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Making Hashes of Arrays

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 be a list.

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";
}```

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`. The normal hash operations (insertion, deletion, iteration, and testing for existence) can now be written in terms of array operations 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;`

The classic application of these data structures is inverting a hash that has many 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 Section 5.8.

Be warned that this:

`@residents = @{ \$phone2name{\$number} };`

causes a runtime exception under `use` `strict` because you’re ...

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