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Re^2: Extracting Unique Characters from a Array

by Fletch (Chancellor)
on Sep 07, 2006 at 17:24 UTC ( #571753=note: print w/replies, xml ) Need Help??

in reply to Re: Extracting Unique Characters from a Array
in thread Extracting Unique Characters from a Array

Yeah, let's turn a single O(n) scan with a hash (the right way to do it) into an O(n log n) sort followed by another O(n) scan. Aside from the added inefficiency you've lost the original ordering if that needed to be preserved.

Update: Ah, a followup did note that the list was to be sorted. Still makes more sense to do the O(n) cull of duplicates first and then the O(n log n) sort of the smaller list.

When in doubt:

#!/usr/bin/perl use Benchmark qw( timethese cmpthese ); use constant SIZE => 6_000; use constant COUNT => 500; my $count = shift || COUNT; my $size = shift || SIZE; my @source = map { int( rand($size) ) } 1 .. $size; cmpthese( $count, { sort_first => sub { my @sorted = sort @source; my $le = undef; my @uniq; for (@sorted) { if ( $le != $_ ) { $le = $_; push @uniq, $_; } } }, cull_first => sub { my %seen; my @uniq = grep { !$seen{$_}++ } @source; my @sorted = sort @uniq; } } ); exit 0; __END__

The cull_first version is from 8-20% faster for lists of from 5 to several thousand items.

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