FAQ 4.41 How can I remove duplicate elements from a list or array?


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4.41: How can I remove duplicate elements from a list or array?

(contributed by brian d foy)

Use a hash. When you think the words "unique" or "duplicated", think
"hash keys".

If you don't care about the order of the elements, you could just create
the hash then extract the keys. It's not important how you create that
hash: just that you use "keys" to get the unique elements.

my %hash = map { $_, 1 } @array;
# or a hash slice: @hash{ @array } = ();
# or a foreach: $hash{$_} = 1 foreach ( @array );

my @unique = keys %hash;

If you want to use a module, try the "uniq" function from
"List::MoreUtils". In list context it returns the unique elements,
preserving their order in the list. In scalar context, it returns the
number of unique elements.

use List::MoreUtils qw(uniq);

my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7

You can also go through each element and skip the ones you've seen
before. Use a hash to keep track. The first time the loop sees an
element, that element has no key in %Seen. The "next" statement creates
the key and immediately uses its value, which is "undef", so the loop
continues to the "push" and increments the value for that key. The next
time the loop sees that same element, its key exists in the hash *and*
the value for that key is true (since it's not 0 or "undef"), so the
next skips that iteration and the loop goes to the next element.

my @unique = ();
my %seen = ();

foreach my $elem ( @array )
next if $seen{ $elem }++;
push @unique, $elem;

You can write this more briefly using a grep, which does the same thing.

my %seen = ();
my @unique = grep { ! $seen{ $_ }++ } @array;


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