How do I store a duplicate value from an array or hash in Perl?

Question

Let's make this very easy. What I want:

@array = qw/one two one/;
my @duplicates = duplicate(@array);
print "@duplicates"; # This should now print 'one'.

How to print duplicate values of a array/hash?

Answer 1

sub duplicate {
    my @args = @_;
    my %items;
    for my $element(@args) {
        $items{$element}++;
    }
    return grep {$items{$_} > 1} keys %items;
}

Answer 2

# assumes inputs can be hash keys
@a = (1, 2, 3, 3, 4, 4, 5);

# keep count for each unique input
%h = ();
map { $h{$_}++  } @a;

# duplicate inputs have count > 1
@dupes = grep { $h{$_} > 1 } keys %h;

# should print 3, 4
print join(", ", sort @dupes), "\n";

Answer 3

The extra verbose, extra readable version of what you want to do:

sub duplicate {
   my %value_hash;
   foreach my $val (@_) {
     $value_hash{$val} +=1;
   }
   my @arr;
   while (my ($val, $num) = each(%value_hash)) {
     if ($num > 1) {
        push(@arr, $val)
     }
   }
  return @arr;
}

This can be shortened considerably, but I intentionally left it verbose so that you can follow along.

I didn't test it, though, so watch out for my typos.

Answer 4

Use a dictionary, put the value in the key, and the count in the value.

Ah, just noticed you've tagged as perl

while ([...]) {
 $hash{[dbvalue]}++
}

Answer 5

Unspecified in the question is the order in which the duplicates should be returned.

I can think of several possibilities: don't care; by order of first/second/last occurrence in the input list; sorted.

Answer 6

I'm going golfing!

sub duplicate {
    my %count;
    grep $count{$_}++, @_;
}

@array = qw/one two one/;
my @duplicates = duplicate(@array);
print "@duplicates"; # This should now print 'one'.

# or if returning *exactly* 1 occurrence of each duplicated item is important
sub duplicate {
    my %count;
    grep ++$count{$_} == 2, @_;
}