Declare and populate a hash table in one step in Perl

Question

Currently when I want to build a look-up table I use:

my $has_field = {};
map { $has_field->{$_} = 1 } @fields;

Is there a way I can do inline initialization in a single step? (i.e. populate it at the same time I'm declaring it?)

Answer 1

Just use your map to create a list then drop into a hash reference like:

my $has_field = { map { $_ => 1 } @fields };

Answer 2

Update: sorry, this doesn't do what you want exactly, as you still have to declare $has_field first.

You could use a hash slice:

@{$has_field}{@fields} = (1)x@fields;

The right hand side is using the x operator to repeat one by the scalar value of @fields (i.e. the number of elements in your array). Another option in the same vein:

@{$has_field}{@fields} = map {1} @fields;

Answer 3

Where I've tested it smart match can be 2 to 5 times as fast as creating a lookup hash and testing for the value once. So unless you're going to reuse the hash a good number of times, it's best to do a smart match:

if ( $cand_field ~~ \@fields ) { 
   do_with_field( $cand_field );
}

It's a good thing to remember that since 5.10, Perl now has a way native to ask "is this untested value any of these known values", it's smart match.