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By the end of this, developers should be able to:

• Create a Ruby Hash using both the implicit ({}) and new constructors.
• Assign a value to, modify, and delete a value in a Ruby Hash using a specified key.
• Access a value in a Ruby Hash using a specified key.
• Obtain an Array of keys from a Ruby Hash.
• Explain why Symbols are preferred over Strings as keys in a Ruby Hash.

In Ruby, "A Hash is a dictionary-like collection of unique keys and their values". In sharp contrast to JavaScript, Ruby Hashes are not the most general object in the language, but are instances of a specialized class for key/value storage.

A Symbol is a sequence of characters that is stored at most once in any instance of the Ruby interpreter.

In Ruby, strings are compared a character at a time, but symbols are compared by object_id. This makes comparing symbols fast and therefore much more performant than strings when used as keys in a Hash.

Each of the following pairs of operations are equivalent:

> 'bob'.equal? 'bob'
=> false
> 'bob'.object_id == 'bob'.object_id
=> false
> 'bob'.eql? 'bob'
=> true
> 'bob' == 'bob'
=> true

But all of the following operations are equivalent:

> :bob.equal? :bob
=> true
> :bob.object_id == :bob.object_id
=> true
> :bob.eql? :bob
=> true
> :bob == :bob
=> true

We can create a symbol with arbitrary characters if we surround the characters with quotes (either :'<characters>' or '<characters>'.to_sym):

> :'&foo'.equal? '&foo'.to_sym
=> true

How does this compare to keys in JavaScript?

Let's look at different ways to create a Hash.

> apartment = {}
=> {}
> apartment = Hash.new
=> {}
> apartment = Hash.new('')
=> {}
> apartment[:address]
=> ""
> apartment[:address] = { street: '255 Long Road', city: 'Awesomeville', bedrooms: 3}
=> {:street=>"255 Long Road", :city=>"Awesomeville", :bedrooms=>3}
> apartment.merge({rent: 1000})
=> {:street=>"255 Long Road", :city=>"Awesomeville", :bedrooms=>3, :rent=>1000}

Picking sensible defaults may not always be easy.

Let's use the different forms of Hash::new to create some hashes in bin/code_along.rb.

In bin/lab.rb create a hash using the shorthand syntax for assigning the keys :sq_ft :pets_allowed with a type-appropriate value of your choice. Then assign a default of [] to the hash and make sure that accessing non-existing keys return the default.

> apartment[:occupants] = []
=> []
> lee = {name: "Lee", age: 24, dog: "Fluffy"}
=> {:name=>"Lee", :age=>24, :dog=>"Fluffy"}
> adrian = {name: "Lee", age: 24, cat: "Scratchy"}
=> {:name=>"Adrian", :age=>25, :cat: "Scratchy"}
> apartment[:occupants].push(lee, adrian)
=> [{:name=>"Lee", :age=>24, :dog=>"Fluffy"}, {:name=>"Lee", :age=>24, :cat=>"Scratchy"}]
> apartment[:occupants][1].delete(:cat)
=> "Scratchy"
> apartment[:rent] += 150
=> 1150

Add roommate Bo to :occupants in the hash in bin/lab.rb. Append one or more properties of your choosing to the roommate objects, such as :job or :education.

To get an Array of the keys that have been set in a hash, use Hash#keys.

> apartment.keys
=> [:address, :city, :bedrooms, :occupants, :rent]

What if we wanted to instantiate our new hash with this default right off the bat? Checkout the Ruby docs on new hashes with default blocks.

Then, in bin/lab.rb initialize a new hash using Hash.new with a block that sets the default value (without using .default) of all keys to the string "Sorry, <keyname> does not exist".

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