nested-hashes.md

JumpStart Live (JSL)

Day 6

Nested Hashes

Learning Goals

By the end of this lesson students will be able to:

• Build a nested data structure involving hashes and arrays
• Access element in a nested data structure
• Navigate a nested data structure using a loop

An Array of Hashes

If Ada was building a student database application we might want to keep track of a student"s ID number, email address, classroom & name. We could store them in 3 parallel arrays like this:

student_names = ["Alice Jones", "Ada Lovelace", "Brook Zoolander"]
student_emails = ["Alice@adadev.org", "Ada@adadevlorg", "brook@adadev.org"]
student_ids = [111, 112, 113]

student_names.length.times do |index|
  puts "ID: #{student_ids[index]}: #{student_names[index]} - #{student_emails[index]}"
end

However as this application grows it could become tedious to keep each student's name, email address and id number on the same index. Instead we could create 1 array with each element being a hash containing the fields for the same student.

students = [
  {
    name: "Alice Jones",
    id: 111,
    email: "Alice@adadev.org",
  },
  {
    name: "Ada Lovelace",
    id: 112,
    email: "ada@adadev.org",
  },
  {
    name: "Brook Zoolander",
    id: 113,
    email: "brook@adadev.org",
  }
]

students.each do |student|
  puts "ID: #{student[:id]}: #{student[:name]} - #{student[:email]}"
end

By using one array of hashes we contain each student's data in one element of the array. We can also use a .each loop to iterate through the array without using an index.

Question How can you, without looping print

• Ada's student email?
• Brook's ID number?
• Alice's full name?

Solution

# Ada's student email
puts students[1][:email]

# Brook's ID number
puts students[-1][:id]

# Alice's Full name
puts students.first[:name]

A Hash of Arrays

What about if Ada wanted to store their student lists by cohort name. In other words they wanted to look up students by cohort name and get a list of student names?

classes = {
  clusters: [
    "Ada Lovelace", "Amahle Elna", "Lucia Santos"
  ],
  containers: [
    "Dee del Rosario", "Devin Helmgren", "Becca Elenzil"
  ],
}

Question How would you loop through and print out all the containers?

Solution

classes[:containers].each do |student|
  puts student
end

Question How could you add, "Christine Martin" to the clusters?

Solution

classes[:clusters] << "Christine Martin"

Design Exercise

Your instructor, Ada, is running a short-term rental property and wants to keep track of their guests. Each guest has a name, id number, check-in date and check-out date (strings). Ada has two rental properties a condo and a townhouse.

How would you organize the data below in Ruby:

ID	Property	Guest Name	Check-in	Check-out
1	Condo	Marie D'Ancanto	2014-08-14	2014-08-16
2	Townhouse	Barbara Wilson	2014-09-20	2014-09-21
3	Condo	Kitty Pryde	2015-12-14	2015-12-27
4	Condo	Patience Phillips	2016-01-02	2016-01-03
5	Townhouse	Wanda Maximoff	2016-01-06	2016-01-14

Resources

• Learn.co Nested Hashes
• Ruby Hash Documentation