Ruby syntax for calling methods in classes

Question

I'm working on an exercise where I have to create a roman to arabic number converter. As far as I can tell, the code below is totally kosher, but I keep getting an error when I run my tests. Ruby thinks there's an undefined method or variable on line 37 (noted by comment below).

I'm wondering if my snytax is off or if it's something else. Suggestions?

class ArabicNumeral

  def replace_troublesome_roman_numerals(letters)
   tough_mappings = {"CM" => "DCCCC",  "CD" => "CCCC", "XC" => "LXXXX", "XL" => "XXXX", "IX"=> "VIIII", "IV" => "IIII"}
   tough_mappings.each { |roman, arabic|   letters = letters.gsub(roman, arabic) } 
   letters
  end

  def convert_and_add(letters)
    digits = { "M" => 1000, "CM" => 900, "D" => 500, "C" => 100, "XC" => 90, "L" => 50, "XL" => 40, "X" => 10, "IX" => 9, "V" => 5, "IV" =>  4, "I" => 1}
    letters = letters.split("")
    letters.inject(0) do |sum, letter|
      arabic = digits[letter]
      sum += arabic
    end  
  end

  def self.convert(letters)
    roman_string = replace_troublesome_roman_numerals(letters)  ###LINE 37!
    arabic_number = convert_and_add(roman_string)
    arabic_number
  end
end

Charles Caldwell · Accepted Answer · 2012-04-25 20:44:40Z

3

The problem here is the method you are calling on line 37. replace_troublesome_roman_numerals(letters). The problem is the method self.convert(letters) is a class method. You can call it like this:

ArabicNumeral.convert(letters)

However, it contains a call to an instance variable (being that replace_troublesome_roman_numerals(letters) I mentioned earlier.

def self.convert(letters)
    roman_string = ArabicNumeral.new.replace_troublesome_roman_numerals(letters)
    ArabicNumeral.new.convert_and_add(roman_string)

end

This creates an instance of ArabicNumeral and calls the method you need without saving it to a variable and taking up memory. I also removed the variable arabic_number from your method because you are calling convert_and_add(roman_string), adding it to the variable, and then returning the variable. since convert_and_add(roman_string) is the last thing handled by the method, it will return this anyway without the variable.

If you never plan on using those methods in an instance of ArabicNumeral then I would suggest making all the methods class level or wrapping them in a Module that you would include in your projects. If you don't plan on using them outside of the ArabicNumeral class at all, consider putting them behind a protected or private while leaving convert(letters) available.

class ArabicNumberal

def self.convert(letters)
   # Code...
end

private

def self.replace_troublesome_roman_numerals(letters)
   # Code...
end

def self.convert_and_add(roman_string)
   # Code...
end

end

edited Apr 25, 2012 at 20:44

answered Apr 25, 2012 at 20:33

Charles Caldwell

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1 Comment

Ben Downey Over a year ago

Thanks! I ended up bring all the methods up to "class level." I popped "self." in front of each method name and now all my tests pass.

Omar · Accepted Answer · 2012-04-25 20:26:07Z

Ok... First of all, you're trying to use an instance method from a class one.

The problem could be solved by changing the method convert from:

def self.convert(letters)
    roman_string = replace_troublesome_roman_numerals(letters)  ###LINE 37!
    arabic_number = convert_and_add(roman_string)
    arabic_number
end

To:

def convert(letters)
    roman_string = replace_troublesome_roman_numerals(letters)  ###LINE 37!
    arabic_number = convert_and_add(roman_string)
    arabic_number
end

Then you'll need to create an instance and call convert method:

x = ArabicalNumeral.new()
x.convert('param')

And that's it.

By the way, I suggest you to add a constructor method (in Ruby is named initialize).

The complete script below:

class ArabicNumeral

  def replace_troublesome_roman_numerals(letters)
   tough_mappings = {"CM" => "DCCCC",  "CD" => "CCCC", "XC" => "LXXXX", "XL" => "XXXX", "IX"=> "VIIII", "IV" => "IIII"}
   tough_mappings.each { |roman, arabic|   letters = letters.gsub(roman, arabic) } 
   letters
  end

  def convert_and_add(letters)
    digits = { "M" => 1000, "CM" => 900, "D" => 500, "C" => 100, "XC" => 90, "L" => 50, "XL" => 40, "X" => 10, "IX" => 9, "V" => 5, "IV" =>  4, "I" => 1}
    letters = letters.split("")
    letters.inject(0) do |sum, letter|
      arabic = digits[letter]
      sum += arabic
    end  
  end

  def convert(letters)
    roman_string = replace_troublesome_roman_numerals(letters)  ###LINE 37!
    arabic_number = convert_and_add(roman_string)
    arabic_number
  end
end

x = ArabicNumeral.new()
puts x.convert('MDC')

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