I ran some benchmarks and I the real answer is that
while <..> has been finely tuned. On a large file it is almost as fast as
foreach over a list.
/dev/null
Rate for whyfor while while_list for_list
for 374878/s -- -13% -15% -55% -62%
whyfor 433016/s 16% -- -2% -48% -56%
while 442469/s 18% 2% -- -47% -55%
while_list 833025/s 122% 92% 88% -- -16%
for_list 991880/s 165% 129% 124% 19% --
/usr/share/dict/words
Rate for while_list whyfor while for_list
for 4.80/s -- -35% -37% -38% -43%
while_list 7.40/s 54% -- -2% -5% -13%
whyfor 7.57/s 58% 2% -- -2% -11%
while 7.75/s 62% 5% 2% -- -9%
for_list 8.48/s 77% 15% 12% 9% --
/etc/passwd
Rate for whyfor while while_list for_list
for 14440/s -- -13% -16% -24% -49%
whyfor 16599/s 15% -- -4% -12% -42%
while 17224/s 19% 4% -- -9% -39%
while_list 18915/s 31% 14% 10% -- -33%
for_list 28442/s 97% 71% 65% 50% --
#!/usr/bin/perl
use strict;
use Benchmark qw( cmpthese );
foreach my $file qw ( /dev/null /usr/share/dict/words /etc/passwd ) {
open my $IN1, '<', $file or die "could not open $file";
my @list = <$IN1>;
seek( $IN1, 0, 0 );
print "$file\n";
cmpthese(
-5,
{
for_list => sub {
my %counts = ();
++$counts{$_} for @list;
die unless keys %counts == @list;
},
while_list => sub {
my $x = 0;
my %counts = ();
++$counts{$_} while defined( $_ = $list[ $x++ ] );
die unless keys %counts == @list;
},
for => sub {
seek( $IN1, 0, 0 );
my %counts = ();
++$counts{$_} for <$IN1>;
die unless keys %counts == @list;
},
while => sub {
seek( $IN1, 0, 0 );
my %counts = ();
++$counts{$_} while <$IN1>;
die unless keys %counts == @list;
},
whyfor => sub {
seek( $IN1, 0, 0 );
my %counts = ();
for ( ; defined( $_ = <$IN1> ) ; ) { ++$counts{$_}; }
die unless keys %counts == @list;
},
}
);
}
-- gam3
A picture is worth a thousand words, but takes 200K.