Enumerable

Iterates over a collection, passing the current element and the memo to the block. Handy for building up hashes or reducing collections down to one object. Examples:

  %w(foo bar).each_with_object({}) { |str, hsh| hsh[str] = str.upcase }
  # => {'foo' => 'FOO', 'bar' => 'BAR'}

Note that you can’t use immutable objects like numbers, true or false as the memo. You would think the following returns 120, but since the memo is never changed, it does not.

  (1..5).each_with_object(1) { |value, memo| memo *= value } # => 1

    # File lib/active_support/core_ext/enumerable.rb, line 78
78:   def each_with_object(memo, &block)
79:     each do |element|
80:       block.call(element, memo)
81:     end
82:     memo
83:   end

The negative of the Enumerable#include?. Returns true if the collection does not include the object.

     # File lib/active_support/core_ext/enumerable.rb, line 107
107:   def exclude?(object)
108:     !include?(object)
109:   end

Collect an enumerable into sets, grouped by the result of a block. Useful, for example, for grouping records by date.

Example:

  latest_transcripts.group_by(&:day).each do |day, transcripts|
    p "#{day} -> #{transcripts.map(&:class).join(', ')}"
  end
  "2006-03-01 -> Transcript"
  "2006-02-28 -> Transcript"
  "2006-02-27 -> Transcript, Transcript"
  "2006-02-26 -> Transcript, Transcript"
  "2006-02-25 -> Transcript"
  "2006-02-24 -> Transcript, Transcript"
  "2006-02-23 -> Transcript"

    # File lib/active_support/core_ext/enumerable.rb, line 22
22:   def group_by
23:     assoc = ActiveSupport::OrderedHash.new
24: 
25:     each do |element|
26:       key = yield(element)
27: 
28:       if assoc.has_key?(key)
29:         assoc[key] << element
30:       else
31:         assoc[key] = [element]
32:       end
33:     end
34: 
35:     assoc
36:   end

Convert an enumerable to a hash. Examples:

  people.index_by(&:login)
    => { "nextangle" => <Person ...>, "chade-" => <Person ...>, ...}
  people.index_by { |person| "#{person.first_name} #{person.last_name}" }
    => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}

    # File lib/active_support/core_ext/enumerable.rb, line 92
92:   def index_by
93:     inject({}) do |accum, elem|
94:       accum[yield(elem)] = elem
95:       accum
96:     end
97:   end

Returns true if the collection has more than 1 element. Functionally equivalent to collection.size > 1. Works with a block too ala any?, so people.many? { |p| p.age > 26 } # => returns true if more than 1 person is over 26.

     # File lib/active_support/core_ext/enumerable.rb, line 101
101:   def many?(&block)
102:     size = block_given? ? select(&block).size : self.size
103:     size > 1
104:   end

Calculates a sum from the elements. Examples:

 payments.sum { |p| p.price * p.tax_rate }
 payments.sum(&:price)

The latter is a shortcut for:

 payments.inject(0) { |sum, p| sum + p.price }

It can also calculate the sum without the use of a block.

 [5, 15, 10].sum # => 30
 ["foo", "bar"].sum # => "foobar"
 [[1, 2], [3, 1, 5]].sum => [1, 2, 3, 1, 5]

The default sum of an empty list is zero. You can override this default:

 [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)

    # File lib/active_support/core_ext/enumerable.rb, line 57
57:   def sum(identity = 0, &block)
58:     if block_given?
59:       map(&block).sum(identity)
60:     else
61:       inject { |sum, element| sum + element } || identity
62:     end
63:   end