Hash Basics

Learning Goals

Define a hash in Ruby
Use common methods to work with hashes

Introduction

You've already encountered Ruby hashes and learned some of their characteristics in the lesson on Ruby data types. As a refresher:

Hashes are Ruby's equivalent of a plain old JavaScript object. They are composed of key/value pairs. Each key points to a specific value, just like a word and a definition in a regular dictionary.

In this lesson, we'll explore hashes in more detail, and discuss what makes a hash in Ruby distinct from an object in JavaScript. Make sure to follow along with the examples using IRB.

Creating a Hash

There are a few ways of writing hashes. You can create a hash by simply writing key/value pairs enclosed in curly braces:

{ key1: "value1", key2: "value2" }

Using the JSON-style syntax above will create a hash with Symbols for keys. To access data from this hash, you can use the bracket notation and pass in the symbol for they key you are trying to access:

my_hash = { key1: "value1", key2: "value2" }
my_hash[:key2]
# => "value2"

Trying to access a key that does not exist will return nil:

my_hash[:key3]
# => nil

You can also use the bracket notation to assign a new value to a key:

my_hash[:key2] = 42
my_hash
# => {:key1=>"value1", :key2=>42}

Or to assign a new key-value pair:

my_hash[:key3] = true
my_hash
# => {:key1=>"value1", :key2=>42, :key3=>true}

Unlike JavaScript, you cannot use the dot notation to access keys on hashes — only the bracket notation will work:

my_hash = { key1: "value1", key2: "value2" }
my_hash.key2
# NoMethodError (undefined method `key2' for {:key1=>"value1", :key2=>"value2"}:Hash)

This is because dot notation has a different meaning in Ruby — dot notation is only used for calling methods on objects, which is why the code above throws a NoMethodError.

Even the "bracket notation" used for Hashes and Arrays in Ruby are technically methods defined on those classes! my_hash[:key2] is syntactic sugar (a nicer way of writing) the same thing as a method call: my_hash.[](:key2).

You can also create hashes with Strings for keys:

{ "i'm a key" => "i'm a value!", "key2" => "value2" }

This syntax is known as the hash rocket syntax, and is useful if you need String keys for Symbols; however, in general, using Symbols for keys is preferred.

Like JavaScript, you can also create nested hashes in Ruby, which can be useful to represent complex data structures:

user = {
  name: "Duane",
  address: {
    street: "123 Fake Street",
    city: "Queens",
    state: "NY"
  }
}
user[:address][:city]
# => "Queens"

Common Hash Methods

Ruby has a few common methods for working with hashes. In addition to using bracket notation to access values, you can also delete values from a hash with the delete method by passing in a key:

pizza_toppings = { cheese: true, topping1: "mushroom", topping2: "sausage" }
pizza_toppings.delete(:topping2)
pizza_toppings
# => {:cheese=>true, :topping1=>"mushroom"}

It also has methods for accessing an array of all the keys or an array of all the values, similar to JavaScript's Object.keys and Object.values methods:

pizza_toppings.keys
# => [:cheese, :topping1]
pizza_toppings.values
# => [true, "mushroom"]

It has the #empty? method to check if there are no key-value pairs defined on the hash:

pizza_toppings.empty?
# => false
{}.empty?
# => true

Lastly, it has the #merge method to join together multiple hashes:

more_toppings = { topping2: "peppers", topping3: "onions" }
pizza_toppings.merge(more_toppings)
# => {:cheese=>true, :topping1=>"mushroom", :topping2=>"peppers", :topping3=>"onions"}

We'll explore a few more methods you can use with hashes to iterate over all the key-value pairs in upcoming lessons, but you can also view all the methods in the Ruby Hash documentation.

Conclusion

Hashes are one of the most common data structures you'll work with in Ruby, along with Arrays. You've already seen what kind of scenarios you'd use objects in JavaScript, so you should be comfortable using hashes in similar situations in your Ruby code. Keep the documentation handy so you can take advantage of some of the special methods Ruby has to offer as well!

Resources

Hash documentation

Gbanks1438 / phase-3-hash-basics