Question

a = [4, 3, 2, nil]
a.max_by { |v| v * 2 } => NoMethodError: undefined method `*' for nil:NilClass

How to overload max_by to ignore nil values?

Answer 1

Welcome to Ruby: there are so many ways to solve the problem!

A very simple solution is:

a.max_by { |v| v.to_f * 2 }

since nil coerces to float as 0. This doesn’t handle negative values, but since nil is only a single instance of a class called NilClass, now as with all classes in Ruby we can open it an let it learn a littly maths:

class NilClass
  # overloading * operator
  def *(y)
    # returning negative infinity: Ruby 1.8.7
    -1.0/0.0
    # returning negative infinity: Ruby 1.9.2
    # -Float::INFINITY
  end
end

now we have

a.max_by { |v| v * 2 }

returning 4.