Ruby Hashes

A hash is an unordered, dictionary-like collection of pairs of keys and values. They can be of any type and you can intermix different types inside a particular hash, as long as all the keys are unique.

To create a hash use curly braces:

frequency = { "a" => 25, "b" => true, 0 => "nope" }

Retrieving values

Values can be retrieved from a hash using the relevant key (this is why the keys must be unique):

frequency["b"]  # true

frequency[0]  # "nope"

If a non-existent key is used, nil is returned:

frequency[1]  # nil

Hashes also have a fetch method that will return the value for an associated key:

frequency.fetch("b")  # true

Adding new values

New values can be added by assigning them to a new key:

frequency["c"] = 25.4
frequency  # {"a"=>25, "b"=>nil, 0=>"nope", "c"=>25.4}

Deleting keys

An existing key can be removed from an array:

frequency.delete("a")
frequency  # {"b"=>true, 0=>"nope", "c"=>25.4}

Using symbols for keys

A very common usage of hashes is with symbols as keys. Since symbols are unique, they are a great candidate for the keys of a hash (which must be unique):

frequency = { :a => 25, :b => true, :c => "nope" }

NB: Given the name restrictions for symbols, :0 won't be a valid symbol name.

In fact, the usage of symbols in hashes is so common, that Ruby provides a shorter syntax for it (notice that the printed value below is identical with the one above):

frequency = { a: 25, b: true, c: "nope" }

Symbols are to be used as keys to retrieve the associated value:

frequency[:a]  # 25

Basic array methods

Keys and values can be retrieved as arrays (order not guaranteed):

frequency = { :a => 25, :b => true, :c => "nope" }
frequency.keys    # [:a, :b, :c]
frequency.values  # [25, true, "nope"]

A hash can also be transformed into an array of key,value pairs:

frequency.to_a  # [[:a, 25], [:b, true], [:c, "nope"]]

To find out how many key,value pairs our hash has, you can use a similar method the one for arrays: length. It should be very easy for you to understand what the methods below do:

frequency.length   # 3
frequency.empty?   # false
frequency.min      # [:a, 25]
frequency.max      # [:c, "nope"]
frequency.display  # {:a=>25, :b=>true, :c=>"nope"}
frequency.clear
frequency          # {}

To view all methods available for our array, use the methods method:

frequency.methods

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