
    slices:  3 wallclock secs ( 2.96 usr +  0.00 sys =  2.96 CPU) @ 33
+78.38/s (n=10000)
         Rate  items hasheq slices
items  2392/s     --   -29%   -29%
hasheq 3367/s    41%     --    -0%
slices 3378/s    41%     0%     --

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    -- Mike

    --
    just,my${.02}

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