comment on

Hello, my hearties! Support the Pirate Party! Aarhhh, matey! P-) <-- (smiley wearing an eye patch)

I ran some benchmarks with the following code to see if elsif chains were faster than a dispatch table. I pray for your wisdom as to why a chain of elsifs which should be O(N) is faster than a hash operation which ought to be O(1). Is there some Perl Magick going on here?

#!/usr/bin/perl
# elsifbench - which is faster, a chain of elsif or a dispatch table? 
# SSF 112208 042609

use strict;
use Benchmark qw/:all/;

sub random { int rand shift }
my @letters=('A'..'Z','a'..'z','0'..'9');
my $nLetters=scalar @letters;

sub elsifchain {
   my $x=$letters[random($nLetters)];
   my $y='xyzzy!' x random(1000);
   if ($x eq 'A') {
      split(/!/,$y);
   }
   elsif ($x eq 'B') {
      split(/!/,$y);
   }
   elsif ($x eq 'C') {
      split(/!/,$y);
   }
   elsif ($x eq 'D') {
      split(/!/,$y);
   }
   elsif ($x eq 'E') {
      split(/!/,$y);
   }
   elsif ($x eq 'F') {
      split(/!/,$y);
   }
   elsif ($x eq 'G') {
      split(/!/,$y);
   }
   elsif ($x eq 'H') {
      split(/!/,$y);
   }
   elsif ($x eq 'I') {
      split(/!/,$y);
   }
   elsif ($x eq 'J') {
      split(/!/,$y);
   }
   elsif ($x eq 'K') {
      split(/!/,$y);
   }
   elsif ($x eq 'L') {
      split(/!/,$y);
   }
   elsif ($x eq 'M') {
      split(/!/,$y);
   }
   elsif ($x eq 'N') {
      split(/!/,$y);
   }
   elsif ($x eq 'O') {
      split(/!/,$y);
   }
   elsif ($x eq 'P') {
      split(/!/,$y);
   }
   elsif ($x eq 'Q') {
      split(/!/,$y);
   }
   elsif ($x eq 'R') {
      split(/!/,$y);
   }
   elsif ($x eq 'S') {
      split(/!/,$y);
   }
   elsif ($x eq 'T') {
      split(/!/,$y);
   }
   elsif ($x eq 'U') {
      split(/!/,$y);
   }
   elsif ($x eq 'V') {
      split(/!/,$y);
   }
   elsif ($x eq 'W') {
      split(/!/,$y);
   }
   elsif ($x eq 'X') {
      split(/!/,$y);
   }
   elsif ($x eq 'Y') {
      split(/!/,$y);
   }
   elsif ($x eq 'Z') {
      split(/!/,$y);
   }
   elsif ($x eq 'a') {
      split(/!/,$y);
   }
   elsif ($x eq 'b') {
      split(/!/,$y);
   }
   elsif ($x eq 'c') {
      split(/!/,$y);
   }
   elsif ($x eq 'd') {
      split(/!/,$y);
   }
   elsif ($x eq 'e') {
      split(/!/,$y);
   }
   elsif ($x eq 'f') {
      split(/!/,$y);
   }
   elsif ($x eq 'g') {
      split(/!/,$y);
   }
   elsif ($x eq 'h') {
      split(/!/,$y);
   }
   elsif ($x eq 'i') {
      split(/!/,$y);
   }
   elsif ($x eq 'j') {
      split(/!/,$y);
   }
   elsif ($x eq 'k') {
      split(/!/,$y);
   }
   elsif ($x eq 'l') {
      split(/!/,$y);
   }
   elsif ($x eq 'm') {
      split(/!/,$y);
   }
   elsif ($x eq 'n') {
      split(/!/,$y);
   }
   elsif ($x eq 'o') {
      split(/!/,$y);
   }
   elsif ($x eq 'p') {
      split(/!/,$y);
   }
   elsif ($x eq 'q') {
      split(/!/,$y);
   }
   elsif ($x eq 'r') {
      split(/!/,$y);
   }
   elsif ($x eq 's') {
      split(/!/,$y);
   }
   elsif ($x eq 't') {
      split(/!/,$y);
   }
   elsif ($x eq 'u') {
      split(/!/,$y);
   }
   elsif ($x eq 'v') {
      split(/!/,$y);
   }
   elsif ($x eq 'w') {
      split(/!/,$y);
   }
   elsif ($x eq 'x') {
      split(/!/,$y);
   }
   elsif ($x eq 'y') {
      split(/!/,$y);
   }
   elsif ($x eq 'z') {
      split(/!/,$y);
   }
   elsif ($x eq '0') {
      split(/!/,$y);
   }
   elsif ($x eq '1') {
      split(/!/,$y);
   }
   elsif ($x eq '2') {
      split(/!/,$y);
   }
   elsif ($x eq '3') {
      split(/!/,$y);
   }
   elsif ($x eq '4') {
      split(/!/,$y);
   }
   elsif ($x eq '5') {
      split(/!/,$y);
   }
   elsif ($x eq '6') {
      split(/!/,$y);
   }
   elsif ($x eq '7') {
      split(/!/,$y);
   }
   elsif ($x eq '8') {
      split(/!/,$y);
   }
   elsif ($x eq '9') {
      split(/!/,$y);
   }
   else {
      warn "Huh?";
   }
}

sub dispatch {
   my $x=$letters[random($nLetters)];
   my $y='xyzzy!' x random(1000);
   my %dispatch=(
       A=>sub { split(/!/,$y); },
       B=>sub { split(/!/,$y); },
       C=>sub { split(/!/,$y); },
       D=>sub { split(/!/,$y); },
       E=>sub { split(/!/,$y); },
       F=>sub { split(/!/,$y); },
       G=>sub { split(/!/,$y); },
       H=>sub { split(/!/,$y); },
       I=>sub { split(/!/,$y); },
       J=>sub { split(/!/,$y); },
       K=>sub { split(/!/,$y); },
       L=>sub { split(/!/,$y); },
       M=>sub { split(/!/,$y); },
       N=>sub { split(/!/,$y); },
       O=>sub { split(/!/,$y); },
       P=>sub { split(/!/,$y); },
       Q=>sub { split(/!/,$y); },
       R=>sub { split(/!/,$y); },
       S=>sub { split(/!/,$y); },
       T=>sub { split(/!/,$y); },
       U=>sub { split(/!/,$y); },
       V=>sub { split(/!/,$y); },
       W=>sub { split(/!/,$y); },
       X=>sub { split(/!/,$y); },
       Y=>sub { split(/!/,$y); },
       Z=>sub { split(/!/,$y); },
       a=>sub { split(/!/,$y); },
       b=>sub { split(/!/,$y); },
       c=>sub { split(/!/,$y); },
       d=>sub { split(/!/,$y); },
       e=>sub { split(/!/,$y); },
       f=>sub { split(/!/,$y); },
       g=>sub { split(/!/,$y); },
       h=>sub { split(/!/,$y); },
       i=>sub { split(/!/,$y); },
       j=>sub { split(/!/,$y); },
       k=>sub { split(/!/,$y); },
       l=>sub { split(/!/,$y); },
       m=>sub { split(/!/,$y); },
       n=>sub { split(/!/,$y); },
       o=>sub { split(/!/,$y); },
       p=>sub { split(/!/,$y); },
       q=>sub { split(/!/,$y); },
       r=>sub { split(/!/,$y); },
       s=>sub { split(/!/,$y); },
       t=>sub { split(/!/,$y); },
       u=>sub { split(/!/,$y); },
       v=>sub { split(/!/,$y); },
       w=>sub { split(/!/,$y); },
       x=>sub { split(/!/,$y); },
       y=>sub { split(/!/,$y); },
       z=>sub { split(/!/,$y); },
       0=>sub { split(/!/,$y); },
       1=>sub { split(/!/,$y); },
       2=>sub { split(/!/,$y); },
       3=>sub { split(/!/,$y); },
       4=>sub { split(/!/,$y); },
       5=>sub { split(/!/,$y); },
       6=>sub { split(/!/,$y); },
       7=>sub { split(/!/,$y); },
       8=>sub { split(/!/,$y); },
       9=>sub { split(/!/,$y); }
   );
   if (exists $dispatch{$x}) {
      $dispatch{$x}->();
   } else {
      warn "Huh?";
   }
}

cmpthese(100000,{elsif=>\&elsifchain,dispatch=>\&dispatch});

exit;

__END__

A..Z = 26 items

Benchmark: timing 100000 iterations of dispatch, elsif...
  dispatch: 56 wallclock secs (55.97 usr +  0.03 sys = 56.00 CPU) @ 17
+85.71/s (n=100000)
     elsif: 35 wallclock secs (35.02 usr +  0.00 sys = 35.02 CPU) @ 28
+55.51/s (n=100000)
           Rate dispatch    elsif
dispatch 1786/s       --     -37%
elsif    2856/s      60%       --

A..Z, a..z, 0..9 = 62 items

Benchmark: timing 100000 iterations of dispatch, elsif...
  dispatch: 93 wallclock secs (91.94 usr +  0.01 sys = 91.95 CPU) @ 10
+87.55/s (n=100000)
     elsif: 41 wallclock secs (35.22 usr +  0.01 sys = 35.23 CPU) @ 28
+38.49/s (n=100000)
           Rate dispatch    elsif
dispatch 1088/s       --     -62%
elsif    2838/s     161%       --
[download]

Thanks in advance for your comments.

Humbly yours,
Aahhrh!
SSF

In reply to elsif chain vs. dispatch by sflitman

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