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(Ovid) RE: From one beginner to others . . .

by Ovid (Cardinal)
on Jul 15, 2000 at 21:53 UTC ( #22715=note: print w/replies, xml ) Need Help??


in reply to From one beginner to others . . .

I know that gryng has stated that regex is faster than a split for long lines. However, his test case was $testlarge = "a " x 100000;. That's a two-hundred thousand character line. I'm highly inclined to doubt that most people are going to be working with lines of that length. In the real world (no offense to gryng intended), where we're not splitting one hundred thousand repetitions of "a ", we usually use much smaller chunks of data. For those, split is probably the best best.

You might want to consider using Benchmark to figure out what works best. I looked at your regex and realized that after you added the capturing parens and assigned the $digit variables, you were looking at a significant performance hit. You can use Benchmark to analyze these things a bit more carefully.

#!/usr/bin/perl -w use strict; use Benchmark; use vars qw($myvar @results $a $b $c $d); $myvar = "one,two,three,four"; timethese(1000000, { Regex => '($a=$1, $b=$2, $c=$3, $d=$4) if $myvar =~ /^([^,]+),([ +^,]+),([^,]+),([^,]+)$/', Split => '@results = split /,/, $myvar' });
This produced:
Benchmark: timing 1000000 iterations of Regex, Split... Regex: 27 wallclock secs (28.06 usr + 0.00 sys = 28.06 CPU) Split: 16 wallclock secs (16.19 usr + 0.00 sys = 16.19 CPU)
In this case, split was clearly the winner. I'd be interested in seeing some sample data and a code snippet to see how you're getting a regex to outperform a split. The structure of the data is everything when it comes to crafting an efficient regex.

Cheers,
Ovid

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RE: RE: From one beginner to others . . .
by greenhorn (Sexton) on Jul 16, 2000 at 01:54 UTC
    I managed to figure out at least one way to use Benchmark, will wonders never cease. :)
    (Don't know if I have used it in the best possible way, though.)
    Result of running the script shown below:
    Benchmark: timing 30 iterations of REGEXP, SPLIT... REGEXP: 10 wallclock secs ( 9.73 usr + 0.25 sys = 9.98 CPU) SPLIT: 47 wallclock secs (47.20 usr + 0.28 sys = 47.48 CPU)

    The script didn't pass muster with "-w" when I was trying to print matching lines to "NUL" (file handle hadn't been opened). I changed it simply to count matching lines. "-w" is now happy. (Note to self: something else for later study: how to print only to "NUL" w/out complaint from "-w".)

    The source (CSV) file is 13,576 lines long (1,703,397 bytes). Each record has 12 fields; the average length per record is 124 characters. The task is to print only lines whose fourth fields contain "MAPI".

    use strict; use Benchmark; timethese( 30, { REGEXP => 'UsingRegExp', SPLIT => 'UsingSplit' } ); sub UsingRegExp { my $file = 'r:\csv\test.csv'; my $field; my $count = 0; open FH, $file or die "\n $file: $!\n"; while ( <FH> ) { # WANT 4TH FIELD. (NOTE: SOME FIELDS _MIGHT_ BE EMPTY.) ($field) = /^[^,]+,[^,]*,[^,]*,\s*([^,]+)\s*,/; $count++ if lc($field) eq "mapi"; # IGNORE CASE } close FH or die "\n $file: $!\n"; } sub UsingSplit { my $file = 'r:\csv\test.csv'; my @record; my $count = 0; open FH, $file or die "\n $file: $!\n"; while ( <FH> ) { @record = split /\s*,\s*/; $count++ if lc($record[3]) eq "mapi"; # IGNORE CASE } close FH or die "\n $file: $!\n"; }
      I prefer this to whiles:

      <code> map{$r=[split (regex here)];$c+=(lc($r->[9]) eq "mapi")}<FH>; <code>

      I haven't tested this works but I hope you get my drift.

      Brother Frankus.

RE: RE: From one beginner to others . . .
by greenhorn (Sexton) on Jul 16, 2000 at 02:08 UTC
    Hmm. And just when I thought it was safe to go back into the water, I added one more thing into the mix. Both subroutines contain: while ( <FH> ) { .. and below each such line, I added: next if $_ !~ /mapi/io; ("MAPI" is the string being tested for.)

    It's the crudest possible test, of course. But why bother with further processing if the string doesn't appear anywhere in the current line?

    With that statement added, there was a considerable difference in the benchmarked results:

    Benchmark: timing 30 iterations of REGEXP, SPLIT... REGEXP: 14 wallclock secs (14.35 usr + 0.19 sys = 14.54 CPU) SPLIT: 17 wallclock secs (16.01 usr + 0.28 sys = 16.29 CPU)
    I should have read my own pep-talk about trying stuff...
RE: RE: From one beginner to others . . .
by greenhorn (Sexton) on Jul 15, 2000 at 22:22 UTC
    Looks as if those forced line-breaks at 80 chars in the original post weren't such a good idea after all. Sorry.:(

    The question about caching is a good one. I can't say for certain that caching didn't play a part in the performance boost (or in that case I guess I should say a perceived performance boost).

    I later re-created the business end of the routine on another computer and timed both approaches. The results were similar. Haven't yet benchmarked it. I'm still, argh, a bit hazy on exactly how to use Benchmark. But never mind "hazy"--I will get make my way through the haze and try it. (To date the two approaches have been timed using only the 4nt command processor's own timer function--far from exact, to be sure.)

    One noticeable difference between the regular expression I used and the one you used in your example here: I had only one set of parens in it. I don't know if this is likely to make a big difference in performance.

    Thanks for the feedback, folks.

      Only using one capturing paren will improve the performance of your regex as it will not be forced to do as much backreferencing. In playing around with this, I managed to optimize the split by breaking it into a minimal number of segments. In all cases, with my example, split significantly outperformed the regex.
      #!/usr/bin/perl -w use strict; use Benchmark; use vars qw($myvar $result $a $b $c $d); $myvar = "one,two,three,four"; timethese(1000000, { Regex => '$a=$1, $b=$2, $c=$3, $d=$4 if $myvar =~ /^[^,]+,([^,] ++),[^,]+,[^,]+$/', Split1 => '$result = (split /,/, $myvar)[1]', Split2 => '$result = (split /,/, $myvar, 4)[1]', Split3 => '$result = (split /,/, $myvar, 3)[1]' }); Benchmark: timing 1000000 iterations of Regex, Split1, Split2, Split3. +.. Regex: 26 wallclock secs (25.75 usr + 0.00 sys = 25.75 CPU) Split1: 16 wallclock secs (16.31 usr + 0.00 sys = 16.31 CPU) Split2: 16 wallclock secs (16.15 usr + 0.00 sys = 16.15 CPU) Split3: 13 wallclock secs (12.74 usr + 0.00 sys = 12.74 CPU)
      Note the whopping improvement in performance of Split3. In my benchmark, it's approximately twice as fast as the regex.

      Cheers,
      Ovid

        But your comparison isn't fair. You let the regex do way much work than needed. There's no need to parse the entire line, and only assign if there are exactly four fields - you aren't doing that for the split cases either. Also, you only have one set of parens, yet you do four assignments. Picking a simpler regex, and doing just one assignment improves the speed with 50%!
        $a=$1 if $myvar =~ /^[^,]+,([^,]+)/

        Still not as fast as the split, but it shows that proper Benchmarking is an art.

        -- Abigail

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