0.rb

enum = Enumerator.new do |yielder|
  yielder << "a"
  yielder << "b"
  yielder << "c"
end

enum.next # "a"
enum.next # "b"
enum.next # "c"
enum.next # StopIteration: iteration reached an end

1.rb

enum = Enumerator.new do |yielder|
  yielder << 1
  yielder << 2
end

loop do
  puts enum.next 
end

puts 'this will be printed out'

2.rb

array = [1, 2, 3]
enum = array.to_enum
enum.each {|n| n * 2} # 2 4 6

3.rb

array = [1, 2, 3]
enum = array.to_enum :map
enum.each {|n| n * 2}                        # [2, 4, 6]
enum.with_index {|n, index| [index, n ** 2]} # [[0, 1], [1, 4], [2, 9], [3, 16]]

4.rb

array = [1, 2, 3, 4]
enum = array.select
enum.each {|n| n % 2 == 0}          # [2, 4]

hash = { one: 1, two: 2 }
enum = hash.map
enum.each {|_, value| value * 10}  # [10, 20]

range = 1..10
enum = range.drop_while
enum.each {|n| n < 7}               # [7, 8, 9, 10]

file = File.new 'Rakefile'
enum = file.reject
enum.each {|line| line.length < 10} # ["#!/usr/bin/env rake\n",
                                       "# Add your own tasks in files placed in lib/tasks ending in .rake,\n",
                                       ...]

Struct.new 'Foo', :name, :age
foo = Struct::Foo.new 'foo', 42
enum = foo.cycle
enum.take 5                         # ["foo", 42, "foo", 42, "foo"]

5.rb

enum = 5.times
enum.map { [] }      # [[], [], [], [], []]

enum = 1.upto 10
enum.inject(:+)      # 55

enum = 5.downto 0
enum.map {|n| n * 2} # [10, 8, 6, 4, 2, 0]

6.rb

enum = "hello".each_char
enum.map &:upcase                    # ["H", "E", "L", "L", "O"]

enum = "abc".each_byte
enum.map {|n| n}                     # [97, 98, 99]

enum = "one\ntwo\nthree".each_line
enum.map {|line| line.chomp.reverse} # ["eno", "owt", "eerht"]

enum = 'a'.upto 'e'
enum.grep /[aeiou]/                  # ["a", "e"]

enum = "one two three".gsub /\b\w+\b/
enum.next # "one"
enum.next # "two"
enum.next # "three"

7.rb

enum = Enumerator.new do |yielder|
  n = 0
  loop do
    yielder << n
    n += 1
  end
end

enum.first 10  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

8.rb

enum.select {|n| n % 20 == 0}.first 5   # never returns

9.rb

class Enumerator 
  def lazy_select(&blk)
    self.class.new do |yielder| 
      each do |n| 
        yielder << n if blk[n]
      end 
    end 
  end 
end

enum.lazy_select {|n| n % 20 == 0}.first 5 # [0, 20, 40, 60, 80]