dot_product.rb

# Recursive
def dot_product(xs, ys, acc = 0)
  if xs.empty? or ys.empty?
    acc
  else
    dot_product(xs.drop(1), ys.drop(1), acc + xs.first * ys.first)
  end
end

# More idiomatic Ruby
def dot_product(xs, ys)
  xs.zip(ys).map{|n,m| n * m}.reduce(:+)
end