Nested Hash Iteration

Objectives

  1. Iterate over a nested hash

Iterating Over Nested Hashes

At this point you should be familiar with iterating over hashes that have one level — a series of key/value pairs on a single tier. For example:

jon_snow = {
  name: "Jon",
  email: "jon_snow@thewall.we"
}

What happens when we want to iterate over a multidimensional hash like the one below? Let's iterate over our nested hash one level at a time; iterating over the first level of our hash would look like this:

contacts = {
  "Jon Snow" => {
    name: "Jon",
    email: "jon_snow@thewall.we",
    favorite_ice_cream_flavors: ["chocolate", "vanilla", "mint chip"],
    knows: nil
  },
  "Freddy Mercury" => {
    name: "Freddy",
    email: "freddy@mercury.com",
    favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
  }
}

contacts.each do |person, data|
  puts "#{person}: #{data}"
end

This should return:

Jon Snow:
{ :name=>"Jon",
  :email=>"jon_snow@thewall.we",
  :favorite_ice_cream_flavors=>["chocolate", "vanilla", "mint chip"],
  :knows=>nil
}

Freddy Mercury:
{ :name=>"Freddy",
:email=>"freddy@mercury.com",
:favorite_ice_cream_flavors=>["strawberry", "cookie dough", "mint chip"]
}

On the first level, the keys are our contacts' names, "Jon Snow" and "Freddy Mercury", and our values are the hashes that contain a series of key/value pairs describing them.

Let's iterate over the second level of our contacts hash. In order to access the key/value pairs of the second tier (i.e. the name, email, and other data about each contact), we need to iterate down into that level. So, we pick up where we left off with the previous iteration and we keep going:

contacts.each do |person, data|
  #at this level, "person" is Jon Snow or Freddy Mercury and "data" is a hash of
  #key/value pairs to iterate over the "data" hash, we can use the following line:

  data.each do |attribute, value|
    puts "#{attribute}: #{value}"
  end
end

That should output the following:

name: Jon
email: jon_snow@thewall.we
favorite_ice_cream_flavors: ["chocolate", "vanilla", "mint chip"]
knows: nil

name: Freddy
email: freddy@mercury.com
favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]

Let's take it one step further and print out just the favorite ice cream flavors. Once again, we'll need to iterate down into that level of the hash, then we can access the favorite ice cream array and print out the flavors:

contacts.each do |person, data|
  #at this level, "person" is Jon Snow or Freddy and "data" is a hash of
  #key/value pairs to iterate over the "data" hash, we can use the following
  #line:

  data.each do |attribute, value|
    #at this level, "attribute" describes the key of :name, :email,
    #:favorite_ice_cream_flavors, or :knows we need to first check and see if
    #the key is :favorite_ice_cream_flavors, if it is, that means the VALUE is
    #an array that we can iterate over to print out each element

    if attribute == :favorite_ice_cream_flavors
      value.each do |flavor|
        # here, each index element in an ice cream flavor string
        puts "#{flavor}"
      end
    end
  end
end

This should output:

chocolate
vanilla
mint chip
strawberry
cookie dough
mint chip
Unlock your future in tech
Learn to code.

Learn about Flatiron School's Mission

With a new take on education that falls somewhere between self-taught prodigy and four-year computer science degree, the Flatiron School promises to turn students with little programming experience into developers.

In the six months since the Manhattan coding school was acquired by WeWork, it has spawned locations in Washington, D.C., Brooklyn, and London. Now, WeWork is opening a fourth Flatiron School location, this time in Houston.

Adam Enbar, Flatiron School's cofounder, believes now is the time to grow. "How the world is changing has impacted working and learning in very similar ways. We think education fundamentally is about one thing: enabling people to pursue a better life."

Learn. Love. Code.
Students come to Flatiron School to change their lives. Join our driven community of career-changers and master the skills you need to become a software engineer or a data scientist.
Find Us