Question

When compare two version strings, to_f does not work well:

 > "1.5.8".to_f > "1.5.7".to_f
  => false

string comparison is better, but not always correct:

 > "1.5.8" > "1.5.7"
  => true 

 > "1.5.8" > "1.5.9"
  => false 

 > "1.5.8" > "1.5.10" # oops!
  => true 

How to compare version strings properly?

Answer 1

An idea: create a Object#compare_by method that behaves like compare (aka the spaceship operator Object#<=>) but takes a custom block:

class Object
  def compare_by(other)
    yield(self) <=> yield(other)
  end
end

>> "1.5.2".compare_by("1.5.7") { |s| s.split(".").map(&:to_i) }
#=> -1

You can also take a more specific approach still based on the compare method:

class String
  def compare_by_fields(other, fieldsep = ".")
    cmp = proc { |s| s.split(fieldsep).map(&:to_i) }
    cmp.call(self) <=> cmp.call(other)
  end
end

>> "1.5.8".compare_by_fields("1.5.8")
#=> 0