Ruby Cookbook, 2nd Edition

Chapter 6. Hashes

Hashes and arrays are the two basic “aggregate” data types supported by most modern programming lagnguages. The basic interface of a hash is similar to that of an array. The difference is that while an array stores items according to a numeric index, the index of a hash can be any object at all.

Arrays and strings have been built into programming languages for decades, but built-in hashes are a relatively recent development. Now that they’re around, it’s hard to live without them: they’re at least as useful as arrays.

You can create a hash by calling Hash.new or by using one of the special syntaxes Hash[] or {}. With the Hash[] syntax, you pass in the initial elements as comma-separated object references. With the {} syntax, you pass in the initial contents as comma-separated key-value pairs:

empty = Hash.new                            # => {}
empty ={}                                   # => {}
numbers = { :two => 2, :eight => 8}         # => {:two=>2, :eight=>8}
numbers = Hash[:two, 2, :eight, 8]          # => {:two=>2, :eight=>8}

In Ruby 2.1, a new syntax was introduced for declaring hashes that saves you a few characters:

    numbers = { two: 2, eight: 8 }              # => {:two=>2, :eight=>8}

Once the hash is created, you can do hash lookups and element assignments using the same syntax you would use to view and modify array elements:

numbers[:two]                              # => 2
numbers[:ten] = 10                         # => 10
numbers                                    # => {:two=>2, :eight=>8, :ten=>10}

You can get an array containing the keys or values of a hash with Hash#keys or Hash#values. You can get the entire hash as ...