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11.2. Making Hashes of Arrays
11.2.1. Problem
For each key in a hash, only one scalar
value is allowed, but you'd like to use one key to store and retrieve
multiple values. That is, you'd like the value to produce a list.
11.2.2. Solution
Use references to
arrays as the hash values. Use push to append:push(@{ $hash{"KEYNAME"} }, "new value");
Then, dereference the value as an array reference when printing out
the hash:foreach $string (keys %hash) {
print "$string: @{$hash{$string}}\n";
}
11.2.3. Discussion
You can only store scalar values in a hash. References, however, are
scalars. This solves the problem of storing multiple values for one
key by making $hash{$key} a reference to an array
containing the values for $key. Normal hash
operations acting on individual scalar values (insertion, deletion,
iteration, and testing for existence) are now written with array
operations acting on lists of values (like push,
splice, and foreach).Here's how to give a key many values:$hash{"a key"} = [ 3, 4, 5 ]; # anonymous array
Once you have a key with many values, here's how to use them:@values = @{ $hash{"a key"} };
To append a new value to the array of values associated with a
particular key, use push:push @{ $hash{"a key"} }, $value;
One common application of such data structures is inverting a hash
that may have several keys with the same associated value. When
inverted, you end up with a hash that has many values for the same
key. This is addressed in Recipe 5.9.Be warned that this:@residents = @{ $phone2name{$number} };
causes a runtime exception under use
strict because you're dereferencing an undefined
reference where autovivification won't occur. You must do this
instead:@residents = exists( $phone2name{$number} )
? @{ $phone2name{$number} }
: ( );
11.2.4. See Also
The section on "Hashes of Arrays" in Chapter 9 of
Programming Perl and in
perldsc(1); the section on "Symbolic References"
in Chapter 8 of Programming Perl; Recipe 5.9;
Tie Example: Make a Hash
That Always Appends in Recipe 13.15
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| 11.1. Taking References to Arrays |  | 11.3. Taking References to Hashes |
Copyright © 2003 O'Reilly & Associates. All rights reserved.
