How do I remove duplicate items from an array in Perl?

159

I have an array in Perl:

my @my_array = ("one","two","three","two","three");

How do I remove the duplicates from the array?

This question is tagged with perl arrays unique duplicates

~ Asked on 2008-08-11 10:04:32

11 Answers


169

You can do something like this as demonstrated in perlfaq4:

sub uniq {
    my %seen;
    grep !$seen{$_}++, @_;
}

my @array = qw(one two three two three);
my @filtered = uniq(@array);

print "@filtered\n";

Outputs:

one two three

If you want to use a module, try the uniq function from List::MoreUtils

~ Answered on 2008-08-11 10:16:22


123

The Perl documentation comes with a nice collection of FAQs. Your question is frequently asked:

% perldoc -q duplicate

The answer, copy and pasted from the output of the command above, appears below:

Found in /usr/local/lib/perl5/5.10.0/pods/perlfaq4.pod
 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;

~ Answered on 2008-08-11 14:27:46


70

Install List::MoreUtils from CPAN

Then in your code:

use strict;
use warnings;
use List::MoreUtils qw(uniq);

my @dup_list = qw(1 1 1 2 3 4 4);

my @uniq_list = uniq(@dup_list);

~ Answered on 2008-08-31 10:01:18


24

My usual way of doing this is:

my %unique = ();
foreach my $item (@myarray)
{
    $unique{$item} ++;
}
my @myuniquearray = keys %unique;

If you use a hash and add the items to the hash. You also have the bonus of knowing how many times each item appears in the list.

~ Answered on 2008-08-11 10:18:45


9

Can be done with a simple Perl one liner.

my @in=qw(1 3 4  6 2 4  3 2 6  3 2 3 4 4 3 2 5 5 32 3); #Sample data 
my @out=keys %{{ map{$_=>1}@in}}; # Perform PFM
print join ' ', sort{$a<=>$b} @out;# Print data back out sorted and in order.

The PFM block does this:

Data in @in is fed into MAP. MAP builds an anonymous hash. Keys are extracted from the hash and feed into @out

~ Answered on 2011-11-09 21:23:25


8

The variable @array is the list with duplicate elements

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

~ Answered on 2010-10-23 16:18:36


8

Method 1: Use a hash

Logic: A hash can have only unique keys, so iterate over array, assign any value to each element of array, keeping element as key of that hash. Return keys of the hash, its your unique array.

my @unique = keys {map {$_ => 1} @array};

Method 2: Extension of method 1 for reusability

Better to make a subroutine if we are supposed to use this functionality multiple times in our code.

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

Method 3: Use module List::MoreUtils

use List::MoreUtils qw(uniq);
my @unique = uniq(@array);

~ Answered on 2017-05-09 15:29:44


4

That last one was pretty good. I'd just tweak it a bit:

my @arr;
my @uniqarr;

foreach my $var ( @arr ){
  if ( ! grep( /$var/, @uniqarr ) ){
     push( @uniqarr, $var );
  }
}

I think this is probably the most readable way to do it.

~ Answered on 2009-01-23 23:35:59


1

Previous answers pretty much summarize the possible ways of accomplishing this task.

However, I suggest a modification for those who don't care about counting the duplicates, but do care about order.

my @record = qw( yeah I mean uh right right uh yeah so well right I maybe );
my %record;
print grep !$record{$_} && ++$record{$_}, @record;

Note that the previously suggested grep !$seen{$_}++ ... increments $seen{$_} before negating, so the increment occurs regardless of whether it has already been %seen or not. The above, however, short-circuits when $record{$_} is true, leaving what's been heard once 'off the %record'.

You could also go for this ridiculousness, which takes advantage of autovivification and existence of hash keys:

...
grep !(exists $record{$_} || undef $record{$_}), @record;

That, however, might lead to some confusion.

And if you care about neither order or duplicate count, you could for another hack using hash slices and the trick I just mentioned:

...
undef @record{@record};
keys %record; # your record, now probably scrambled but at least deduped

~ Answered on 2019-01-02 00:38:37


0

Try this, seems the uniq function needs a sorted list to work properly.

use strict;

# Helper function to remove duplicates in a list.
sub uniq {
  my %seen;
  grep !$seen{$_}++, @_;
}

my @teststrings = ("one", "two", "three", "one");

my @filtered = uniq @teststrings;
print "uniq: @filtered\n";
my @sorted = sort @teststrings;
print "sort: @sorted\n";
my @sortedfiltered = uniq sort @teststrings;
print "uniq sort : @sortedfiltered\n";

~ Answered on 2015-05-26 01:56:44


0

Using concept of unique hash keys :

my @array  = ("a","b","c","b","a","d","c","a","d");
my %hash   = map { $_ => 1 } @array;
my @unique = keys %hash;
print "@unique","\n";

Output: a c b d

~ Answered on 2017-03-30 09:47:16


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