Enumerable

each_with_object(memo)

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

Source: show | on GitHub

    # File activesupport/lib/active_support/core_ext/enumerable.rb, line 79
79:   def each_with_object(memo)
80:     return to_enum :each_with_object, memo unless block_given?
81:     each do |element|
82:       yield element, memo
83:     end
84:     memo
85:   end

exclude?(object)

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

Source: show | on GitHub

     # File activesupport/lib/active_support/core_ext/enumerable.rb, line 114
114:   def exclude?(object)
115:     !include?(object)
116:   end

group_by()

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"

Source: show | on GitHub

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

index_by()

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 ...>, ...}

Source: show | on GitHub

    # File activesupport/lib/active_support/core_ext/enumerable.rb, line 94
94:   def index_by
95:     return to_enum :index_by unless block_given?
96:     Hash[map { |elem| [yield(elem), elem] }]
97:   end

many?()

Returns true if the enumerable has more than 1 element. Functionally equivalent to enum.to_a.size > 1. Can be called with a block too, much like any?, so people.many? { |p| p.age > 26 } returns true if more than one person is over 26.

Source: show | on GitHub

     # File activesupport/lib/active_support/core_ext/enumerable.rb, line 101
101:   def many?
102:     cnt = 0
103:     if block_given?
104:       any? do |element|
105:         cnt += 1 if yield element
106:         cnt > 1
107:       end
108:     else
109:       any?{ (cnt += 1) > 1 }
110:     end
111:   end

sum(identity = 0, &block)

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)

Source: show | on GitHub

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

Module Enumerable