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Re^5: multicolumn extraction

by GrandFather (Cardinal)
on Jun 03, 2012 at 22:33 UTC ( #974196=note: print w/ replies, xml ) Need Help??


in reply to Re^4: multicolumn extraction
in thread multicolumn extraction

Significantly faster is on the order of a few tens of seconds if run several times a day over a long period, or about half an hour if run just once. For almost all practical use cases the trivial difference you demonstrate is just that - trivial. A, maybe useful, little extra juice can be squeezed out by stopping the split early rather than just slicing the result to avoid copying a few extra list elements:

use strict; use warnings; use Benchmark qw(cmpthese); my $kFName = 'delme.txt'; test(); sub test { my $entry = 'a' x 18; open my $fOut, '>', $kFName or die "Can't create $kFName: $!\n"; print $fOut "$entry\t" x 19, "\n" for 1 .. 10000; close $fOut; cmpthese( -5, { splitAll => sub {splitAll()}, splitLimit => sub {splitLimit()}, splitSlice => sub {splitSlice()}, } ); } sub splitAll { open my $fIn, '<', $kFName or die "Can't open $kFName: $!\n"; while (<$fIn>) { my @columns = split /\t/; } close $fIn; } sub splitSlice { open my $fIn, '<', $kFName or die "Can't open $kFName: $!\n"; while (<$fIn>) { my @columns = (split /\t/)[1 .. 2]; } close $fIn; } sub splitLimit { open my $fIn, '<', $kFName or die "Can't open $kFName: $!\n"; while (<$fIn>) { my @columns = (split /\t/, $_, 4)[1 ..2]; } close $fIn; }

Prints:

Rate splitAll splitSlice splitLimit splitAll 5.60/s -- -36% -73% splitSlice 8.75/s 56% -- -59% splitLimit 21.1/s 276% 141% --

However, even the worst performing variant is still so fast that it simple not worth worrying about even if you were running it several thousand times a day every day of the year. And not of these solutions is actually useful for parsing CSV. To do that in a reasonably robust way you should really use something like Text:CSV, which is about ten times slower than any of the benchmarked solutions, but has the huge advantage that it may actually give correct results for anything other than the trivial test data used by this test.

True laziness is hard work


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Re^6: multicolumn extraction
by Kenosis (Priest) on Jun 03, 2012 at 23:00 UTC

    Your example and explanation are well done, suggesting that the difference between the two splits I timed--even if 'statistically significant'--really makes little or no practical difference.

    I see, now, that these timing routines can be a red herring that detracts from achieving both correct results and readability.

    Thanks, GrandFather.

Re^6: multicolumn extraction
by sauoq (Abbot) on Jun 04, 2012 at 20:08 UTC
    And not of these solutions is actually useful for parsing CSV.

    True enough! But then, he didn't say it was CSV. He said it was "tab delimited".

    CSV is a rather more robust format that permits fields in which you can escape your separator, have embedded newlines, etc. "Tab delimited" generally means a file where records are delimited by newlines, fields are delimited by tabs, and fields may not contain newlines or tabs. It's common. And splitting on tabs works very nicely for it without requiring additional dependencies and overhead.

    -sauoq
    "My two cents aren't worth a dime.";

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