Hashes

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🗂 If your repl.it is feeling cluttered, you may want to start a new project for this next part of class!

Goals

• Use Ruby syntax to declare variables that store Hashes
• Explain use cases for Hashes
• Access elements by key
• Create and work with more robust student lists

Organizing Data

Say we need to monitor data on student lunches. We could keep track of how many students bring a sack lunch and how many purchase lunch. To start, let’s use this array:

student_lunches = ["sack", "sack", "sack", "sack", "sack", "sack", "sack", "sack", "purchase", "purchase", "purchase", "purchase", "purchase", "purchase", "purchase", "purchase", "purchase", "purchase"]

Hashes

A hash is another Ruby data type that lets us store multiple pieces of data. The difference between a hash and an array is that hashes allow us to have associative relationships between peices of data. Let’s look at this example:

student_lunches = {
  "sack": 8,
  "purchase": 10
}

print student_lunches

Make sure to observe the syntax Ruby expects:

• To open/close the Hash, we use curly braces {}
• Hashes are made up of key/value pairs. In the example above, "sack" is a key that is associated with the value of 8
• Each key should be in quotes followed by a colon
• Each key/value pair should be separated by a comma
• Like Arrays, we can have as many key/value pairs as we like/need to in a Hash

Here’s another example, a simplified Hash holding data about a Kahoot. This example show how various data types can be stored as values.

kahoot1 = {
  "title": "Fractions Review",
  "num_questions": 10,
  "author": "ameseee",
  "favorited": false
}

print kahoot1

Access Values

Given my Kahoot Hash, let’s say I want to print out a sentence that includes the title and num_questions values.

We can use bracket notation to access the values that are associated with a specific key.

kahoot1 = {
  "title": "Fractions Review",
  "num_questions": 10,
  "author": "ameseee",
  "favorited": false
}

puts kahoot1[:title]

puts "This quiz is titled #{kahoot1[:title]}! Have fun!"

Nested Collections

Both Arrays and Hashes are classified as collections - they are data types that allow us to store potentially many pieces of data.

A nested collection is defined as a “set of sets”. As a Kahoot user, I have a list of Kahoots (Array). Each Kahoot within that list has multiple pieces of data associated with it, so is best represented with a Hash. So, my Array of Hashes is considered a nested collection.

Here’s the syntax:

kahoots = [
  {
  "title": "Fractions Review",
  "num_questions": 10,
  "author": "ameseee",
  "favorited": false
  },
  {
  "title": "Fractions Warm Up",
  "num_questions": 8,
  "author": "ameseee",
  "favorited": true
  },
  {
  "title": "Fractions Final CFU",
  "num_questions": 15,
  "author": "ameseee",
  "favorited": true
  },
]

While this looks like a LOT of code and at first glance is quite complex, we can break it down:

• kahoots is a variable that stores an Array
• The kahoots Array has 3 elements. Each of them is a Hash
• Each Hash element has 4 key/value pairs and the keys are consistent

Iterating over an Array of Hashes

We can now put all the pieces together! We have built more robust student lists, so iterating over and array and filtering for pieces of information associated with students may be more helpful.

Let’s start with each. In the code below, we are using each to print out a short sentence for every kahoot:

kahoots.each do |kahoot|
  puts "#{kahoot[:title]} has #{kahoot[:num_questions]} questions."
end

Getting a little spicier, let’s say I want to list out all kahoots that have been favorited.

#  step 1: filter out favorites and store them in an new array
favorited = kahoots.select do |kahoot|
  kahoot[:favorited] == true
end

#  step 2: iterate over favorites and print sentence for EACH of them
favorites.each do |favorite|
  puts "#{favorite[:title]} is a favorite!"
end

Next: Wrap Up