Building Nested Hashes

Overview

We'll take a closer look at nested, or multidimensional, hashes and how to add data to them.

Objectives

1. Add a new value to a key in a nested hash.

2. Add new key/value pairs to a nested hash.

Adding Information to Nested Hashes

As a refresher, we were used to seeing single level hashes that look like this:

jon_snow = {
  name: "Jon",
  email: "[e-mail]"
}

However, what if we want to have a more complicated system that can catalog several people? A nested hash will be particularly useful in this case.

Let's say we have the following nested hash:

contacts = {
  "Jon Snow" => {
    name: "Jon",
    email: "[e-mail]",
    favorite_ice_cream_flavors: ["chocolate", "vanilla"]  },
  "Freddy Mercury" => {
    name: "Freddy",
    email: "[e-mail]",
    favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
  }
}

Your good buddy Jon Snow has just tried mint chip ice cream for the first time. He loved it a lot and now you need to add "mint chip" to his list of favorite ice creams. We already know how to access the array of ice cream flavors that constitute the value of the :favorite_ice_cream_flavors key:

contacts["Jon Snow"][:favorite_ice_cream_flavors]
#  => ["chocolate", "vanilla"]

How can we add a flavor to the list? Well, :favorite_ice_cream_flavors is an array, so we can use the same syntax as above to access that array along with our old friend << (the shovel method) to add an item to the array. There are two ways we can do this. Let's take a look.

####Option 1:

contacts["Jon Snow"][:favorite_ice_cream_flavors] << "mint chip"

####Option 2:

If the above option isn't clear, we can break it down step by step.

• First, grab the value (which itself is another hash) of the "Jon Snow" key, and store it in a variable called jon_snow: jon_snow = contacts["Jon Snow"]

• Second, grab the value of Jon Snow's favorite ice cream, and store that array in a variable: jons_fav_ice_cream = jon_snow[:favorite_ice_cream_flavors]

• Third, add the new ice cream to jons_fav_ice_cream using the shovel (<<) operator: jons_fav_ice_cream << "mint chip"

jon_snow = contacts["Jon Snow"]
jons_fav_ice_cream = jon_snow[:favorite_ice_cream_flavors]
jons_fav_ice_cream << "mint chip"

puts contacts
#  > {
  "Jon Snow" => {
    name: "Jon",
    email: "[e-mail]",
    favorite_ice_cream_flavors: ["chocolate", "vanilla", "mint chip"]
  },
  "Freddy Mercury" => {
    name: "Freddy",
    email: "[e-mail]",
    favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
  }
}

Adding Key/Value Pairs to Nested Hashes

Now let's say that you want to add a whole new key/value pair to your Jon Snow contact—his address. In a simpler hash with just one level of key/value pairs, we've already learned how to add new key/value pairs. Here's a reminder:

hash = {first: "first value!", second: "second value!"}
hash[:third] = "third value!"

puts hash
#  > {first: "first value!", second: "second value!", third: "third value!"}

In a nested hash, we can add new key/value pairs in a similar way. We need to first access the level of the hash to which we want to add a key value pair, and then we can create a new key value on the second level using the same notation. Since we want the "Jon Snow" key to include a new key/value pair of his address, the implementation should look like this:

contacts["Jon Snow"][:address] = "The Lord Commander's Rooms, The Wall, Westeros"

puts contacts
  #  >
  {
    "Jon Snow" => {
     :name=>"Jon",
     :email=>"[e-mail]",
     :favorite_ice_cream_flavors=>["chocolate", "vanilla", "mint chip"],
     :address=>"The Lord Commander's Rooms, The Wall, Westeros"
    },
    "Freddy Mercury"=> {
     :name=>"Freddy",
     :email=>"[e-mail]",
     :favorite_ice_cream_flavors=> ["cookie dough", "mint chip"]
    }
  }

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View Building Nested Hashes on Learn.co and start learning to code for free.