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Re^2: pop sort strangeness

by Brovnik (Hermit)
on Nov 15, 2004 at 16:18 UTC ( #407861=note: print w/ replies, xml ) Need Help??


in reply to Re: pop sort strangeness
in thread pop sort strangeness

my $max = pop @{[ sort @vals ]};
is the answer I was looking for, thanks. I realise it is inefficient, just came across the error and didn't immediately twig the list/array issue.


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Re^3: pop sort strangeness
by davido (Archbishop) on Nov 15, 2004 at 16:27 UTC

    Indeed, it is very inefficient, by comparison to simply doing a linear search. First, you're sorting the list. That's O(N log N) for Perl's default sort type. Next, you're taking a copy of the list (yes, you're copying it) for the purpose of creating the anonymous array. That's about O(N). Next, you're dereferencing the array (a pretty quick action), and then you're popping something off the array (which is also pretty quick, but still is another step).

    So what you've got is O( N + N log N ), when you could just have O(N). That's not so good. And as someone else already pointed out, sort @vals does a string sort, not a numeric sort, so 11 will be sorted next to 1 instead of next to 12. If you must use the sort routine, at least change it to doing a numeric sort:

    my $max = pop @{[ sort { $a <=> $b } @vals ]};

    And here's a linear search for max.

    my $max = $vals[0]; $max = ( $_ > $max ) ? $_ : $max foreach @vals;

    This version is a little terse, on purpose, to demonstrate that it's possible to do the linear search in just slightly more keystrokes than the sort pop method.


    Dave

      Just for reference, here's some timing results on my machine for the following code snippet...
      #!/usr/bin/perl -w #Initialization @vals = reverse "a".."daaaa"; #O(NlogN) my $max = pop @{[ sort @vals ]}; #O(N) #my $max = $vals[0]; #$max = ( $_ gt $max ) ? $_ : $max foreach @vals; $e=@vals; print "elements=$e max=$max\n";
      Commenting out everything but the initialization section (i.e. not finding any maximum at all), this snippet executes in about 4.4 seconds on my machine. Using the sort to find the max, it takes about 6.2 seconds. Finally, using the linear method, it executes in about 5.1 seconds. So, for the 1,846,183 elements in this example, the sort method is about 2.5 times slower (6.2-4.4)/(5.1-4.4) than the linear search method. But the test is still dominated by just generating the large array. So you might not notice the efficiency gain if your data set is smaller than 1.8 million elements.


      -- All code is 100% tested and functional unless otherwise noted.
        Here are my results, first with your "a" .. "daaa", and then with a much smaller dataset ("a".."daa", only 2731 values), and finally with just "a" .. "aa" (27 values).
        #!/usr/bin/perl use strict; use warnings; use List::Util qw/ maxstr /; use Benchmark qw/ cmpthese /; my @vals = reverse "a" .. "daa"; cmpthese( -10, { sorting => sub {my $max = pop @{[sort @vals]}}, iterating => sub { my $max = $vals[0]; $max = ( $_ gt $max ) ? $_ : $max foreach @vals; }, list_util => sub {my $max = maxstr @vals} }); __END__ Rate sorting iterating list_utils sorting 0.659/s -- -42% -82% iterating 1.13/s 72% -- -69% list_util 3.67/s 458% 224% -- Rate sorting iterating list_utils sorting 537/s -- -32% -80% iterating 789/s 47% -- -70% list_util 2661/s 395% 237% -- Rate sorting iterating list_utils sorting 50180/s -- -25% -78% iterating 66991/s 34% -- -70% list_utils 225804/s 350% 237% --
        So in all cases, using the built in List::Util proceedure was quite a bit more than twice as fast as either of the others. And it's also much simpler to use, so it seems like something of a no-brainer to me.

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