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Re^2: sort an array with +ve & -ve numbers in it

by eric256 (Parson)
on Apr 03, 2009 at 13:16 UTC ( #755264=note: print w/ replies, xml ) Need Help??


in reply to Re: sort an array with +ve & -ve numbers in it
in thread sort an array with +ve & -ve numbers in it

Ummm...isn't a sort a bit easier and simpler?

my @array = ( 99, 67, 0, -100, -38, 98); my ($min, $max) = (sort @array)[0,-1];

___________
Eric Hodges


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Re^3: sort an array with +ve & -ve numbers in it
by philipbailey (Chaplain) on Apr 03, 2009 at 14:38 UTC
    Easier and simpler, but less efficient (O n log n, perhaps) than jwkrahn's approach (O n). Of course this won't matter for small data sets.
      Easier and simpler, but less efficient

      Woah! I would benchmark that before making such a statement. I expect you'll need a seriously huge list before the O(n log n) starts to lose to your straight O(n).

      There's no debate that it will, but the problem is the 0(n) algorithm is using a comparatively large number of slow ops, where as the sort compiles down into a single op, and there you're running at C speed (simple {$a <=> $b} blocks are recognised and special-cased during the parse). This will drown out the extra cost for a long, long time (that is: for a long list of values). On my machine, the cross-over occurs between 100 000 and 1 000 000 elements (and I had to run the million element benchmark for 15 seconds in order to give it enough time to settle down)

      Rate with_scan_1 with_sort_1 with_scan_1 35662/s -- -53% with_sort_1 76332/s 114% -- Rate with_scan_2 with_sort_2 with_scan_2 6838/s -- -40% with_sort_2 11437/s 67% -- Rate with_scan_3 with_sort_3 with_scan_3 759/s -- -11% with_sort_3 853/s 12% -- Rate with_sort_4 with_scan_4 with_sort_4 62.6/s -- -18% with_scan_4 76.2/s 22% -- Rate with_sort_5 with_scan_5 with_sort_5 3.38/s -- -47% with_scan_5 6.38/s 89% -- s/iter with_sort_6 with_scan_6 with_sort_6 4.88 -- -66% with_scan_6 1.65 196% --

      And since either choice is crazy fast enough for me, I'd throw my lot in with the more succinct version -- less chance of introducing semantic mistakes and off-by-one errors). For instance, I had to think for a little while about how you initialised $min and $max...

      • another intruder with the mooring in the heart of the Perl

Re^3: sort an array with +ve & -ve numbers in it
by lostjimmy (Chaplain) on Apr 03, 2009 at 15:26 UTC

    As already stated by ikegami, you really need to use the numerical sort: sort {$a <=> $b} @array

    Try changing your data set to my @array = ( -10, 99, 67, 0, -100, -38, 98) and using that sort routine. This is probably the problem the OP is having. You're going to get -10 as the minimum.

      Very true, corrected code:

      my @array = ( 99, 67, 0, -100, -38, 98); my ($min, $max) = (sort {$a <=> $b} @array)[0,-1];

      Update: Cut n paste error fixed, thanks plobsing and ikegami


      ___________
      Eric Hodges

        Typo alert: you have 2 sorts. The result is that you sort alphabetically after having sorted numerically.

        Unfortunately, your test dataset doesn't show the problem. Try adding 100 to the data.

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