in reply to slow CGI's

I don't know to what extent Perl is optimised, but I suppose accessing a member of an array is somewhat slower than accessing a private variable. And especially if the indexing takes place inside a loop. So how about this:

if ( defined($array[0]) ) {
    my $array0 = $array[0];
    foreach ( @numbers ) {
        next unless $array0 == $_;
        print "FOUND<p>";
    }
}

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--bwana147

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Re: Re: slow CGI's
by chromatic (Archbishop) on Jul 07, 2003 at 18:14 UTC

    I downvoted this node because I think it is an inappropriate suggestion. Rule #1 of benchmarking is don't guess. Rule #2 of benchmarking is optimize what really matters.

    The biggest potential problem I can see with this code (without seeing the data set) is that the loop tends to O(n/2) performance. If there are a lot of numbers in the array, it will take a while to find the right number. A hash lookup, tending to O(1) performance, would be much more performant.

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      Interestingly, benchmarks do not bear this assumption out. (perl 5.8.0 on a celeron lots of other stuff going on at the same time, but tried multiple times, very similar results). 
      #!/usr/local/bin/perl
use Benchmark;


for (1.. 100000) {
  $rand = rand * 100;
  push @rvs, $rand;
  $last = $rand;
}
$array[0] = $rand;

timethese ( 100, {grepit => \&grepit,
           hashit => \&hashit});

sub grepit {
  return 1 if (grep /$array[0]/, @rvs);

}

sub hashit {
  @hash{@rvs} = (1) x @rvs;
  return 1 if ($hash{$array[0]});
}

      [download]
      Benchmark: timing 100 iterations of grepit, hashit...
    grepit: 19 wallclock secs 
       (19.36 usr +  0.03 sys = 19.39 CPU) @  5.16/s (n=100)
    hashit: 28 wallclock secs 
       (27.84 usr +  0.04 sys = 27.88 CPU) @  3.59/s (n=100)
the number is 0.359661571026372
Grepit found the number
Hashit found the number

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