Taking References to Hashes

Problem

You need to manipulate a hash by reference. This might be because it was passed into a function that way or because it's part of a larger data structure.

Solution

To get a hash reference:

$href = \%hash; $anon_hash = {
 "key1" => "value1", "key2" => "value2", ... };
 $anon_hash_copy = {
 %hash };

To dereference a hash reference:

%hash = %$href; $value = $href->{$key};
 @slice = @$href{$key1, $key2, $key3};
 # note: no arrow! @keys = keys %$href;

To check whether something is a hash reference:

if (ref($someref) ne 'HASH') {
 die "Expected a hash reference, not $someref\n";
}

Discussion

This example prints out all the keys and values in two predefined hashes:

foreach $href ( \%ENV, \%INC ) {
 # OR: for $href ( \(%ENV,%INC) ) {
 foreach $key ( keys %$href ) {
 print "$key => $href->{$key}\n";
}
}

Accessing slices of hashes by reference works just as it does with slices of arrays by reference. For example:

@values = @$hash_ref{"key1", "key2", "key3"};
for $val (@$hash_ref{"key1", "key2", "key3"}) {
 $val += 7; # add 7 to each value in hash slice
}

See Also

; of Perl Developing; perlref (1);