pick.rb

class Array
  def pick(number)
    result = []
    original = self.clone

    if number < original.size
      number.times { result << original.delete_at(Kernel.rand(original.size)) }
    else
      result = original
    end

    result
  end
end

[1, 2, 3, 4].pick(2)
=> [1, 2]

[1, 2, 3, 4].pick(2)
=> [4, 3]