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Re: Fastest way to sort a list of integers into 0,1,2,3,-3,-2,-1

by swl (Vicar)
on Feb 06, 2019 at 02:42 UTC ( #1229446=note: print w/replies, xml ) Need Help??


in reply to Fastest way to sort a list of integers into 0,1,2,3,-3,-2,-1

edit - this is essentially the same as Tybalt's entry in 1229444, which I missed when posting.

Here's one that is adapted from Discipulus and haukex3. Using a binary search will speed up the search for the inflection point, while pushing and splicing in one pass avoids repeated pushes. It should scale better as the data increases in size.

The binary search needs a recent version of List::MoreUtils. I've specified the latest here, but one could go somewhat older I think. One could also use List::BinarySearch::XS

swl => sub { my @list = @input; use List::MoreUtils 0.428; @list = sort {$a<=>$b} @list; my $i = List::MoreUtils::bsearchidx {$_ <=> 0} @list; push @list, splice @list, 0, $i-1; Compare(\@list,\@output) or die "@list" if DO_CHECK; }

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Re^2: Fastest way to sort a list of integers into 0,1,2,3,-3,-2,-1
by tybalt89 (Prior) on Feb 06, 2019 at 06:31 UTC

    Would this work if

    1) There is no 0 in the list ?

    2) There are several 0, Would it pick the correct (earliest) one ?

      Thanks for checking.

      It fails if there were three or more zeroes (the test case provided has two zeroes), and if there are no zeroes.

      Adding some extra checks seems to fix the issues, and is still close to try1a for speed given the provided test data (sometimes as fast but usually a few percent slower).

      swl2 => sub { my @list = @input; use List::MoreUtils 0.428; @list = sort {$a<=>$b} @list; my $i = List::MoreUtils::bsearchidx {$_ <=> 0} @list; if ($i < 0) { # no zero $i = List::MoreUtils::firstidx {$_ >= 0} @list; } else { $i-- while !$list[$i]; $i++; } push @list, splice @list, 0, $i; Compare(\@list,\@output) or die "@list" if DO_CHECK; },

      It will be slower in cases where there are no zeroes, although List::BinarySearch::binsearch_pos could be used to find an insert point for zero in such cases.

        Another update to handle lists where all values are >= 0 (as flagged in 1229492). Also using the while loop instead of List::MoreUtils::firstidx based on the benchmarks in that post.

        EDIT - added check for upper bound being >=0, borrowing from code in 1229437.

        swl3 => sub { my @list = @input; use List::MoreUtils 0.428; @list = sort {$a<=>$b} @list; if ($list[0] < 0 && $list[-1] >= 0) { my $i = List::MoreUtils::bsearchidx {$_ <=> 0} @list; if ($i < 0) { # no zero, find first positive $i = 0; $i++ while ($list[$i]<0); } else { # find start of zeroes $i-- while !$list[$i]; $i++; } push @list, splice @list, 0, $i; } Compare(\@list,\@output) or die "@list" if DO_CHECK; },
Re^2: Fastest way to sort a list of integers into 0,1,2,3,-3,-2,-1
by swl (Vicar) on Feb 06, 2019 at 02:54 UTC

    And if non-core modules aren't permitted, then this adaptation of haukex3 can be used:

    swl2 => sub { my @list = @input; @list = sort {$a<=>$b} @list; my $i = 0; $i++ while ($list[$i]<0); push @list, splice @list, 0, $i; Compare(\@list,\@output) or die "@list" if DO_CHECK; },

      Updated for lists that are all negative, following Discipulus4 in 1229492.

      swl_pp2 => sub { my @list = @input; @list = sort {$a<=>$b} @list; if ($list[0] < 0 && $list[-1] >= 0) { my $i = 0; $i++ while ($list[$i]<0); push @list, splice @list, 0, $i; } Compare(\@list,\@output) or die "@list" if DO_CHECK; },

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