Enumerables

Introduction

Checkout the Enumerable Documentation. Bookmark this page! Look at the "Methods" column on the left-hand side.

The Enumerable module contains a set of methods that are 'mixed in' to various other classes. You don't have to understand how this works, but you need to know where and how to use Enumerable methods.

The Enumerable methods are extremely useful methods that can be used with ruby collections. All of the following can use Enumerable methods:

• Arrays
• Ranges
• Hashes

Generally, if the object has an #each method, then it probably has access to the Enumerable module.

For example, let's say you have an array of integers and you want to double each value. There's a handy Enumerable method we can use, called #map:

    [1, 2, 3].map { |el| el * 2 } # ==> [2, 4, 6]

Code Blocks

Code blocks used with Array and Range Enumerable methods use a single argument, but code blocks used with Hash and Enumerable methods use two arguments: one for keys and one for values.

    [1, 2, 3].select { |el| ... }          # ==> single argument in code block
    (0...100).select { |el| ... }          # ==> single argument in code block
    {"a" => "b"}.select { |key, val| ... } # ==> two arguments in code block

Useful Methods

#map

Returns an array with each element in the original collection is "mapped" by the given code block

    squares = (1..5).map { |el| el ** 2 } # ==> [1, 4, 9, 16, 25]

#select

Returns a new collection but only with the elements whose code block evaluated to true

    dictionary = {
      "apple" => "fruit",
      "ant" => "a bug",
      "banana" => "yellow fruit",
      "cat" => "fluffy animal"
    }

    a_words = dictionary.select do |key, val|
      key.start_with?("a")
    end

    p a_words # ==> { "apple" => "fruit", "ant" => "a bug"}

#reject

Returns a new collection but omits the the elements whose code block evaluated to true

    no_tens = (1..100).reject do |el|
      el % 10 == 0
    end

#any?

Returns true / false based on whether any of the elements in the collection force the code block to evaluate true

    words = ["pan", "pot", "spatula", "knife", "fork"]
    words.any? { |kitchen_item| kitchen_item == "knife" } # ==> true

    # At least one of the words is "knife"

#all?

Returns true / false based on whether all of the elements in the collection force the code block to evaluate true

    objects = [2, 4, 9, "5", 6]
    objects.all? { |obj| obj.is_a?(Integer) }
    # ==> false

    # Not all of the objects are Integers

#none?

Returns true / false based on whether none of the elements in the collection force the code block to evaluate true

    numbers = [12, 14, 16, 23]
    objects.none? { |n| n < 10 } # ==> true

    # None of the numbers are less than 10

#count

Counts the number of elements that evaluate true for a particular code block

    [1, 4, 5, 4, 3, 5, 6, 7].count { |n| n.even? } # ==> 3

    # There are 3 even numbers in the array

#with_index

Some enumerable methods are chainable with the #with_index method. Let's say we wanted to map an array to a new value where n ~~> n * index(n)

We could accomplish this using the #with_index method.

    result = [1, 5, 6, 8].map.with_index do |el, idx|
      el * idx
    end

    result # ==> [0, 5, 12, 24]

Try these in pry!