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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 = = { "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

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