(hashes were added in v1.1)

A hash is a variable type that maps one set of values to another, much like an array maps indexes to values. In a hash each pair of values is called a key/value pair, where the key value can be used to lookup the other value. Unlike an array the key value can be of any type, not just an integer.

To create a new hash you use the Hash object type command (see more below), eg:

$favorites = Hash.new('animal', 'Bunny')

To define a key/value pair in a hash you use curly braces to enclose the key:

$favorites{'color'} = 'cerulean'

In this example the hash now maps the key 'color' to the string 'cerulean'. Mixing types in a hash is perfectly valid:

$map{7} = 'Heaven7'

$map{0.5} = '1/2'

$map{'PI'} = 3.14

To retrieve a value from an existing hash you use the same curly brace notation:

$favColor = $favorites{'color'}

If the key is not currently defined by the hash then the value returned is the @undefined value.

Warning: hash key equality comparisons do not make full use of value coercion that would apply when using other kinds of equality tests. It is best to always use the same type of value (eg: text vs float) for a key when setting and getting a value from a hash. An example that illustrates the problem:

$hash{3.14} = 'Pie!' # key is a numeric type

$failed = $hash{"3.14"} # key is now a text type, failure

Value Semantics

By default hashes use value semantics, which means a copy of the hash is made each time it is assigned. This ensures each variable is independent from one another. For example:

$original = Hash.new("a", 123, "z", 999)

$copy = $original # a copy of $original is implicitly made

$copy{"m"} = '😱' # only modifies $copy, leaving $original intact

Prompt $original, $copy # shows two different hashes