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📖 NAME

Perl FAQ – Data Manipulation and Error Handling (from perlfaq4 and perlfaq7)

🚀 Quick Reference

Use Case	Command	Description
Remove duplicates from list/array (order not preserved)	my %hash = map { $_, 1 } @array; my @unique = keys %hash;	Use a hash to extract unique keys, ignoring order.
Remove duplicates from list/array (order preserved)	use List::MoreUtils qw(uniq); my @unique = uniq(@array);	Module uniq preserves order; in scalar context returns count.
Remove duplicates manually with grep	my %seen; my @unique = grep { !$seen{$_}++ } @array;	Iterates once, using a hash to track seen elements.
Catch undefined variables at compile time	use warnings FATAL => qw(uninitialized);	Promotes uninitialized warnings to fatal errors.
Catch undefined functions/methods at runtime	sub AUTOLOAD { ... }	Define AUTOLOAD to handle undefined subroutine calls.

🔍 Found in /usr/share/perl/5.34/pod/perlfaq4.pod

🔍 Found in /usr/share/perl/5.34/pod/perlfaq7.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;

⚠️ How can I catch accesses to undefined variables, functions, or methods?

The AUTOLOAD method, discussed in "Autoloading" in perlsub lets you capture calls to undefined functions and methods.

When it comes to undefined variables that would trigger a warning under "use warnings", you can promote the warning to an error.

use warnings FATAL => qw(uninitialized);

📖 NAME 🚀 Quick Reference 🧹 How can I remove duplicate elements from a list or array? ⚠️ How can I catch accesses to undefined variables, functions, or methods?