Enumerable

group_by()

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

e.g.

latest_transcripts.group_by(&:day).each do |day, transcripts| 
  p "#{day} -> #{transcripts.map(&:class) * ', '}"
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 rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 17
def group_by
  inject({}) do |groups, element|
    (groups[yield(element)] ||= []) << element
    groups
  end
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 rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 55
def index_by
  inject({}) do |accum, elem|
    accum[yield(elem)] = elem
    accum
  end
end

sum(identity = 0, &block)

Calculates a sum from the elements. Examples:

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

This is instead of payments.inject { |sum, p| sum + p.price }

Also calculates sums without the use of a block:

[5, 15, 10].sum # => 30

The default identity (sum of an empty list) is zero. However, you can override this default:

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

Source: show

# File rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 39
def sum(identity = 0, &block)
  return identity unless size > 0
  if block_given?
    map(&block).sum
  else
    inject { |sum, element| sum + element }
  end
end

Module Enumerable