ENumerators 

Enumerable #all? #any? #chunk #collect #collect_concat #count #cycle #detect #drop #drop_while #each_cons #each_entry #each_slice #each_with_index #each_with_object #entries #find #find_all #find_index #first #flat_map #grep #group_by #include? #inject #lazy #map #max #max_by #min_by #minmax #minmax_by #none? #one? #partition #reduce #reject #reverse_each #select #slice_before #sort #sort_by #take #take_while #to_a #zip 

Enumerator #each #feed #next #next_values #peek #peek_values #rewind #size #with_index #with_object 

enumerator = [1, 2, 3].each # => # enumerator = [1, 2, 3].to_enum # => # enumerator = [1, 2, 3].enum_for(:each) # => # 

def counter yield 1 yield 2 yield 3 end enumerator = enum_for(:counter) enumerator.to_a # => [1, 2, 3] 

with_object with_index 

letters = ['a', 'a', 'a', 'b', 'b', 'c'] counts = letters.inject({}) do |memo, letter| memo[letter] ||= 0 memo[letter] += 1 memo end # => {"a"=>3, "b"=>2, "c"=>1} 

letters = ['a', 'a', 'a', 'b', 'b', 'c'] counts = letters.each.with_object({}) do |letter, memo| memo[letter] ||= 0 memo[letter] += 1 end # => {"a"=>3, "b"=>2, "c"=>1} 

range = (1..10) range.map.with_index {|n, i| n * i } # => [0, 2, 6, 12, 20, 30, 42, 56, 72, 90] range.select.with_index {|_, i| i.even? } # => [1, 3, 5, 7, 9] 

Iterator 

enumerator = [1, 2, 3].each # => # enumerator.next # => 1 enumerator.next # => 2 enumerator.next # => 3 enumerator.next # StopIteration: iteration reached an end 

enumerator = [1, 2, 3].to_enum # => # enumerator.peek # => 1 enumerator.peek # => 2 enumerator.peek # => 3 enumerator.next # => 1 

loop do i = enumerator.next puts i # `loop` silently rescues StopIteration end 

Generator 

fibonacci = Enumerator.new(Float::INFINITY) do |yielder| a, b = 0, 1 loop do yielder.yield a a, b = b, (a + b) end end 

class SumOfNaturalNumbers < Enumerator def initialize super(Float::INFINITY) do |yielder| n = 1 loop do yielder.yield (n * (n + 1)) / 2 n += 1 end end end end 

def fibonacci a, b = 0, 1 loop do yield a a, b = b, (a + b) end end enumerator = enum_for(:fibonacci) # => # 

LAZY 

require 'prime' primes = Prime.instance primes .select {|i| i.to_s.end_with?('3') } .take(10) # infinite loop 

require 'prime' primes = Prime.instance primes .lazy .select {|i| i.to_s.end_with?('3') } .take(10) .to_a # => [3, 13, 23, 43, 53, 73, 83, 103, 113, 163] 

File.open('/usr/share/dict/words') .each_line .lazy .map(&:chomp) .take_while {|line| line.length < 10 } .to_a 

mine = ->(repository) { repository.owner == 'olly' } cutoff = (Time.now - (365 * 24 * 60 * 60)).to_datetime year_old = ->(repository) { repository.commits.first.date > cutoff } client.repositories .lazy .select(&mine) .select(&year_old) .take(10) .each {|repository| puts repository.name }