Dynamically create a Numeric given a class

Question

Given a string '123', I can create a Float or Integer:

x = Float('123.45')
y = Integer('123')

As an exercise in dynamic typing, I want to extend Numeric, the top-level number class, with a method that converts the number to a string, reverses it, and then back to its original type.

This would allow me to do this:

x = (123).reverse
// x == 321

or this:

y = (54.321).reverse
// y == 123.45

One implementation looks like this: (this works correctly)

class Numeric
  def reverse
    str = self.to_s.reverse

    if self.is_a?(Float)
      return Float(str) # or str.to_f
    elsif self.is_a?(Integer)
      return Integer(str) # or str.to_i
    end
  end
end

But I want to create the result dynamically rather than checking a list of types. I tried using Class.new():

class Numeric
  def reverse
    str = self.to_s.reverse
    self.class.new(str)
  end
end

I thought this would work since I can call Float('123.45') or Integer('123'). However, I get these errors:

irb(main):047:0> (54.321).reverse
NoMethodError: undefined method `new' for Float:Class
    from (irb):44:in `reverse'
    from (irb):47
    from /Users/aaron/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'
irb(main):048:0> (123).reverse
NoMethodError: undefined method `new' for Fixnum:Class
    from (irb):44:in `reverse'
    from (irb):48
    from /Users/aaron/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'

So I have two questions:

How does Float('123.45') or Integer('123') work if they don't implement new?
How can I implement Numeric.reverse() without any conditionals?

I know there are other cases where this won't work (like negative numbers), but I'm not concerned with those issues (yet).

It also doesn't work for Float::INFINITY. Also a good answer to the first question can be found here. — cremno
– cremno, Commented Jun 30, 2015 at 0:41
And since they are just methods, you can dynamically call them with Object#send. But then it still won't work for integers since they're not actually instances of Integer. — cremno
– cremno, Commented Jun 30, 2015 at 0:44

Piotr Kruczek · Accepted Answer · 2015-06-30 09:08:21Z

1

How does Float('123.45') or Integer('123') work if they don't implement new?

They call the Kernel methods Kernel::Float and Kernel::Integer respectively, more info here
How can I implement Numeric.reverse() without any conditionals?

Like this:
```
class Numeric
  def reverse
    str = self.to_s.reverse
    method(self.class.to_s).call(str)
  end
end
```
The method method converts string or symbol to a method, which then can be called using call and passing the string you want to it.

Example: method((123.2).class.to_s).call('321') # => 321.0

answered Jun 30, 2015 at 9:08

Piotr Kruczek

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10 Comments

Vamsi Krishna Over a year ago

What if the number has leading zeros...say '0112'

Piotr Kruczek Over a year ago

While casting it on Float or Integer all leading zeros will be omitted as expected

Vamsi Krishna Over a year ago

(0112).reverse NameError: undefined method 'Fixnum' for class 'Fixnum'

gunn Over a year ago

@VamsiKrishna that's just because 112.class == Fixnum

Piotr Kruczek Over a year ago

Thats true, Kernel doesn't have a Fixnum method, just an Integer. Not sure yet how to get around that..

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