in reply to Re: Simple Matching
in thread Simple Matching

Thank you both. I was looking at Friedl's book on RegEx and wrapping myself up in eval's when both of you replied. Thank you. I am working on optimizing a bit of code and have not needed eval yet. Is there a workaround for this specific example, e.g., "grep -c"? Counting characters in a string should be fast and simple.

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Re^3: Simple Matching
by ikegami (Patriarch) on Sep 28, 2009 at 05:14 UTC

    It's only getting compiled once, just as if it was outside of eval, so what's the problem?

    If you wanted to reuse the counter, make a sub if it: 

    $counters{$_} = eval "sub { \$_[0] =~ tr/\Q$_\E// }";

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      Thanks again, Putting the eval into Method 2 below is (much) slower than any other match (as noted by BrowserUK). Brute force (Method 3) is significantly faster than either 1 or 2, but does not let me loop through an array to check match counts. I have tried "grep -c" within perl and it works, but is not as fast as brute force. Is there a faster way? After 15+ years of Perl I have not needed to worry about cutting edge optimizing (in Perl) ... ergo my ignorance. Thanks for help again. 
      my $flush = 0;
my @suits = ( "D", "S", "C", "H" );
my $long_hand = ("2H 3D 4C 5S 5H)";

# Method 1:  43 microseconds
#    (My initial code ... seemed slow ... ergo benchmark)
foreach ( @suits ) {
    @m = ( $long_hand =~ m/$_/g );
    $flush = 1 if  ($#m eq 4 );
}

# Method 2: 126 microseconds (definitely bottleneck)
foreach ( @suits ) {
    my $count = ( eval "\$long_hand =~ tr/$_//" );
    $flush = 1 if  ($count eq 5 );
}

#Method 3: accounts for only 3(!) microseconds of code
# Ugly but functional for very few patterns to check
# BUT what if I had 10k patterns to check
$flush = 1 if ( $long_hand =~ tr/D// eq 5);
$flush = 1 if ( $long_hand =~ tr/S// eq 5);
$flush = 1 if ( $long_hand =~ tr/C// eq 5);
$flush = 1 if ( $long_hand =~ tr/H// eq 5);

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[d/l]
        How does this compare speed-wise: 
        >perl -wMstrict -le
"my @suits = qw(D S C H);
 my $suits = join '', @suits;
 my $flush = qr{ ([$suits]) (?: [^$suits]+ \1){4} }xms;
 print $_, $_ =~ $flush ? ' flush' : ' nope'
   for '2H 3D 4C 5S 5H', '1H 2H 3H 4H 5H';
"
2H 3D 4C 5S 5H nope
1H 2H 3H 4H 5H flush

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        Update: Might be faster if possessive: 
        >perl -wMstrict -le
"my @suits = qw(D S C H);
 my $suits = join '', @suits;
 my $flush = qr{ ([$suits]) (?: (?> [^$suits]+) \1){4} }xms;
 print $_, $_ =~ $flush ? ' flush' : ' nope'
   for '2H 3D 4C 5S 5H', '1H 2H 3H 4H 5H';
"
2H 3D 4C 5S 5H nope
1H 2H 3H 4H 5H flush

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        5.10 has possessive modifier: 
        >perl -wMstrict -le
"my @suits = qw(D S C H);
 my $suits = join '', @suits;
 my $flush = qr{ ([$suits]) (?: [^$suits]++ \1){4} }xms;
 print $_, $_ =~ $flush ? ' flush' : ' nope'
   for '2H 3D 4C 5S 5H', '1H 2H 3H 4H 5H';
"
2H 3D 4C 5S 5H nope
1H 2H 3H 4H 5H flush

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        Bugfix: Changed  (?> [^$suits])+ to  (?> [^$suits]+) per ikegami Re^6: Simple Matching.
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        It is a simple fact of life that if you can hard code a lookup table, it will be faster than building it on-the-fly, or choosing one conditionally.

        If you need best possible speed--and that is a big if--then the choice to store 10,000 lookup tables is a speed -v- memory trade-off that you might choose to make.

        If youreally need absolute best speed, then C or assembler are your only choices. But the surprising thing about Perl--the redeeming feature for all it faults--is that it rarely make you make that choice. Beyond the realms of either highly algorithmically complex; or no-alternative, cpu intensive--Perl's quirks, irregularities and non-orthogonality's are tailored--by experience and practice, over orthodoxy and dogma--to produce best possible results in best possible time.....

        with the proviso--and it is a big one--that you are not afraid to consider perfectionism, passe; correctness. contrived; elitism, irrelevant; and practicality paramount. Beyond the rarefied atmospheres of academic and dogmatic perfection, code that works for the common case. today; is far preferable to either perfection tomorrow.

        Strive for perfection; but don't exclude a solution today, for perfection tomorrow. Tomorrow rarely (I'm pragmatic; "never" is not a part of my vocabulary :) ever comes.

        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
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