Enumerable

each_with_object(memo, &block)

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

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 78
def each_with_object(memo, &block)
  each do |element|
    block.call(element, memo)
  end
  memo
end

exclude?(object)

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

Source: show

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 104
def exclude?(object)
  !include?(object)
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

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 22
def group_by
  assoc = ActiveSupport::OrderedHash.new

  each do |element|
    key = yield(element)

    if assoc.has_key?(key)
      assoc[key] << element
    else
      assoc[key] = [element]
    end
  end

  assoc
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

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 92
def index_by
  Hash[map { |elem| [yield(elem), elem] }]
end

many?(&block)

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

Source: show

# File activesupport/lib/active_support/core_ext/enumerable.rb, line 98
def many?(&block)
  size = block_given? ? count(&block) : self.size
  size > 1
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

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

Module Enumerable