The document discusses the Enumerable module in Ruby, which provides collection-oriented methods like each, map, grep, find, all?, any?, and inject. It explains that classes like Array, Hash, and a custom MyCollection class can mix in Enumerable to gain these collection methods. It provides examples of using Enumerable methods on a vehicles array to select, transform, and iterate over elements in complex ways.

1. ENUMERABLE
A powerful Ruby Module for Collections

2. WHY USE ENUMERABLES
• Ruby's Enumerable module has methods for all kinds of tasks
which operate on a collection.
• If you can imagine a use for the #each method other than simply
iterating, there is a good chance a method exists to do what you
have in mind.

3. WHAT DOES ENUMERABLE
MEAN?
• Collection objects (instances of Array, Hash, etc) typically “mixin”
the Enumerable module
• The Enumerable module gives objects of collection classes
additional collection-specific behaviors.
• The class requiring the Enumerable module must have an #each
method because the additional collection-specific behaviors given
by Enumerable are defined in terms of #each

4. MIXING IN ENUMERABLE
class MyCollection
include Enumerable
#lots of code
def each
#more code
end
end

5. VIEW ALL CLASSES MIXING IN
ENUMERABLE
ObjectSpace.each_object(Class) do |cl|
puts cl if cl < Enumerable
end

6. Enumerable::Enumerator
Struct::Tms
Dir
File
IO
Range
Struct
Hash
Array
String
Struct::Group
Struct::Passwd
MyCollection
StringIO
Gem::SourceIndex
YAML::Set
YAML::Pairs
YAML::Omap
YAML::SpecialHas

7. TEST AN INSTANCE OR CLASS
>> a = [1,2,3]
=> [1, 2, 3]
>> a.respond_to? :any?
=> true
>> a.is_a? Enumerable
=> true
>> Array < Enumerable
=> true

8. ENUMERABLE METHODS
• Collection Behavior

9. EACH
• Classes that include the Enumerable module must have an #each
method.
• The #each method yields items to a supplied code block, one
at a time
• Different Classes define #each differently
• Array: #each yields each element
• Hash: each yields #each key/value pair as a two-element array
>> v_names = %w(car truck bike)
=> ["car", "truck", "bike"]
>> v_names.each do |vehicle|
?> puts vehicle
>> end

10. MAP
• The map method modifies each member according to
instructions in a block and returns the modified collection of
members.
>> v_names.map { |v| v.upcase}
=> ["CAR", "TRUCK", "BIKE"]

11. GREP
• The grep method 'searches' for members using a regular
expression.
>> v_names.grep /a/
=> ["car"]
>> v_names.grep(/a/) { |v| v.upcase}
=> ["CAR"]

12. FIND
>> v_names.find { |v| v.size > 3}
=> "truck"
>> v_names.find { |v| v.size > 2}
=> "car"
>> v_names.find do |v|
v.size > 3 && v.size < 5
end
=> "bike"

13. ALL?
• The all? method returns true if all of the members of a collection
satisfy the evaluation of the block. Otherwise it returns false.
>> v_names.all? { |v| v.length > 2}
=> true
>> v_names.all? { |v| v.length > 10}
=> false

14. ANY?
• The any? method returns true if any of the members of a
collection satisfy the evaluation of the block. Otherwise it returns
false.
>> v_names.any? { |v| v.length == 3}
=> true
>> v_names.any? { |v| v = "car"}
=> true

15. WORKING WITH COMPLEX
DATA
irb
>> load 'vehicles.rb'
=> true

16. INJECT
>> $vehicles.inject(0) do |total_wheels, v|
?> total_wheels += v[:wheels]
>> end
=> 10
>> $vehicles.inject([]) do |classes, v|
?> classes += v[:classes]
>> end.uniq
=> [:ground, :water, :air]

17. COMPLEX OPERATIONS
>> $vehicles.find do |v|
?> v[:name] =~ /Plane/
>> end[:name]
=> "Plane"
>> $vehicles.find_all do |v|
?> v[:name] =~ /Plane/
>> end.collect { |v| v[:name] }
=> ["Plane", "Sea Plane"]
>> $vehicles.find_all do |v|
?> v[:wheels] > 0
>> end.collect { |v| v[:name] }
=> ["Car", "Truck", "Bike"]

18. COMPLEX OPERATIONS
CONTINUED
>> $vehicles.find_all do |v|
?> v[:classes].include? :ground
>> end.collect { |v| v[:name] }
=> ["Car", "Truck", "Bike", "Sea Plane"]
>> $vehicles.find_all do |v|
?> v[:classes].include? :air
>> end.collect { |v| v[:name] }
=> ["Plane", "Helicopter", "Sea Plane"]