Recipe 11.3 Taking References to Hashes

11.3.1 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.

11.3.2 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"; }

11.3.3 Discussion

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

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

Access slices of hashes by reference as you'd access 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 }

11.3.4 See Also

The Introductionin Chapter 5; Chapter 8 of Programming Perl; perlref(1); Recipe 11.9