Nested Collections

Learning Goals

  • Use multiple types of collections intermingled
  • Apply common hash and array methods
  • Describe strategies for navigating nested collections


  • Key & Value: a paired combination of pieces of data that exist in a hash (think dictionary)
  • Element/Item: a piece of data within an array
  • Collection: a container for 0 or more pieces of data
  • Data Structure: a data organization/storage format that allows for efficient retrieval and modification of pieces of data
  • Nested Array: an array that exists as an element within another (outer) array
  • Nested Hash: a hash that exists as either an element in an array OR the value of a key within another hash


Using mod-1-be-exercises, find answers to the following:

First, try to answer without using pry, then use pry to verify.

animals = {"dogs" => 3, "cats" => 5, "iguanas" => 2}

  • Using the above animals hash, how would you do the following
    • return the amount of dogs
    • add 3 parakeets
    • increase the amount of cats by 2

pet_names = ["Fela", "Spot", "Patch", "Willy"]

  • Using the above pet_names array, how would you do the following
    • add “Claude”
    • access which name is first in the list
    • access which name is last in the list
    • remove “Fela” from the list

Hash and Array Nesting

As our programs get more complex, we’ll sometimes encounter more sophisticated combinations of these structures. Consider the following scenarios:

Array within an Array

numbers = [[1, 2, 3], [4, 5, 6]]


  • what is numbers.count
  • what is numbers.first.count
  • how can I access the element 5
  • how can I add [7,8,9] to the numbers array

Hashes within an Array

food_feelings = [{:pizza => "tasty"}, {:calzone => "also tasty"}]


  • what is food_feelings.count
  • what is food_feelings.first.count
  • how can I access the element "also tasty"
  • how can I change also tasty to super delicious

Hash within a Hash

pets = {:dog => {:name => "Chance", :weight => "45 pounds"},
        :cat => {:name => "Sassy", :weight => "15 pounds"}}


  • what is pets.count
  • what is pets.keys
  • what is pets.values
  • how can I access the element "15 pounds"
  • how can I add :age => 3 to the value of the key :dog

Array within a Hash

pizza_toppings = {veggies: ["green peppers", "jalapeño", "mushrooms"],
                  protein: ["pepperoni", "sausage", "sardines"],
                  fruit: ["pineapple"]}


  • What is pizza_toppings.count
  • What is pizza_toppings.values
  • How can I access the element "pineapple"
  • How can I add the element "olives" to the key "veggies"

Checks for Understanding

  1. Name one common hash method and one common array method
  2. What can you ask yourself while working on nested collections to help you strategize navigating nested collections?


Using the mod-1-be-exercises repo, navigate to the challenges directory. Follow the instructions and work through the Denver Biscuit Company API response.

Extra Practice

1: State Capitals

You have 2 hashes, one which maps state names to state abbreviations, and one which maps state abbreviations to their capital:

states = {"Oregon" => "OR",
          "Alabama" => "AL",
          "New Jersey" => "NJ",
          "Colorado" => "CO"}

capitals = {"OR" => "Salem",
            "AL" => "Montgomery",
            "NJ" => "Trenton",
            "CO" => "Denver"}
  • Level 1: Write some code which given a state name (“Alabama”) outputs the state abbreviation
  • Level 2: Write some code which given a state name (“Oregon”) outputs its capital (“Salem”)
  • Level 3: Handle the case when a state’s information is not known by returning “Unknown”
  • Level 4: Let’s go the other way. Given a capital name (“Denver”), return the state name for which it is the capital (“Colorado”)
  • Level 5: Write some code to turn these two hashes into one nested hash which looks like this:

      state_info = {
         "Oregon" => {abbreviation: "OR", capital: "Salem"},
         "Alabama" => {abbreviation: "AL", capital: "Montgomery"},
         "New Jersey" => {abbreviation: "NJ", capital: "Trenton"},
         "Colorado" => {abbreviation: "CO", capital: "Denver"}

2: Age Ordering

You have age data for a group of people:

ages = [
  ['Frank', 33],
  ['Stacy', 15],
  ['Juan', 24],
  ['Dom', 32],
  ['Steve', 24],
  ['Jill', 24]
  • Level 1: Write code that’ll output the ages (and only the ages) for the data set
  • Level 2: Write code that’ll output the names (and only the names) in order by ascending age
  • Level 3: Output the name with the age, like Juan (24)
  • Level 4: Write code to automatically build a hash with the age as the key and an array of names as the value (all the people who are that age). e.g. {24 => ['Juan', 'Steve', 'Jill']...}

More Practice

You can also work through the data-types/collections exercises in ruby-exercises

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