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Re^2: Request to correct the perl code for getting substrings

by jwkrahn (Monsignor)
on Apr 19, 2012 at 22:17 UTC ( #966046=note: print w/ replies, xml ) Need Help??


in reply to Re: Request to correct the perl code for getting substrings
in thread Request to correct the perl code for getting substrings

Don't use $&, it will slow down every regular expression in the program.

my $string = "CCATGNNNTAACCNNATGNNTAGCC"; while ( $string =~ /([AG]TG.*?T[AG][AG])/ig ) { print "matched at $-[0]: $1\n"; }


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Re^3: Request to correct the perl code for getting substrings
by oldtomas (Novice) on Apr 20, 2012 at 06:57 UTC
    Don't use $&, it will slow down every regular expression in the program
    You're quite right, Sir ;-)
    #!/usr/bin/perl -w use Benchmark; my $string = "CCATGNNNTAACCNNATGNNTAGCC" x 10000; timethese(10, { 'matchamp' => sub {matchamp($string)}, 'matchpar' => sub {matchpar($string)}, }); sub matchamp { my $string = shift; my $matchlen = 0; while($string =~ /[AG]TG.*?[AG][AG]/g) { $matchlen += length($&); # Actually reference $& to keep Perl from + cheating } return $matchlen; } sub matchpar { my $string = shift; my $matchlen = 0; while ( $string =~ /([AG]TG.*?T[AG][AG])/g ) { $matchlen += length($1); # Reference $1, for fairness } return $matchlen; }
    yields:
    Benchmark: timing 10 iterations of matchamp, matchpar... matchamp: 24 wallclock secs (23.48 usr + 0.00 sys = 23.48 CPU) @ 0 +.43/s (n=10) matchpar: 26 wallclock secs (25.70 usr + 0.00 sys = 25.70 CPU) @ 0 +.39/s (n=10)

    The "$&" leg seems a tad faster, right?

    BUT...

    removing every trace of matchamp in the above program yields:

    Benchmark: timing 10 iterations of matchpar... matchpar: 1 wallclock secs ( 0.54 usr + 0.00 sys = 0.54 CPU) @ 18 +.52/s (n=10)

    That's a speedup by a factor ~26. I'm impressed.

    Thanks for pointing that out

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