RE: Return a Unique Array

Using hashes can take up a lot of memory, especially with long lists. Here's one that acts like sort -u :

my @list = sort(@list);
my @unique;
my $i=0;

$unique[0] = $list[0];

foreach my $item(@list) {
    unless($item eq $unique[$i]) {
        push(@unique,$item);
        $i++;
    }
}
[download]

@unique is now a sorted unique list.

Edit kudra, 2001-09-10 Added code tags

Comment on RE: Return a Unique Array Download Code

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RE: RE: Return a Unique Array by btrott (Parson) on Mar 23, 2000 at 22:20 UTC
That's nice. Although it is slower than using a hash. Comparing your code against this routine: `sub sort_unique_hash { my %hash; @hash{@_} = (); return sort keys %hash; }` [download] gives (where "foreach" is your code and "hash" is the above): `Benchmark: timing 100000 iterations of foreach, hash... foreach: 14 secs (13.50 usr 0.00 sys = 13.50 cpu) hash: 6 secs ( 5.85 usr 0.00 sys = 5.85 cpu)` [download] This is with: `@list = qw/foo bar baz foo quack bar baz/;` [download] The difference is even more marked if you have rather odd arrays that you want to sort -u. For example, running the code against this array: `@list = ('foo') x 1000;` [download] gives `Benchmark: timing 1000 iterations of foreach, hash... foreach: 12 secs (11.82 usr 0.00 sys = 11.82 cpu) hash: 1 secs ( 0.88 usr 0.00 sys = 0.88 cpu)` [download] Presumably because your code sorts the list before it selects only the unique elements, and the hash approach only sorts the unique list--so you're sorting 1000 elements, and the hash approach just sorts 1. Just something to think about.	[reply] [d/l] [select]
Re^2: Return a Unique Array by Anonymous Monk on Apr 10, 2014 at 16:16 UTC
Should be: my $i=1 otherwise you overwrite the first element and are then missing the first element.	[reply]
Re^3: Return a Unique Array by Anonymous Monk on Apr 11, 2014 at 06:49 UTC
Oops, no it shouldn't, my mistake. $i=0 is correct.	[reply]