gistfile1.txt

class Array
  def split_into(n)
    (0...(s = (self.size.to_f/n).ceil)-1).inject([]) { |m, i| m << self.slice(s * i, s); m }
  end
end

class Array
  def split_into(n)
    size = (self.size.to_f/n).ceil
    result = []
    n.times do |x|
      result << self.slice(size * x, size)
    end
    result
  end
end

[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18].split_into(3)