Re: Speed comparison of foreach vs grep + map

[ikegami]

As Corion pointed out, the switch from !$_ to /0/ is overpowering the other differences and invalidating your test.

But other improvements can be made.

---

There's no reason to use an intermediate array in the latter snippet. This avoids the creation of 5 million scalars.

my @e =
   grep !$_,
      map $_ % 2,
         @c;

You could even eliminate the grep.

my @e =
   map $_ % 2 ? () : 0,
      @c;

---

Similar improvements can be made for the foreach version.

my @e;
for my $c ( @c ) {
   my $d = $c % 2;
   push @e, $d if !$d;
}

my @e;
for my $c ( @c ) {
   push @e, 0 if !( $c % 2 );
}

---

Finally, I'm curious how these compare:

my @e = ( 0 ) x grep !( $_ % 2 ), @c;

my @e; push @e, 0 for 1 .. grep !( $_ % 2 ), @c;

[ikegami]

Benchmarks:

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw( cmpthese );

my @c = 1 .. 1_000_000;

cmpthese( -3, {
   for => sub {
      my @e;
      for my $c ( @c ) {
         my $d = $c % 2;
         push @e, $d if !$d;
      }
   },

   for2 => sub {
      my @e;
      for my $c ( @c ) {
         push @e, 0 if !( $c % 2 );
      }
   },

   map_grep => sub {
      my @e =
         grep !$_,
            map $_ % 2,
               @c;
   },

   map => sub {
      my @e =
         map $_ % 2 ? () : 0,
            @c;
   },

   grep_x => sub {
      my @e = ( 0 ) x grep !( $_ % 2 ), @c;
   },

   grep_for => sub {
      my @e; push @e, 0 for 1 .. grep !( $_ % 2 ), @c;
   },
} );

           Rate map_grep      for grep_for      map     for2   grep_x
map_grep 10.6/s       --     -27%     -43%     -47%     -51%     -54%
for      14.6/s      38%       --     -21%     -27%     -33%     -36%
grep_for 18.4/s      75%      27%       --      -7%     -15%     -20%
map      19.9/s      88%      36%       8%       --      -9%     -13%
for2     21.7/s     106%      49%      18%       9%       --      -5%
grep_x   22.9/s     117%      57%      24%      15%       6%       --

           Rate map_grep      for grep_for      map     for2   grep_x
map_grep 12.2/s       --     -16%     -34%     -40%     -45%     -49%
for      14.6/s      20%       --     -22%     -29%     -35%     -39%
grep_for 18.6/s      52%      27%       --      -9%     -17%     -23%
map      20.5/s      68%      40%      10%       --      -8%     -15%
for2     22.4/s      83%      53%      20%       9%       --      -7%
grep_x   24.1/s      97%      65%      29%      17%       8%       --

           Rate map_grep      for grep_for      map     for2   grep_x
map_grep 12.1/s       --     -19%     -31%     -36%     -45%     -49%
for      14.8/s      23%       --     -15%     -21%     -32%     -37%
grep_for 17.4/s      44%      17%       --      -8%     -21%     -27%
map      18.8/s      56%      27%       8%       --     -14%     -21%
for2     21.9/s      82%      48%      26%      16%       --      -8%
grep_x   23.7/s      97%      60%      37%      26%       8%       --