in reply to Why is "for" much slower than "while"?
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.
A picture is worth a thousand words, but takes 200K.
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Re^2: Why is "for" much slower than "while"?
by Anonymous Monk on Jan 17, 2010 at 01:52 UTC | |
Re^2: Why is "for" much slower than "while"?
by Xiong (Hermit) on Jan 24, 2010 at 00:08 UTC |
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