Out of interest, I ran the code through Devel::NYTProf, setting the number of iterations to 200,000. The sort functions in the hash-based methods are the main slow-points, with populating the hash coming a close second (i.e. @ha{0..$#array} = @array;).

haukex has already shown that pre-initialising the hash does not change the order by much.

Another point is that the hash slice subs don't need to get the results using a slice - they can just get the values directly.

The code below adds two subs that sort the values call on %ha.

When run on my system (Strawberry perl 5.28) both variants are about 20%-25% faster than their respective originals. The top few subs are, unsurprisingly, unchanged. I have not done prehash_local but would expect the same relative difference.

use warnings;
use strict;
use Benchmark qw/cmpthese/;

my @array = ( 'a'..'t' );
my %hash = map { $_ => $array[$_] } 0..$#array;
my $index = 3;
my $expect = 'abcefghijklmnopqrst';
use constant TEST => 0;

cmpthese(-2, {
    splice => sub {
        my @output = @array;
        splice @output, $index, 1;
        join("", @output) eq $expect or die if TEST;
    },
    hash_slice => sub {
        my %ha; @ha{0..$#array} = @array;
        delete $ha{$index};
        my @output = @ha{ sort {$a <=> $b} keys %ha };
        join("", @output) eq $expect or die if TEST;
    },
    h_slice_2 => sub {
        my %ha; @ha{0..$#array} = @array;
        delete $ha{$index};
        my @output = sort values %ha;
        join("", @output) eq $expect or die if TEST;
    },
    swap => sub {
        my @output = @array;
        $output[$index] = pop @output;
        join("", sort @output) eq $expect or die if TEST;
    },
    prehash_del => sub {
        my %ha = %hash;
        delete $ha{$index};
        my @output = @ha{ sort {$a <=> $b} keys %ha };
        join("", @output) eq $expect or die if TEST;
    },
    prehash_del_2 => sub {
        my %ha = %hash;
        delete $ha{$index};
        my @output = sort values %ha;
        join("", @output) eq $expect or die if TEST;
    },
    prehash_local => sub {
        delete local $hash{$index};
        my @output = @hash{ sort {$a <=> $b} keys %hash };
        join("", @output) eq $expect or die if TEST;
    },
    prehash_grep => sub {
        my @output = @hash{ sort {$a <=> $b}
            grep { $_ != $index } keys %hash };
        join("", @output) eq $expect or die if TEST;
    },
    outhash => sub {
        my %output = %hash;
        delete $output{$index};
        join("", @output{ sort {$a <=> $b} keys %output }) eq $expect
            or die if TEST;
    },
});

__END__

                  Rate hash_slice h_slice_2 prehash_del prehash_del_2 
+prehash_local prehash_grep outhash swap splice
hash_slice     50341/s         --      -20%        -21%          -36% 
+         -52%         -59%    -67% -79%   -79%
h_slice_2      62792/s        25%        --         -2%          -20% 
+         -40%         -48%    -58% -74%   -74%
prehash_del    63796/s        27%        2%          --          -19% 
+         -39%         -48%    -58% -74%   -74%
prehash_del_2  78613/s        56%       25%         23%            -- 
+         -25%         -35%    -48% -68%   -68%
prehash_local 104733/s       108%       67%         64%           33% 
+           --         -14%    -31% -57%   -57%
prehash_grep  121518/s       141%       94%         90%           55% 
+          16%           --    -19% -50%   -50%
outhash       150826/s       200%      140%        136%           92% 
+          44%          24%      -- -38%   -38%
swap          243449/s       384%      288%        282%          210% 
+         132%         100%     61%   --    -1%
splice        244776/s       386%      290%        284%          211% 
+         134%         101%     62%   1%     --

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In reply to Re^3: Fastest way to "pick without replacement" by swl
in thread Fastest way to "pick without replacement" by haukex

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