What the heck is Ruby Hash ??

You can think of Ruby Hash is kind of an Array without the numerical indexes. You access the Hash values with Keys. A Hash is a data structure used to store data in the form of UNIQUE key-value pairs.

A Hash has certain similarities to an Array, but:

✅ An Array index is always an Integer
✅ A Hash key can be (almost) any object

We are gonna dive 🤿 into the Hash world in Ruby. But not too deep, just at the level where most people dive in :p.

note: I encourage you to open irb in your favourite terminal app. Make sure you get your hands 🙌 dirty.

TL;DR

• 1️⃣ Hash Data Syntax
• 2️⃣ Common use case of using Hash
• 3️⃣ How to Create a Hash
• 4️⃣ How to create or update a Hash value
• 5️⃣ Values in a Ruby Hash
• 6️⃣ Hash Entry Order
• 7️⃣ How to delete a Hash entry
• 8️⃣ How to Access Values From a Hash
• 9️⃣ Extract a nested Hash value
• 🔟 Hash Keys 101
• 1️⃣1️⃣ Default Values
• 1️⃣2️⃣ How to Merge Two Ruby Hashes
• 1️⃣3️⃣ Multiple Hash values for one key
• 1️⃣4️⃣ Get All Keys and Values From a Hash
• 1️⃣5️⃣ Check if key exists in hash
• 1️⃣6️⃣ How to change a Hash to Array

✅ Hash Data Syntax

Hash has three Syntax style as Ruby 3.0.

1) hash rocket =>

hashy = {:foo => 0, :bar => 1, :baz => 2}
hashy # => {:foo=>0, :bar=>1, :baz=>2}

2) JSON-style syntax

hashy = {foo: 0, bar: 1, baz: 2}
hashy # => {:foo=>0, :bar=>1, :baz=>2} 

note: The Hash key become a Symbol

note: You will get an error if you use the key that's not a bareword or a String

# Raises SyntaxError (syntax error, unexpected ':', expecting =>):
hashy = {0: 'zero'}

3) String

hashy = {'foo': 0, 'bar': 1, 'baz': 2}
hashy # => {:foo=>0, :bar=>1, :baz=>2} 

note: same like number 2. But the key is change to the String, instead of symbol.

And you can mix the styles;

hashy = {foo: 0, :bar => 1, 'baz': 2}
hashy # => {:foo=>0, :bar=>1, :baz=>2}

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✅ Common use case of using Hash

1) Give names to objects

book = {author: "Daniel J. Levitin", title: "This is Your Brain on Music}"
book # => {:author=>"Daniel J. Levitin", :title=>"This is Your Brain on Music"}

2) Give names to method arguments

def some_method(hash)
  p hash
end
some_method({foo: 0, bar: 1, baz: 2}) # => {:foo=>0, :bar=>1, :baz=>2}

3) initialize an object

class Book
  attr_accessor :author, :title
  def initialize(hash)
    self.author = hash[:author]
    self.title = hash[:title]
  end
end
book1 = Book.new(author: 'Daniel J. Levitin', title: "'This is Your Brain on Music')"
book1 # => #<Book: @author="Daniel J. Levitin", @title="This is Your Brain on Music">

4) Get the frequency from a list of numbers

numbers = [1, 1, 1, 2, 4, 65, 55, 54, 55]
freq_hash = numbers.each_with_object(Hash.new(0)) { |number, hash| hash[number] += 1 }

puts "#{freq_hash}"
# {1=>3, 2=>1, 4=>1, 65=>1, 55=>2, 54=>1}

Another tricks is using tally method.

numbers = [1, 1, 1, 2, 4, 65, 55, 54, 55]
numbers.tally
# output
# {1=>3, 2=>1, 4=>1, 65=>1, 55=>2, 54=>1}

note: This is for Ruby 2.7+ only. :P

5) Specify routes in Ruby on Rails

This is a line from the config/routes.rb file, the Router in a Rails application:

# defines a GET route mapped to the new action in the PostsController

get '/posts/new', to: 'posts#new'

The example of the above could be rewritten as:

get('/posts/new', { to: 'posts#new' })

The Hash is the second parameter passed to the get(...) method.

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✅ How to Create a Hash

Here are three ways to create a Hash:

1) Method Hash.new

You can create a Hash by calling method Hash.new

h = Hash.new #Define empty hash
h # => {}
h.class # => Hash

h[:first] = 10
h[:second] = 20
h[:third] = 30

puts h # => {:first=>10, :second=>20, :third=>30}

2) Method Hash[]

You can create a Hash by calling method Hash[].

Create an empty Hash:

h = Hash[]
h # => {}

Create a Hash with initial entries:

h = Hash[foo: 0, bar: 1, baz: 2]
h # => {:foo=>0, :bar=>1, :baz=>2}

3) Literal form {}

You can create a Hash by using its literal form (curly braces).

Create an empty Hash:

h = {}
h # => {}

Create a Hash with initial entries:

h = {foo: 0, bar: 1, baz: 2}
h # => {:foo=>0, :bar=>1, :baz=>2}

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✅ How to create or update a Hash value

We can use instance method []=:

h = {foo: 0, bar: 1, baz: 2}
h[:bat] = 3 # => 3
h # => {:foo=>0, :bar=>1, :baz=>2, :bat=>3}

h[:foo] = 4 # => 4
h # => {:foo=>4, :bar=>1, :baz=>2, :bat=>3}

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✅ Values in a Ruby Hash

Values can be any Ruby Object. Including:

• Strings
• Integers & Floats
• Arrays

note Keys are unique, you can only have one :foo key, or one :bar key. When you add the same key twice, the latter will override the former value.

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✅ Hash Entry Order

A Hash object presents its entries in the order of their creation.

A new Hash has its initial ordering per the given entries:

h = Hash[foo: 0, bar: 1]
h # => {:foo=>0, :bar=>1}

New entries are added at the end:

h[:baz] = 2
h # => {:foo=>0, :bar=>1, :baz=>2}

Updating a value does not affect the order:

h[:baz] = 3
h # => {:foo=>0, :bar=>1, :baz=>3}

But re-creating a deleted entry can affect the order:

h.delete(:foo)
h[:foo] = 5
h # => {:bar=>1, :baz=>3, :foo=>5}

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✅ How to delete a Hash entry

The simplest way is using instance method delete:

h = {foo: 0, bar: 1, baz: 2}
h.delete(:bar) # => 1
h # => {:foo=>0, :baz=>2}

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✅ How to Access Values From a Hash

Hash value/element are access by particular key.

The simplest way to retrieve a Hash value is using instance method [] :

h = {foo: 0, bar: 1, baz: 2}
h[:foo] # => 0

We also can use fetch method. It does same as the square bracket lookup [], but it will raise an error if the key is not defined:

$ irb
> dictionary = { "one" => "satu" } #Malay language 🇲🇾
> dictionary.fetch("one")
=> "satu"
> dictionary.fetch("two")
KeyError: key not found: "two"

We can prevent this error using the default value.

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✅ Extract a nested Hash value

dig is handy for nested Hash.

h = { foo: {bar: {baz: 11}}}

h.dig(:foo, :bar, :baz)   # => 11
h.dig(:foo, :zot, :xyz)   # => nil

g = { foo: [10, 11, 12] }
g.dig(:foo, 1)            # => 11

note: This is only for Ruby 2.3+

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✅ Hash Keys 101

Hash Key Equivalence

From Documentation: Two objects are treated as the same hash key when their hash value is identical and the two objects are eql? to each other.

irb> g = {foo: 1}
=> {:foo=>1}
irb> h = {foo: 1}
=> {:foo=>1}
irb> g.eql?(h)
=> true

Modifying an Active Hash Key

Modifying a Hash key while it is in use damages the hash index.

This Hash has keys that are Arrays:

a0 = [ :foo, :bar ]
a1 = [ :baz, :bat ]
h = {a0 => 0, a1 => 1}
h.include?(a0) # => true
h[a0] # => 0
a0.hash # => 110002110

Modifying array element a0[0] changes its hash value:

a0[0] = :bam
a0.hash # => 1069447059

And damages the Hash index:

h.include?(a0) # => false
h[a0] # => nil

You can repair the hash index using method rehash:

h.rehash # => {[:bam, :bar]=>0, [:baz, :bat]=>1}
h.include?(a0) # => true
h[a0] # => 0

A String key is always safe. That's because an unfrozen String passed as a key will be replaced by a duplicated and frozen String:

s = 'foo'
s.frozen? # => false
h = {s => 0}
first_key = h.keys.first
first_key.frozen? # => true

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✅ Default values

By default, accessing a key which has not been added to the hash returns nil, meaning it is always safe to attempt to look up a key's value:

my_hash = {}

my_hash[:name] # => nil

Hashes can also contain keys in strings. If you try to access them normally it will just return a nil, instead you access them by their string keys:

my_hash = { "name" => "asyraf" }

my_hash[:name]    # => nil
my_hash["name"]   # => asyraf

You can retrieve the default value with method default:

h = Hash.new
h.default # => nil

You can set the default value by passing an argument to method Hash.new or with method default=

h = Hash.new(100)
h.default # => 100

h.default = 99
h.default # => 99

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✅ How to Merge Two Ruby Hashes

I think you can guess the method name :p. Tadaaaa, we will use merge method.

defaults    = { a: 1, b: 2, c: 3 }
preferences = { c: 4 }

defaults.merge!(preferences)
# {:a=>1, :b=>2, :c=>4}

Notice that because keys are unique, newer values overwrite older values.

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✅ Multiple Hash values for one key

Malaysia = {
  food: [
    "Nasi Lemak",
    "Roti Canai"
  ],
  city: [
    "Kuala Lumpur",
    "Malacca City",
    "George Town"
  ]
}

Malaysia[:city][1]

Where Malaysia[:city] gives you an array & [1] gives you the 2nd element from that array.

The key is a symbol & the values are arrays. When you access the hash, you get an array back which you access normally, like any other array.

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✅ Get All Keys and Values From a Hash

If you want a list of all the keys, good news, there is a method for that!

Here it is:

{ foo: 1, bar: 2 }.keys
# [:foo, :bar]

There’s also a method for values:

{ foo: 1, bar: 2 }.values
# [1, 2]

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✅ Check if key exists in hash

If you want to know if a key exists in a hash, use the key? method.

hash_data = {'Country'=>"Malaysia",'Food'=>"Nasi Lemak",}
puts hash_data.key?("Country") #true
puts hash_data.key?("Code") #false

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✅ How to change a Hash to Array

We can do Hash 🔁 Array. Converting a hash of key/value pairs into an array will produce an array containing nested arrays for pair:

{ :a => 1, :b => 2 }.to_a # => [[:a, 1], [:b, 2]]

In the opposite direction a Hash can be created from an array of the same format:

[[:x, 3], [:y, 4]].to_h # => { :x => 3, :y => 4 }

Similarly, Hashes can be initialized using Hash[] and a list of alternating keys and values:

Hash[:a, 1, :b, 2] # => { :a => 1, :b => 2 }

Or from an array of arrays with two values each:

Hash[ [[:x, 3], [:y, 4]] ] # => { :x => 3, :y => 4 }

Hashes can be converted back to an Array of alternating keys and values using flatten():

{ :a => 1, :b => 2 }.flatten # => [:a, 1, :b, 2]

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Super cool

Pretty cool, hah... 😎

There are other cool stuff on Ruby Hash. But as i said, this is just a level of most of people dive in.

Maybe for specific tricks like Hash iteration, sorting and other methods for future articles :P.

And probably this is my notes about Ruby Hash, I will add new stuff as I get new knowledge or experience with it.

Thank you for traveling with me 🚀, I hope you enjoyed the journey 🔥.

The End

Resources:

1, 2, 3, 4, 5, 6, 7