- For for (X..Y), Perl uses a counting loop.
- For for (reverse constX..constY), Perl builds a flattened list at compile-time and iterates over it in reverse.
- For for (reverse X..Y), Perl builds a flattened list at run-time and iterates over it in reverse.
So far, I've found 6 different kinds of for loops in Perl. You can find them in an earlier node.
#!/usr/bin/perl --
use strict;
use warnings;
use Benchmark qw(cmpthese);
my $BIG = 100_000;
cmpthese(-3, {
c_f => \&c_f,
c_r => \&c_r,
p_fv => \&p_fc,
p_fc => \&p_fv,
p_rc => \&p_rc,
p_rv => \&p_rv,
});
sub c_f { for (my $i = 0; $i < $BIG; $i++ ) { 1 } }
sub c_r { for (my $i = $BIG; $i-- > 0; ) { 1 } }
sub p_fc { for my $i ( 0..100_000-1 ) { 1 } }
sub p_fv { for my $i ( 0..$BIG-1 ) { 1 } }
sub p_rc { for my $i ( reverse 0..100_000-1 ) { 1 } }
sub p_rv { for my $i ( reverse 0..$BIG-1 ) { 1 } }
__END__
Rate p_rv c_r c_f p_rc p_fv p_fc
p_rv 28.5/s -- -37% -44% -59% -64% -65% Builds list and loops.
c_r 45.0/s 58% -- -11% -35% -44% -44%
c_f 50.7/s 78% 13% -- -27% -37% -37%
p_rc 69.7/s 144% 55% 37% -- -13% -14% Loops over pre-built array.
p_fv 80.0/s 180% 78% 58% 15% -- -1% Counting loop
p_fc 80.7/s 183% 79% 59% 16% 1% -- Counting loop
If you add "()," to the front of any of the lists, you'll notice that p_* will become as slow as p_rv.
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