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    <p>Even know <em>Premature optimisation is the root of all evil</em></p> <pre><code>{ local $\ = "\n"; print foreach @numbers; } </code></pre> <p>but some expectations can be wrong. Test is little bit weird because output can make some weird side-effects and order can be important.</p> <pre><code>#!/usr/bin/env perl use strict; use warnings; use Benchmark qw(:all :hireswallclock); use constant Numbers =&gt; 10000; my @numbers = (1 .. Numbers); sub no_out (&amp;) { local *STDOUT; open STDOUT, '&gt;', '/dev/null'; my $result = shift()-&gt;(); close STDOUT; return $result; }; my %tests = ( loop1 =&gt; sub { foreach my $current (@numbers) { print "$current\n"; } }, loop2 =&gt; sub { foreach (@numbers) { print "$_\n"; } }, loop3 =&gt; sub { local $\ = "\n"; print foreach @numbers; } ); sub permutations { return [ map { my $a = $_; my @f = grep {$a ne $_} @_; map { [$a, @$_] } @{ permutations( @f ) } } @_ ] if @_; return [[]]; } foreach my $p ( @{ permutations( keys %tests ) } ) { my $result = { map { $_ =&gt; no_out { sleep 1; countit( 2, $tests{$_} ) } } @$p }; cmpthese($result); } </code></pre> <p>One can expect that loop2 should be faster than loop1</p> <pre><code> Rate loop2 loop1 loop3 loop2 322/s -- -2% -34% loop1 328/s 2% -- -33% loop3 486/s 51% 48% -- Rate loop2 loop1 loop3 loop2 322/s -- -0% -34% loop1 323/s 0% -- -34% loop3 486/s 51% 50% -- Rate loop2 loop1 loop3 loop2 323/s -- -0% -33% loop1 324/s 0% -- -33% loop3 484/s 50% 49% -- Rate loop2 loop1 loop3 loop2 317/s -- -3% -35% loop1 328/s 3% -- -33% loop3 488/s 54% 49% -- Rate loop2 loop1 loop3 loop2 323/s -- -2% -34% loop1 329/s 2% -- -33% loop3 489/s 51% 49% -- Rate loop2 loop1 loop3 loop2 325/s -- -1% -33% loop1 329/s 1% -- -32% loop3 488/s 50% 48% -- </code></pre> <p>Sometimes I observed consistently <code>loop1</code> about 15%-20% faster than <code>loop2</code> but I can't determine why.</p> <p>I was observed generated byte-code for loop1 and loop2 and there is difference only one when creating <code>my</code> variable. This variable interior is not allocated and also not copied thus this operation is very cheap. Difference comes I think only from <code>"$_\n"</code> construct which is not cheap. These loops should be very similar</p> <pre><code>for (@numbers) { ... } for my $a (@numbers) { ... } </code></pre> <p>but this loop is more expensive</p> <pre><code>for (@numbers) { my $a = $_; ... } </code></pre> <p>and also </p> <pre><code>print "$a\n"; </code></pre> <p>is more expensive than </p> <pre><code>print $a, "\n"; </code></pre>
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    1. POIs $_ more efficient than a named variable in Perl's foreach?
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    1. USHynek -Pichi- Vychodil
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    1. COI more interested in the general idea of using $_ rather than printing... Anyways I would love if you could explain a bit more the above code :D
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    2. COThis is Perlish :P
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    3. COMain idea to use $_ is avoid my variable creation and allocation. These operation consume some time amount anyway using `for my $a (@numbers) {}` doesn't allocate memory for array members (beware of this, changing $a will affect @numbers).
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