Re^2: Summing Up Array Elements By Group

The first one you wrote was the fastest:

Benchmark: timing 1000000 iterations of tin1, tin2, tin3...
      tin1:  1 wallclock secs ( 0.59 usr +  0.00 sys =  0.59 CPU) @ 16
+86340.64/s (n=1000000)
      tin2:  1 wallclock secs ( 0.75 usr +  0.00 sys =  0.75 CPU) @ 13
+33333.33/s (n=1000000)
      tin3:  3 wallclock secs ( 2.22 usr +  0.00 sys =  2.22 CPU) @ 45
+0450.45/s (n=1000000)
          Rate tin3 tin2 tin1
tin3  450450/s   -- -66% -73%
tin2 1333333/s 196%   -- -21%
tin1 1686341/s 274%  26%   --
[download]

the input to get this was:

$results = timethese(1000000,{
tin1 => '$output[int($_/3)] += $arr[$_] for 0..@arr-1;',
tin2 =>'$out[$_] = ($arr[(3*$_)]+$arr[(3*$_+1)]+$arr[(3*$_+2)]) for 0.
+.(scalar(@arr)/3 - 1);',
tin3=>'for (0..$k) {
    $output[$_] = $arr[$b] + $arr[$b+1] + $arr[$b +2];
    $b += 3;
}
'});
Benchmark::cmpthese( $results );
[download]

Raghu

Comment on Re^2: Summing Up Array Elements By Group Select or Download Code

Replies are listed 'Best First'.
Re^3: Summing Up Array Elements By Group by ivancho (Hermit) on Jan 21, 2009 at 09:40 UTC
clearly the massive differences between tin2 and tin3 must at least alert you that something is wrong with the benchmark... Frankly I have no idea what goes on once you start eval-ing strings in the Benchmark module, but from the presented examples, something seems suspicious. Here is a benchmark by me `use Benchmark qw(cmpthese); my @arr = map {rand} 0..900000; my @output = map {rand} 0..300000;; Benchmark::cmpthese(10,{ tin1 => sub { $output[int($_/3)] += $arr[$_] for 0..@arr-1; }, tin2 => sub { $output[$_] = $arr[3$_] + $arr[3$_+1] + $arr[3*$_+2] for 0..@arr/3; }, tin3=> sub { my $b = 0; for (0..@arr /3) { $output[$_] = $arr[$b] + $arr[$b+1] + $arr[$b +2]; $b += 3; } } });` [download] With result `$ perl ../../../perlmonks/bench-trip.pl Rate tin1 tin2 tin3 tin1 1.81/s -- -41% -46% tin2 3.08/s 70% -- -9% tin3 3.39/s 87% 10% --` [download]	[reply] [d/l] [select]
Re^4: Summing Up Array Elements By Group by ikegami (Patriarch) on Jan 21, 2009 at 12:59 UTC
clearly the massive differences between tin2 and tin3 must at least alert you that something is wrong with the benchmark... The problem is that his `$b` kept growing. You fixed it by readding `my $b = 0;`. Which leads to two the second problem: He didn't use warnings. They don't add a penalty, so use them! If they did add a penalty, all the more reason to use them just like in the real code. By the way, `$output[int($_/3)]` can be written as `$output[$_/3]`	[reply] [d/l] [select]