Enumerable#bucket_by (and #uniq_by and #uniq_by!)

I just learned about Enumerable#partition and immediately had the desire to partition into more than two parts. The #partition method is certainly ideal for implementing a quicksort, but for general bucketing there is a need for something less boolean:

module Enumerable
  def bucket_by
    hash = Hash.new { |h,k| h[k] = [] }
    each { |v| hash[yield(v)] << v }
    hash.default = nil
    hash
  end
end

The bucket_by method takes a block and returns a hash of arrays of objects keyed by the block return values for each element. Simple and convenient. And hey, while we're using blocks for stand-in values and _by suffixes:

module Enumerable
  def uniq_by
    seen = {}
    select { |v|
      key = yield(v)
      (seen[key]) ? nil : (seen[key] = true)
    }
  end
end
class Array
  def select!
    reject! { |v| not yield(v) }
  end
  def uniq_by!
    seen = {}
    select! { |v|
      key = yield(v)
      (seen[key]) ? nil : (seen[key] = true)
    }
  end
end

Enjoy!

Update! See this post with a slightly cleaner implementation.