How do I setup x.to_amethod where x is an integer?

1

I’m going through the Ruby Track on Exercism and the exercise for converting arabic to roman numerals makes assertions on x.to_roman where x is an integer and to_roman is the translation method. I can tackle the translation, but I have no idea of how to design the x.to_method call: it doesn’t seem to involve a class or a module. The tests give to_roman no parameter, so what do I setup so that it knows what integer to translate?

2

Are you looking to add a method to a class? The following adds to_roman as a method on the class of Integers. self in the method is the integer.

2.7.2 :001 > class Integer
2.7.2 :002 >   def to_roman()
2.7.2 :003 >     puts "Current number #{self}"
2.7.2 :004 >   end
2.7.2 :005 > end
 => :to_roman 
2.7.2 :006 > 3.to_roman()
Current number 3
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3

Thank you. I didn’t know that you could add methods to classes not your own. How freely can you do this?

4

Yes, Ruby’s classes are known as ‘Open Classes’, and adding methods like this is ‘monkey patching’. It’s a standard meta-programming technique in Ruby.

You can do this anytime, to any class (at least, I don’t know of any way to stop it):

2.7.2 :001 > class A
2.7.2 :002 >   def me()
2.7.2 :003 >     puts "in A"
2.7.2 :004 >   end
2.7.2 :005 > end
2.7.2 :006 > x = A.new
 => #<A:0x000055c3d6f25338> 
2.7.2 :007 > x.me()
in A
 => nil 
2.7.2 :008 > class A
2.7.2 :009 >   def me_too()
2.7.2 :010 >     puts "also in A"
2.7.2 :011 >   end
2.7.2 :012 > end
 => :me_too 
2.7.2 :013 > x.me_too()
also in A
 => nil

notice this works even though x was created before the method was defined.

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