Okay, here is the max() given in Mastering Algorithms with Perl (O'Reilly):

sub max { # Numbers.
    my $max = shift;
    foreach ( @_ ) { $max = $_ if $_ > $max }
    return $max;
}
[download]

So, modifying that to fit into our existing benchmark/profiling code, it looks like:

foreach ( keys %{$url{'monday'}} ) { $now = $_ if $_ > $now }
[download]

And then, the score:

Benchmark: timing 10000 iterations of bench_grep, bench_if_then, bench_max...
bench_grep: 115 wallclock secs (111.04 usr +  0.01 sys = 111.05 CPU) @ 90.05/s (n=10000)
bench_if_then: 73 wallclock secs (70.02 usr +  0.01 sys = 70.03 CPU) @ 142.80/s (n=10000)
 bench_max: 64 wallclock secs (62.72 usr +  0.01 sys = 62.73 CPU) @ 159.41/s (n=10000)

Well, we have some surprises here! grep comes in last, even though it has the least operations. As you can see, not all operations are equal... but the book technique is the big winner. Which tells me it was worth the $35US.

The results also show that Devel::OpProf is a useful tool, but only as an accessory to Benchmark. On it's own, it can be misleading.

Paris Sinclair    |    4a75737420416e6f74686572
pariss@efn.org    |    205065726c204861636b6572
I wear my Geek Code on my finger.

Very interesting.

I started with the Mastering Perl Algorithms code (I left it in a subroutine), and abandoned it for the ternary when the ternary was faster. Of course, the subroutine entry was the main speed bottleneck.

This is fascinating. In the if/then/else results, we pay a considerably higher cost for the "constant item," "scalar variable" and "scalar assignment" operations. I wonder why that is so... Even the numeric comparison is more expensive in if/then/else than book max.

Also, your grep benchmarked slower than if/then/else. I saw the opposite. Would you post your complete code, to let me compare?

Thanks, you've made a fantastic post!

Russ

My complete code is a CGI that spits out the results inside the tables, so it's a bit ugly. Here I've commented out the CGI line, so it will print out the HTML without the header. You might want to redirect the output to a file, and view it in a browser.

I did wrap each version into a sub, as a max() should usually be used that way.

Also note that this code REQUIRES perl 5.6.0. If you have an old version, you will need to replace "our" with "use vars"

#!/usr/bin/perl
use strict;
#use CGI qw( header ); print header();
use Benchmark;
use Devel::OpProf qw(profile zero_stats stats );
profile(1);    # turn on profiling
our $now = 8; # we'll pretend it's between 8 and 9 PM
our %url = (
        monday => {
        @{[map(($_,1), (1..1000))]}
        }
        );
#start profiling    
$|++;
print "<TABLE>\n";

print "<TR><TD>\n";
my_profile( "grep",\&test_grep ); 
print "</TD><TD>\n";
my_profile( "if/then/else",\&test_if_then ); 
print "</TD><TD>\n";
my_profile( "book max()",\&test_max ); 
print "</TD></TR>\n";

# now lets benchmark
print "<TR><TD COLSPAN=3><PRE>";
timethese(10000, {
    bench_grep => q{
    test_grep();
    },
    bench_if_then => q{
    test_if_then();
    },
    bench_max => q{
    test_max();
    }} 
      );
print "</PRE></TD></TR></TABLE>\n";

sub test_grep {
    $now = (sort grep {$_ <= $now} keys %{$url{'monday'}})[-1];
}

sub test_if_then {
    $now = ($now < $_ && $_ < 8 ? $_ : $now) for keys %{$url{'monday'}
+};
}

sub test_max {
    foreach ( keys %{$url{'monday'}} ) { $now = $_ if $_ > $now }
}

sub my_profile {
    my $title = shift;
    my $test_sub = shift || return;
    zero_stats;
    &$test_sub;
    stats_td( $title );
#    print_stats;
}

sub stats_td {
    my $title = shift;
    my %stats = %{stats()};
    print "<TABLE><TR><TH COLSPAN=2>&nbsp;$title<BR><HR></TH></TR>\n";
    while ( my ( $stat, $value ) = each %stats ) {
    $stat =~ s/</\&lt\;/;
    $stat =~ s/>/\&gt\;/;
    if ( $value ) {
        $value =~ s/>/\&gt\;/;
        $value =~ s/</\&lt\;/;
        print "<TR><TH>$stat</TH><TD>$value</TD></TR>\n";
    }
    }
    print "</TABLE>\n";
}
[download]

I just whipped this up without much planning, so there are some problems with it. One is that the call to stat() should be in my_profile() not stats_td(). But, because of it's location in stats_td(), it is a O(1) error, and so it shouldn't skew the results much.

Paris Sinclair    |    4a75737420416e6f74686572
pariss@efn.org    |    205065726c204861636b6572
I wear my Geek Code on my finger.