Ruby Hashes (versus JavaScript dictionaries)

Instructions

Fork, clone, branch (training), bundle install

Objectives

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

• Constrast Ruby Hashes with JavaScript dictionaries.
• Create a Ruby Hash using both the implicit ({}) and new constructors.
• Assign a value to or modify 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.

Introduction

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.

Ruby Symbols

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.

Demo: Test Equivalency

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?

Creating Ruby Hashes

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

Demo: Hash Creation

> consultant = {}
=> {}
> consultant = Hash.new
=> {}
> consultant = Hash.new('')
=> {}
> consultant.default = ''
=> ""
> consultant[:surname]
=> ""
> consultant
=> {}
> consultant = { given_name: 'Lauren', surname: 'Fazah', height: 67}
=> {:given_name=>"Lauren", :surname=>"Fazah", :height=>67}
> consultant[:weight] += 10
NoMethodError: undefined method ``+'' for nil:NilClass
from (pry):2:in ``__pry__''
> consultant = Hash.new(0).merge({ given_name: 'Lauren', surname: 'Fazah', height: 67})
=> {:given_name=>"Lauren", :surname=>"Fazah", :height=>67}
> consultant[:weight] += 10
=> 10

Picking sensible defaults may not always be easy.

Code Along: Hash::new

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

Lab: Hash Shorthand

In bin/lab.rb create a hash using the shorthand syntax for assigning the keys :education and :experience with the string "GA WDI". Then assign a default of [] to the hash and make sure that accessing non-existing keys return the default.

Assigning and Accessing Elements in a Ruby Hash

Demo: Assigning and Accessing

> consultant[:weight]
=> 0
> consultant[:weight] = 185
=> 185
> consultant[:weight] += 5
=> 190
> consultant[:surname]
=> "Fazah"
> consultant[:surname].prepend 'O\''
=> "O'Fazah"
> consultant[:surname]
=> "O'Fazah"

Lab: Appending

Append " Boston" to both :experience and :education in the hash in bin/lab.rb. Append one or more item of your choosing to the values at each of these keys.

Demo: Hash Keys

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

> consultant.keys
=> [:given_name, :surname, :height, :weight]

Lab: Hash.new Initialized With Default

If you noticed during the demo above, we were able to add a default value of an empty string to the consultant hash using consultant.default = ''.

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".

License

1. All content is licensed under a CC­BY­NC­SA 4.0 license.
2. All software code is licensed under GNU GPLv3. For commercial use or alternative licensing, please contact legal@ga.co.