in reply to coderef assignment with and without eval

As of at least perl 5.6.0 you can use qr to precompile regexps... so something like... 
$re = qr/$regex/;

$_ =~ $re;

[download]
granted that is not what you asked... but it's much easier and more straightforward if you didn't already know about it.

Update actually since 5.005 it seems

Update2 I tried this... 

use Benchmark;

sub make_grep {
    my $p = $_[0];
    eval "sub { grep /$p/o, @_ }";
}

sub make_grep2 {
    my $p = $_[0];
    sub { grep /$p/o, @_ };
}
$re = '(ob|[tTs])';
$a = make_grep($re);
$b = make_grep2($re);
$pat = qr/$re/;

timethese(100000,
{
    a => sub { $a->(qw(this is a test bob and jobe)) },
    b => sub { $b->(qw(this is a test bob and jobe)) },
    c => sub { grep $pat, qw(this is a test bob and jobe) }
});

[download]
...and got this... 
Benchmark: timing 100000 iterations of a, b, c...
         a:  7 wallclock secs ( 5.92 usr +  0.00 sys =  5.92 CPU) @ 16
+891.89/s (
n=100000)
         b:  5 wallclock secs ( 4.57 usr +  0.00 sys =  4.57 CPU) @ 21
+881.84/s (
n=100000)
         c:  0 wallclock secs ( 0.62 usr +  0.00 sys =  0.62 CPU) @ 16
+1290.32/s
(n=100000)

[download]
putting the compiled re to be much faster... maybe I did something wrong...

                - Ant

Replies are listed 'Best First'.
(sacked) Re: Re: coderef assignment with and without eval
by sacked (Hermit) on Jul 24, 2001 at 23:05 UTC
    I believe the qr// case is much faster in your benchmark because it does not involve dereferencing a coderef and then invoking a subroutine. In any case, I like the idea of using qr// so I do not need to use string eval. I thought that since qr// precompiles regexes, the subroutine using it would be just as fast as the subroutine with eval. I found that isn't the case:

    #!/usr/bin/perl -w
#
# compiled regex with qr vs. eval and /o

use strict;
use Benchmark qw(timethese cmpthese);

use vars qw( @words $foo $bar );

@words = qw(heaven evan seven);

sub anon_sub_eval {
  my $pat = shift;
  eval 'sub { grep /$pat/o, @_ }';
}

sub anon_sub_qr {
  my $p   = shift;
  my $pat = qr/$p/;
  eval 'sub { grep /$pat/, @_ }';
}

$foo = anon_sub_eval( q/evan/);
$bar = anon_sub_qr  ( q/evan/);

my $results = timethese( -10,
 {
    anon_sub_eval => sub { $foo->(@words) },
    anon_sub_qr   => sub { $bar->(@words) },
 }
);

cmpthese($results);

__END__
Benchmark: running anon_sub_eval, anon_sub_qr, each for at least 10 CP
+U seconds...
anon_sub_eval: 13 wallclock secs (10.73 usr + -0.01 sys = 10.72 CPU) @
+ 156307.84/s (n=1675620)
anon_sub_qr: 12 wallclock secs (10.40 usr +  0.01 sys = 10.41 CPU) @ 1
+36413.54/s (n=1420065)
                  Rate   anon_sub_qr anon_sub_eval
anon_sub_qr   136414/s            --          -13%
anon_sub_eval 156308/s           15%            --

    [download]
    Does anyone know why the qr// version is slower?

    UPDATE: Thanks to a tip from suaveant, I changed the sub with qr// to not use slashes around the compiled regex: 
    sub anon_sub_qr {
  my $p   = shift;
  my $pat = qr/$p/;
  eval 'sub { grep $pat, @_ }';
}

__END__
Benchmark: running anon_sub_eval, anon_sub_qr, each for at least 10 CP
+U seconds...
anon_sub_eval: 11 wallclock secs (10.66 usr +  0.00 sys = 10.66 CPU) @
+ 158363.41/s (n=1688154)
anon_sub_qr: 11 wallclock secs (10.39 usr +  0.00 sys = 10.39 CPU) @ 2
+25783.45/s (n=2345890)
                  Rate anon_sub_eval   anon_sub_qr
anon_sub_eval 158363/s            --          -30%
anon_sub_qr   225783/s           43%            --

    [download]
    Lesson learned: do not use slashes around a regex precompiled with qr// in a grep expression. (But the difference is nominal in a regular match expression: $f =~ /$re/ vs. $f =~ $re)

    --sacked
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