Convenience methods for Ruby's Number class and 0

Question

I'm writing convenience methods to check if number is positive or negative like so:

class Numeric
  def positive?
    self > 0
  end

  def negative?
    self < 0
  end
end

but in this case I do not know how to handle cases like these:

>> 0.positive?
>> 0.negative?

Update: I've updated the typo in the class name. I used numeric because I needed to check the floats as well.

Answer 1

Ruby 2.3 introduced Numeric#positive? and Numeric#negative?, if you are on Ruby 2.3+, should use these two instead.

Answer 2

If the problem is that you're getting false for both, either you consider 0 to be positive or not. If so, you should have something like:

def positive?
    self >= 0
end

If not, leave it as it is, since 0 is neither positive not negative and you should return false for both.

However if the problem is that you're getting errors with 0.positive? (far more likely), the reason you're getting a problem is because 0 is a FixNum, not a Number. You can see that with the following message:

testprog.rb:12: undefined method `positive?' for 0:Fixnum (NoMethodError)

You should probably add it to Fixnum itself, or Integer, or Numeric, the base class for various numeric types like FixNum and BigNum. Where you inject your convenience methods depends on how widely you want them available.

For example, if you change your code to the following (I'm including test code here):

class Numeric
    def positive?
        self > 0
    end

    def negative?
        self < 0
    end
end

print " 0 positive?: ",  0.positive?,"\n"
print " 0 negative?: ",  0.negative?,"\n"
print " 0 zero?    : ",  0.zero?,"\n\n"

print "99 positive?: ", 99.positive?,"\n"
print "99 negative?: ", 99.negative?,"\n"
print "99 zero?    : ", 99.zero?,"\n\n"

print "-2 positive?: ", -2.positive?,"\n"
print "-2 negative?: ", -2.negative?,"\n"
print "-2 zero?    : ", -2.zero?,"\n\n"

it then works fine, outputting:

 0 positive?: false
 0 negative?: false
 0 zero?    : true

99 positive?: true
99 negative?: false
99 zero?    : false

-2 positive?: false
-2 negative?: true
-2 zero?    : false

as expected.