in reply to Re^2: eval string possibilities
in thread eval string possibilities

I learned about Benchmark only a short while ago. It will probably be a little tricky to factor out things that are not related to the to-eval-or-not-eval question.

In cases like this, I have learned that it's usually better to post bad code than no code at all. A small example written with Benchmark would have given repliers something to work with. They likely would have pointed out any flaws in the benchmark, and posted a modified version.

There is a danger in this, though. Sometimes the code has small unrelated problems that derail the conversation, but I think this is a small price to pay. In my experience, posts containing pertinent code almost always get better replies.

Sorry for the off-topic reply, but I think it will be useful for future posts. Hopefully you can avoid making some of the mistakes that I have made here at the Monastery. :-)

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Re^4: eval string possibilities
by erix (Prior) on Nov 22, 2004 at 18:48 UTC

    Thanks for that advice, and I'll follow it up immediately with the script below, which I made after the OP and which is probably indeed bad code, although I think it does give the time discrepancies that I am looking for (qr much faster than eval). I suppose the main thing I missed all along (I had been told things in CB before), is the slowness of eval. Well, now I know :) 

    #!/usr/bin/perl -w

use strict;
use warnings;

use Benchmark qw /cmpthese/;

cmpthese 10_000 => {
  'do_eval' =>  sub { do_eval() },
  'do_qr'   =>  sub { do_qr() },
};

sub do_eval {
  my $genome  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ('abc', '^qwerty'); # dont care
  my $count   = 0;
  my $code    = 'if ($genome =~ /' . join ('/ && /', @regexes) . '/) {
+ $count++; }';
  eval $code;
  die "Error: $@\n Code:\n$code\n" if ($@);
}

sub do_qr {
  my $string  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ("abc", "^qwerty");  # dont care
  my $count   = 0;
  my @compiled  = map qr/$_/, @regexes;
  for(my $i=0; $i<@regexes; $i++) {
      if($string =~ /$compiled[$i]/){
          $count++;
      }
  }
}

__END__

          Rate do_eval   do_qr
do_eval 2162/s      --    -77%
do_qr   9242/s    328%      --

    [download]
[reply]
[d/l]

      An important part of making benchmark code is to ensure the alternatives do the same thing. Your do_eval subroutine had a minor problem (you only had =~ $genome on the first match, instead of all of them), which I corrected. But then when I looked at the output from the two subroutines, I noticed it was different:

      sub do_eval {
  my $genome  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ('AGT', 'ATC'); # dont care
  my $count   = 0;
  my $code    = 'if ('
              . join(' && ',
                     map { "\$genome =~ /$_/" } @regexes)
              . ') { $count++; }';
  eval $code;
  die "Error: $@\n Code:\n$code\n" if ($@);
  return $count; # returns 1
}

sub do_qr {
  my $string  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ("AGT", "ATC");  # dont care
  my $count   = 0;
  my @compiled  = map qr/$_/, @regexes;
  for(my $i=0; $i<@regexes; $i++) {
      if($string =~ /$compiled[$i]/){
          $count++;
      }
  }
  return $count; # returns 2
}

      [download]

      To fix that, I changed your do_eval sub to the following:

      sub do_eval {
  my $genome  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ('AGT', 'ATC'); # dont care
  my $count   = 0;
  my $code    = join ";",
                map { "\$count++ if \$genome =~ /$_/" } @regexes;
  eval $code;
  die "Error: $@\n Code:\n$code\n" if ($@);
  return $count; # returns 2
}

      [download]

      And I decided to add my own take on the matter, which generates One Big Regex, rather than a bunch of them:

      sub do_genre {
  my $genome  = "AGTATCGATCGATGCATGCTAGCTAGCTAGCTAGCTAGCTAGSTGCTAGCT";
  my @regexes = ("AGT", "ATC");  # dont care
  my $regex   = join "|", map "($_)", @regexes;
  my $count   = () = $genome =~ /$regex/;
  return $count; # returns 2
}

      [download]

      When I run the benchmark, I get the following results:

                   Rate  do_eval    do_qr do_genre
do_eval   15531/s       --     -65%     -90%
do_qr     44671/s     188%       --     -72%
do_genre 157893/s     917%     253%       --

      [download]

      Which just goes to show that the string eval is slow, but the looping is even slower. A different algorithm makes a big difference.

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