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Ruby Programming
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Hashes (sometimes known as associative arrays or dictionaries) are similar to arrays, in that they are indexed collectives of object references.

However, while you index arrays with integers, you can index a hash with objects of any type: strings, regular expressions, and so on. When you store a value in a hash, you actually supply two objects---the key and the value. You can subsequently retrieve the value by indexing the hash with the same key. The values in a hash can be any objects of any type. The example that follows uses hash literals: a list of key => value pairs between braces.

h = { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' }
h.length 3
h['dog'] "canine"
h['cow'] = 'bovine'
h[12]    = 'dodecine'
h['cat'] = 99
h {"cow"=>"bovine", "cat"=>99, 12=>"dodecine", "donkey"=>"asinine", "dog"=>"canine"}

Compared with arrays, hashes have one significant advantage: they can use any object as an index. However, they also have a significant disadvantage: their elements are not ordered, so you cannot easily use a hash as a stack or a queue.

You'll find that hashes are one of the most commonly used data structures in Ruby. A full list of the methods implemented by class Hash starts on page 317.
Ruby Programming
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