• Review Ruby arrays and hashes
• Use Ruby loops to iterate over code blocks.
• Define what a Ruby enumerable method is.
• Use enumerables to traverse, sort and modify collections.
• Identify useful Ruby enumerables, including .each, .map and .select.

One of the most common things we do as developers is to loop through data structures. We understand and learned how to do these things in Javascript. Now we're going to learn how to do them in ruby. We'll start with the basics but then..

We will be going over enumerables in Ruby, one of the most powerful modules that comes included in the language out of the box.

We will get into how to use enumerables later on this lesson, but first just know that enumerables essentially are a more readable, expressive way to work with collections of data in Ruby.

Whenever we talk about data in Ruby, its important to review how Ruby handles groups of data.

Q: What are the different types of collections in Ruby?

fruits = ["apple", "banana", "cherry"]

fruits.length # 3
fruits.push("date")
fruits.length # 4

fruits[0] # apple
fruits[3] # date

fruits[3] = "mango"
fruits[3] # mango

fruits.join(" ") # "apple banana cherry mango"
fruits.join(", ") # "apple, banana, cherry, mango"
fruits.join(" and ") # "apple and banana and cherry and mango "

A: using <<, or the shovel operator: e.g. fruits << "peach"

Hashes are like Javascript Object Literals, but they are a bit more limited:

instructor = {
  name: "Bob",
  age: 30,
  favorite_foods: ["Tater Tots", "Cheese Steaks", "Kale Salad"]
}

# Access values from a hash
instructor[:name] # "Bob"
instructor[:age]  # 30
instructor[:favorite_foods] # ["Tater Tots", "Cheese Steaks", "Kale Salad"]

# Set values to an existing key
instructor[:name] = "Robert"
instructor[:name] # "Robert"

# Add new key-value pairs
instructor[:favorite_color] = "red"
instructor[:favorite_color] # "red"

• What's another "rubyist" way to add items to an array?
• What is one main difference between Ruby's hashes and Javascript's object literals?
• What are some useful methods we can call on collections?
• Where would I go look if I wanted to find more methods?

Another similarity Ruby shares with Javascript is base support for various types of loops.

A: while, do-while, for, for-in, forEach

Let's start by reviewing JS for loops.

Say we had our fruits example from earlier and we wanted to print each individual fruit in the console

var fruits = ["apple", "banana", "cherry"];
for (var i = 0; i < fruits.length; i++) {
  console.log(fruits[i]);
}

The closest equivalent to Javascript's for loop is Ruby's for-in loop

fruits = ["apple", "banana", "cherry"]
for fruit in fruits do
  puts fruit
end

Ruby also has a plethora of other types of loops including:

• loop
  • loop runs uninterrupted until stopped
• while
  • runs the loop while the condition is true
• until
  • runs the loop while the condition is false
• .times (called on a number)
  • runs the loop a specific of times

Further Reading on Ruby loops

10.times.do |i|
  puts i
end

What does this code do? (ST-WG)

break lets us end -- or "break" out of -- a loop.

10.times do |i|
  if i == 5
    break
  end
  puts i
end

next lets us skip to the next iteration of a loop.

10.times do |i|
  if i == 5
    next
  end
  puts i
end

As the new manager of GA's Club Code, you been tasked with creating an automated bouncing system.

The club's rules state:

• Only people 18 and over are allowed in the door
• No more than 8 people should be inside the club at any time

Given a line of people:

people = [
  { name: "Jack", age: 16 },
  { name: "Sam", age: 21 },
  { name: "Jill", age: 23 },
  { name: "Paul", age: 20 },
  { name: "Mike", age: 16 },
  { name: "Stan", age: 70 },
  { name: "Chris", age: 17 },
  { name: "Julie", age: 45 },
  { name: "Suzy", age: 65 },
  { name: "Eli", age: 28 },
  { name: "Katie", age: 50 },
  { name: "Ben", age: 33 }
]

and using what you know about loops and collections in Ruby, write a program that:

• Creates a new array of people inside the club
• Allows the appropriate people into the club
• Stops once 8 people have been admitted

Hints:

• How could you use next and/or break to alter the behavior of a loop?

Bonus

• Determine whether or not a person is to be served
  • Anyone over 18, but under 21 can come in, but they are not to be served adult beverages
  • Create a new key-value pair for each person with served as a boolean

Double Bonus

• Write a function that takes 3 arguments: a list of people, an age limit, and capacity limit.
• It should return a hash that looks like this:

  {
    accepted:
      [ {name: "Jack", age: 22},  {name: "Jill", age: 31}, ...],
    rejected:
      [ {name: "Billy", age: 18},  {name: "Nancy", age: 31}, ...]
  }

A Solution

The Enumerable module provides a set of methods to traverse, search, sort and manipulate collections.

So how are enumerables different than loops you might ask? In general, loops just execute a certain block of code for a given amount of time, while enumerables are used in relation to data collections to more easily control and transform values in data sets.

Since we already have a base understanding of loops and arrays and hashes, there's nothing new conceptually here. But you'll learn to do more with prettier, fewer lines of code.

• Documentation

The king (or queen) of enumerables, and the one you will most likely be using the most.

• Iterates through and performs an action(s) on a collection.
• Note: Does not permanently modify the collection.

If we were to emulate .each using plain ol' Ruby, it would look something like this...

# A loop that prints out the doubled value of each item in an array
numbers = [ 1, 2, 3, 4, 5 ]
i = 0

while(i <= numbers.length) do
  puts numbers[i] * 2
  i = i + 1
end

But using .each looks like this, with the code block format...

numbers = [ 1, 2, 3, 4, 5 ]
numbers.each do |number|
  puts number * 2
end

# Alternate syntax
numbers = [ 1, 2, 3, 4, 5 ]
numbers.each { |number| puts number * 2 }

Visualize: EACH: using this code visualizer let's look at how each operates under the hood

The each method yields a reference to each element in the collection, rather than a reference to the element's numerical index in the array.

WHITEBOARD: You'll notice we wrote out the .each example in two ways: multi-line and single-line.

Multi-line

• .each - method name
• do - keyword that begins block of code
• |number| - iteration variable; represents an individual value in the array
  • Common syntax is to name the variable the singular version of the collection. In this case, use number for numbers.
  • Some enumerables may have more than one of these.
• end - closes code block

Single-line

• .each - method name
• { } - replaces do and end; contains the iteration variable and code block
• |number| - iteration variable

Use each to do the following...

• Say hello to everybody in the below array of names (sample output: Hello Donald!).

  names = [ "Donald", "Daisy", "Huey", "Duey", "Luey" ]

• Print out the squared values of every number in this numbers array.

  numbers = [ 1, 3, 9, 11, 100 ]

• Print out the Celsius values for an array containing Fahrenheit values.

  fahrenheit_temps = [ -128.6, 0, 32, 140, 212 ]

• Insert all the values in the artists array into the ninja_turtles array.

  artists = [ "Leonardo", "Donatello", "Raphael", "Michelangelo" ]
  ninja_turtles = []

• Bonus: Print out every possible combination of the below ice cream flavors and toppings.

  flavors = [ "vanilla", "chocolate", "strawberry", "butter pecan", "cookies and cream", "rainbow" ]
  toppings = [ "gummi bears", "hot fudge", "butterscotch", "rainbow sprinkles", "chocolate sprinkles" ]

Let's get a quick Fist-of-Five on how we feel about each and enumerables so far?

• There are many enumerable methods. All of them look and feel similar to each.

Map is similar to each. It iterates through each element in the collection, but map generates a new collection with values based on the code block. Say we want to double our numbers array and store it in a new array...

numbers = [ 1, 2, 3, 4, 5 ]
doubles = numbers.map do |number|
  number * 2
end
doubles
# => [ 2, 4, 6, 8, 10 ]

# Alternate syntax
numbers = [ 1, 2, 3, 4, 5 ]
doubles = numbers.map { |number| number * 2 }
doubles

NOTE: We did not have to type out any variable assignment in the code block!

VISUALIZE: MAP: using this code visualizer let's look at how map operates under the hood

Use map to do the following...

1. Create an array that appends "Duck" to everybody in this array of first names

first_names = [ "Donald", "Daisy", "Daffy" ]

#=> ["Donald Duck", "Daisy Duck"]

2. Create an array containing the squared values of every number in this array.

numbers = [ 1, 3, 9, 11, 100 ]

# => [1, 9, 81, 121, 10000]

3. Create an array with the Celsius values for these Fahrenheit values.

fahrenheit_temps = [ -128.6, 0, 32, 140, 212 ]

#=> [-89.2, -17.8, 0, 60, 100]

Instructions: Each group will spend 10 minutes using Ruby documentation to look up an assigned enumerable. Prepare the following for your demo:

• your own definition of what it does
• whiteboard an example.
• at a high level, try to find/think of a use case of this enumerable in the wild, doesn't have to be the actual code just conceptually similar
• You can test your example in IRB/Pry.
• Documentation

Groups

• Group 1: Each With Index
• Group 2: Reject
• Group 3: Find
• Group 4: Select
• Group 5: Sort By
• Group 6: Inject/Reduce

Bonus: If you find yourself with extra time, you can:

• Pick out another enumerable that wasn't assigned to a group.
• ...and/or think of another example for your assigned enumerable.

Combine your knowledge of Ruby basics and enumerables to make our old nemesis a piece of cake...

High Card

1. What is the difference between loops and enumerables in Ruby?
2. What are some examples of loops in Ruby that are not in JS?
3. Differentiate between Ruby's each and map methods
4. What is an iteration variable?
5. List 5 useful Ruby enumerable methods

• Intro to Ruby Enumerables
• Ruby Monk on Enumerables
• Ruby Explained: Awesome Enumerables
• Ruby Magic
• Ruby Loops and Iterators