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### Re^4: sort an array with +ve & -ve numbers in it

by grinder (Bishop)
 on Apr 03, 2009 at 21:52 UTC ( #755350=note: print w/replies, xml ) Need Help??

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...

#! /usr/local/bin/perl use strict; use warnings; use Benchmark 'cmpthese'; my @s1 = (999, -999, map {rand(900)-900} 0..10); my @s2 = (999, -999, map {rand(900)-900} 0..100); my @s3 = (999, -999, map {rand(900)-900} 0..1000); my @s4 = (999, -999, map {rand(900)-900} 0..10000); my @s5 = (999, -999, map {rand(900)-900} 0..100000); my @s6 = (999, -999, map {rand(900)-900} 0..1000000); sub with_scan { my ( \$min, \$max ) = @_; for my \$element ( @_ ) { \$min = \$element if \$min > \$element; \$max = \$element if \$max < \$element; } return (\$min, \$max); } sub with_sort { return (sort {\$a <=> \$b} @_)[0,-1]; } print join(',', with_scan(@s1)), \$/; print join(',', with_sort(@s1)), \$/; cmpthese( -3, { 'with_sort_1' => sub {with_sort(@s1)}, 'with_scan_1' => sub {with_scan(@s1)}, } ); cmpthese( -3, { 'with_sort_2' => sub {with_sort(@s2)}, 'with_scan_2' => sub {with_scan(@s2)}, } ); cmpthese( -3, { 'with_sort_3' => sub {with_sort(@s3)}, 'with_scan_3' => sub {with_scan(@s3)}, } ); cmpthese( -3, { 'with_sort_4' => sub {with_sort(@s4)}, 'with_scan_4' => sub {with_scan(@s4)}, } ); cmpthese( -3, { 'with_sort_5' => sub {with_sort(@s5)}, 'with_scan_5' => sub {with_scan(@s5)}, } ); cmpthese( -15, { 'with_sort_6' => sub {with_sort(@s6)}, 'with_scan_6' => sub {with_scan(@s6)}, } );

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