Nested Hashes Lab

Learning Goals

Recognize vocabulary term: "array of hashes" ("AoH")
Recognize that "arrays of hashes" are contact lists or spreadsheets
Create an Array of Hashes
Read scalar data from an Array of Hashes
Modify scalar data in an Array of Hashes

Introduction

Having worked with Arrays of Arrays, you're going to be pleasantly surprised that most of the syntax works exactly the same between "arrays of arrays" to "arrays of hashes."

Key Image: Contact List or Spreadsheet

To help you when reading the rest of this lesson, keep in mind that an Array of Hashes is well expressed by a contact list (or, if you're old enough, a Rolodex). Take a card, add some standard fields to it (first name, family name, phone number, fax number, email address, etc.) and fill the fields in. Take another card and do the same. Take another card and do the same. Those three cards could go in an Array, an Array of Hashes.

If you think about a contact list, it's also easy to export it to a spreadsheet like a Google Sheet or an Excel Sheet. You could take the fields on the card and make them the columns across the top of the spreadsheet. Then for each member of the contact list you could make a new row for it.

Oftentimes when a business friend who doesn't know how to code asks for programming help, they'll want to give us data out of a spreadsheet. Our first move is to turn that spreadsheet into an AoH.

From Hash to Array of Hashes

Recall that a Hash is like a table, but in code form. It is a way for your program to store pairs of data as elements in a collection.

We call names in a Hash that we use to point to data, keys. By providing a Hash name and a key, we can uniquely identify a value.

Multiple Hashes inside of an Array would mean you could find the zeroth, first, second, etc. element of that Array and get back a little dictionary. You could then ask that dictionary for the :family_name field's content and get back "Smith" or "Huang" or "Dayal."

Recognize Vocabulary Term: "Array of Hashes"

Nested Hashes have been common data structures since at least Alexandrian Egypt (the library there famously had books of locations of books). As such, there are lots of words that mean "Array of Hashes." Here's a quick reference. You don't need to memorize these, but if you're trying to model a problem as a programmer, you might see someone use these words.

AoH: Array of Hashes
field: The key / value pair of a Hash e.g. "the :family_name field"

Create an AoH

Let's make a simple list of our favorite directors

# Simple Hashes

director_1 = {
  name: "Ida Lupino",
  favorite_movie: "The Hitch-Hiker"
}

director_2 = {
  name: "Orson Welles",
  favorite_movie: "Citizen Kane"
}

director_3 = {
  name: "Andrej Tarkovsky",
  favorite_movie: "Solaris"
}

film_fest = [
  director_1,
  director_2,
  director_3,
] # => [{:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"}, {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"}, {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}]

If we don't need to use director_1, director_2, director_3, then we don't need to assign them before nesting them in film_fest We can write a nested AoH using literal formatting:

# AoH in one go
film_fest = [{:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"}, {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"}, {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}]

# Same as above, but using whitespace to make it easier for humans to read

film_fest = [
  {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"},
  {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"},
  {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}
]

Since Ruby doesn't care about whitespace, we'd advise you to write an AoH like the second example.

Take a moment to reflect, can you "see" the spreadsheet there in the Array literal? There's more repetition than you'd see in a real spreadsheet, but the idea of intersecting coordinates between an order (0, 1, 2, 3) and a key (:name or :favorite_movie) should feel a little bit like a spreadsheet.

It's good to keep those coordinates in mind as we start trying to retrieve values in this next section!

Read Scalar Data From a Nested Hash

One of the defining features of AoHs is that we use coordinates to target unique elements as if they were in a grid where one part of the coordinate is a number and the other is a Hash key. To read an element from a nested Array, simply provide the coordinates.

directors = [
  {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"},
  {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"},
  {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}
]

directors[0] #=> {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"}
directors[0][:name] #=> "Ida Lupino"

The first set of brackets refers to the "row" of "spreadsheet" of the AoH. The second set of brackets refer to the key used to get a value out of the Hash.

Modify Scalar Data In an Array of Hashes

The same technique of using coordinates applies both to reading and modification. If you can "target" an element you can read it or update it.

directors = [
  {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"},
  {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"},
  {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}
]

directors[0] #=> {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"}
directors[0][:name] #=> "Ida Lupino"
directors[0][:name] = "John Ford" #=> "John Ford" 
directors[0][:favorite_movie] = "The Grapes of Wrath" #=> "The Grapes of Wrath" 
directors[0] #=> {:name=>"John Ford", :favorite_movie=>"The Grapes of Wrath" }
directors #=> => [{:name=>"John Ford", :favorite_movie=>"The Grapes of Wrath"}, {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"}, {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}]

You can also replace inner Hashes with new Hashes:

directors = [
  {:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"},
  {:name=>"Orson Welles", :favorite_movie=>"Citizen Kane"},
  {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}
]

directors[1] = { name: "Cecil B. DeMille", favorite_movie: "Samson and Delilah" }
directors #=> => [{:name=>"Ida Lupino", :favorite_movie=>"The Hitch-Hiker"}, {:name=>"Cecil B. DeMille", :favorite_movie=>"Samson and Delilah"}, {:name=>"Andrej Tarkovsky", :favorite_movie=>"Solaris"}]

Conclusion

Congratulations, you've learned to use your second nested data structure: the Array of Hashes. You've seen that you can build them by building Arrays filled with variables that point to Hashes, or that you can use literal notation to build them out.

You've seen that you can use coordinates to look up elements in the AoH as well as update those elements. In the lab associated with this material, you'll have a chance to make sure you've understood the basics.

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