Ruby Enumerable

Very basic talk on Ruby enumerable module.

Transcript

1. In The Wild Example of classes that inherit from Enumerable: Array # [1,2,3] Hash # { a: “hash” } Range # (3..1) File # File.new('myfile.txt') Dir # Dir.pwd Struct # Struct.new(:attrib1, :attrib2)
2. any?, all?, include?, none? ["is", "there"].any? { |word| word > 4 } # => true [1,2,3].all? { |i| i.is_a? Integer } # => true ["does", "this"].include? { |a| a == "stutter" } # => false ["I", "have"].none? { |word| word == "none" } # => true
3. map, select, reject, inject select returns a new array [1, "two", 3, "four"].select { |element| element.is_a? String } # => ["two", "four"] reject does opposite of select [1, "two", 3, "four"].reject { |element| element.is_a? String } # => [1, 3] map applies block to each element and returns new array ["DONT", "SHOUT"].map { |word| word.downcase } # => ["dont", "shout"] inject...well, does this ["pay", "attention"].inject(0) do |counter, word| counter += word.length end
4. cycle, zip, partition cycle applies block to each element n times (note: forever as default!) ["a", "b", "c"].cycle { |word| puts word } # => "a", "b", "c", "a", "b" ... forever ["a", "b", "c"].cycle(2) { |word| puts word } # => "a", "b", "c", "a", "b", "c" zip merges elements of multiple arrays [1,3,5].zip([2,4,6]) # => [[1, 2], [3, 4], [5, 6]] partition returns two arrays. First where passed block is true. Second for rest. (1..6).partition? { |i| i.even? } # => [[2, 4, 6], [1, 3, 5]]
5. These are aliased methods (uncommon → common): collect → map detect → find find_all → select reduce → inject
6. Mixin Example class DogEnumerator include Enumerable def initialize(dogs) @dogs = dogs end def each(&block) @dogs.each do |dog| if block_given? block.call(dog) else yield dog end end end end dogs = DogEnumerator.new(["Scooby", "Pluto"]) dogs.include? "Scooby" # => true • each(...) method to the left must be defined for class to behave correctly. • When you include Enumerable module you get a bunch of helpful methods that allow your class to behave like an enumerator. • Array inherits from Enumerable and makes heavy use of this functionality so all your favourite methods are available (any?, all?, include?, select) • Note: there are differences. Bang/dangerous methods are not available in Enumerable. Why?
7. Enumerable and comparisons When using Enumerable max/min/sort functionality, we need to additionally implement a three-way comparison method '<=>'. class FilmEnumerator include Enumerable def initialize @films = [ "1984", "2001: Space Odyssey", "1492: Conquest of Paradise" ] end def each... (references @films) def <=>(this, other) # turn back this.match(/\d{4}/)[0].to_i <=> other.match(/\d{4}/)[0].to_i end end fe = FilmEnumerator.new fe.sort # => ["1492: Conquest of Paradise", "1984", "2001: Space Odyssey"] '<=>' is called a spaceship operator/method, which has the following returns: a <=> b if a < b then return -1 if a == b then return 0 if a > b then return 1
8. Suggested Reading • http://ruby-doc.org/core-2.1.3/Enumerable.html • Punish yourself: • Enumerable vs Enumerator • Tutorials: • http://ruby.bastardsbook.com/chapters/enumerables/ • https://practicingruby.com/articles/building-enumerable- and-enumerator
9. THANKS! Don't you dare ask me any questions (unless they're very very easy)