Ruby Monstas

Hashes

Definition

  • A Hash is another type of collection (like Array)
  • It consists of keys and values
  • It is sometimes referred as a Dictionary

Creating Hashes

To create a Hash, we use curly braces {} 

hash = {}
# => {}

Rules

Hashes always have pairs of keys and values

hash = { key: "value", another_key: "another value" }
# => {:key=>"value", :another_key=>"another value"}
  • A key can only exist once
  • Keys are usually symbols (we learn about it later!)
  • Values can exist multiple times
  • Pairs are separated by comma ,

By the way...

hash = { key: "value", another_key: "another value" }

can also be written like this

hash = { :key => "value", :another_key => "another value" }

It's just a different way to write it,
whereas key: "value" is more common

An example

movie = {
  name: "The Matrix",
  released: 1999,
  imdb_rating: 8.7
}
#  => {:name=>"The Matrix", :released=>1999, :imdb_rating=>8.7}

Reading from hashes

We can use the key in [square brackets] to read a value

hash[:key]
# => "value"

Reading from movie

movie[:name]
# => "The Matrix"

movie[:released]
# => 1999

Updating values

We can assign a new value to a key

hash[:key] = "new value"
# => "new value"

Updating a value for movie

movie[:imdb_rating] = 9.3
# => 9.3

movie
# => {:name=>"The Matrix", :released=>1999, :imdb_rating=>9.3}

Adding new key/value pairs

The same way we update values,
we can add new key/value pairs

hash[:new_key] = "yet another new value"
# => "yet another new value"

Adding a key/value pair to movie

movie[:tomatometer] = "88%"
# => "88%"

movie
# => {:name=>"The Matrix", :released=>1999, :imdb_rating=>9.3,
#     :tomatometer=>"88%"}

Deleting key/value pairs

We use the method .delete(:key) to delete a pair

hash.delete(:new_key)
# => "yet another new value"

Removing a key/value pair from movie

movie.delete(:tomatometer)
# => "88%"

movie
# => {:name=>"The Matrix", :released=>1999, :imdb_rating=>9.3}

Summary

# creating new hash
movie = { name: "WALL·E", released: 2008, imdb_rating: 4.8 }

# updating values
movie[:imdb_rating] = 8.4

# adding new key/value pairs
movie[:tomatometer] = "88%"

# deleting key/value pairs
movie.delete(:tomatometer)

movie
# => {:name=>"WALL·E", :released=>2008, :imdb_rating=>8.4}

When to use hashes

We mainly use Hashes for two purposes

When there are multiple variables that logically belong to each other
person = {
  name: 'Harry',
  age: 14,
  hair_color: 'Black'
}
As a lookup structure, aka dictionary
airports = {
  CDG: 'Paris',
  JFK: 'New York',
  YYZ: 'Toronto'
}

Additional methods

There are some useful methods for hashes

  • .keys
  • .values
  • .key?(key)
  • .has_value?(value)

Additional methods examples

movie = { name: "Fargo", released: 1996, imdb_rating: 8.1 }

movie.keys              # => [:name, :released, :imdb_rating]
movie.values            # => ["Fargo", 1996, 8.1]

movie.key?(:name)       # => true
movie.key?(:director)   # => false

movie.has_value?(8.1)   # => true
movie.has_value?(2019)  # => false

Additional Resources

What questions do you have?