in reply to Re: efficiency & style
in thread efficiency & style

When seeking to match a series of strings like this there is no reason not to push the loop down to the RE engine. First you: 
my %service = qw(
    0050    www
    0016    ssh
    1A0B    irc
    0015    ftp
);
my $match_lst = join "|", keys %service;
my $match = qr/:($match_lst)\s/;

[download]
and then you change the loop to: 
foreach my $line (grep /\b01\s/, `cat /proc/net/tcp`  ) {
    ++$count{$service{$1}} while $line =~ /$match/g;
}

[download]
If you want you can speed it up even more by optimizing the RE as I did in RE (tilly) 4: SAS log scanner.

Replies are listed 'Best First'.
RE: RE (tilly) 2: efficiency & style
by dchetlin (Friar) on Sep 23, 2000 at 14:13 UTC
    Hmm. I disbelieve that using alternation is as efficient as looping over a list of patterns. I believe the following benchmark backs me up: 

    tilly gives: 1600
chetlin gives: 1600
Benchmark: running chetlin, tilly, each for at least 5 CPU seconds...
   chetlin:  9 wallclock secs ( 5.52 usr +  0.00 sys =  5.52 CPU) @ 33
+3.70/s (n=1842)
     tilly: 10 wallclock secs ( 5.09 usr +  0.00 sys =  5.09 CPU) @ 10
+4.52/s (n=532)

    [download]

    Here's the code for it; do feel free to slap me around if I made a thinko: 

    my @patterns=qw/foo bar baz blarch/;
my $tilly=qr/(@{[join "|",@patterns]})/;
my @chetlin=map qr/$_/,@patterns;
my $target="foo baz blarcy foo blarch"x400;

sub tilly {
    my $count;
    $count++ while ($target =~ /$tilly/g);
    print STDERR "tilly gives: $count\n" if ((caller)[1]!~/eval/);
}

sub chetlin {
    my $count;
    for (@chetlin) {$count++ while ($target =~ /$_/g) }
    print STDERR "chetlin gives: $count\n" if ((caller)[1]!~/eval/);
}

tilly();
chetlin();

use Benchmark;

timethese(-5, { tilly   => \&tilly,
                chetlin => \&chetlin,
});

    [download]

    In general, my credo is to avoid alternation at all costs. I would be interested in seeing what a benchmark of your optimized alternation (ref. the pointer you gave above) would give.

    -dlc

[reply]
[d/l]
[select]
      /:(00(?:1[56]|50)|1A0B)\s/

      [download]

      Is hand optimized. The automated approach would produce (5|6) instead of [56].

              - tye (but my friends call me "Tye")
[reply]
[d/l]
[select]
      Better, but not as good as yours.

      The last time we did this we found that the tried approach was significantly faster on data sets with matches to be found. See RE (tilly) 4: SAS log scanner for details.

      Live and learn.

[reply]

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