Pointer to element


There’s a gem in Perl Monks that I always struggle finding… again: Recursively walk a hash to get to an element.

From time to time I hit this problem in Perl: traverse a hash of hashes to get to a specific item, with the path provided via an array of keys. Or, as laid out by the post itself:

How do I start with %myhash, and a list of keys qw(bedrock flintstone fred), and get access to the value at $myhash{bedrock}{flintstone}{fred}?

Recursively walk a hash to get to an element gives the answer, one that I always struggle finding when I need it:

sub pointer_to_element {
  require List::Util;
  return List::Util::reduce(sub { \($$a->{$b}) }, \shift, @_);

It returns a reference to a slot in a hash, which can be used to either read or write a value in the hash:

my %hash = (hello => {to => 'everyone'});
my $scalar_ref = pointer_to_element(\%hash, qw< hello to >);
print $$scalar_ref, "\n";    # prints "everyone"
$$scalar_ref = 'everybody';
print "$hash{hello}{to}\n";  # prints "everybody"

merlyn gets every possible credit for this clever piece of code, of course.

Now… I hope it will not take me long to find it again in the future!

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