Ruby: Nested Arrays vs. Hash


So if you have been following my blog or paying attention to when these blogs are updated, you would have known that today was my very first day at DevBootCamp. To see a snapshot of what we did today, please check out this post, whereas this particular post will deal with the types of challenges we faced today.


Today we were given a set of challenges that all had one common factor, using Ruby to convert from one thing into something else. As there are no actual 'primitive' types in Ruby, we dealt with converting different types of objects.

One of the problems we were given was the Roman Numerals challenge, where we would create a method that would take an integer and convert it to a roman numeral. Since roman numerals are instances of Strings in ruby, we were basically asked to convert Integers into Strings.

The actual problem is as follows:

Create a method to_roman that takes an integer parameter and outputs its equivalent as a Roman Numeral.

A test that you can use to determine if your solution worked is as follows:

Since the integers that we are used to seeing in the western world are known as arabic numerals, we will call integers as arabic_numeral for our solutions.

Nested Array Solution

The first type of solution for this problem is to create a nested array. We would go about our solution like so.

Line 3 creates an array with nested_arrays as elements. Each nested_array within array has two elements; nested_array[0] is the arabic numeral and nested_array[1] refers to its corresponding roman numeral.

Line 8 sets up a .each loop for each nested_array element in array

Lines 9-10 creates an if conditional within the each loop where if the input arabic_numeral is greater than or equal to the arabic numeral in each nested_array, the corresponding roman numeral would be pushed to the result string. (arabic_numeral/nested_array[0]) on line 10 tells us how many times we should display the corresponding roman numeral, as the arabic numeral 4 => IIII, not just one I.

Line 11 removes the unneccessary roman numerals from the result string. Because of our conditional on line 9 and multiplier (arabic_numeral/nested_array[0]) on line 10, we will get extra roman numerals in our string. Ex: 10 will return XIXVVIVIVIIIIIIIIII whereas it should return X. the -= operator will cause our arabic_numeral to be set to 0 as it loops, and in this way we remove all the roman numerals after X. This only works if the nested_array elements in array are arranged in descending order, since each will loop through elements from beginning to end.

Hash Solution

The second type of solution for this problem is to create a nested array. We would go about our solution like so.

We set up a hash on lines 3-17 where the keys are arabic numbers and values are roman numerals. Line 18 will loop through the hash using an each method, and line 19 evaluates our arabic_numeral parameter against all our arabic_keys.If true, we will add the corresponding roman_value to our result string multiplied by the number of times it needs to be displayed. The modulo equals operator on line 21 sets new arabic_numeral to the remainder of being divided by our arabic key. This will limit the unneccessary roman numerals that would have attached after our correct numeral.

Special Case: method .divmod()

We can also use a special hash method called .divmod() to simplify our solution like so. (Credits to Tommy Dugger for showing me this solution

The num.divmod(integer) method will take whatever number its called upon and divide by argument integer. It returns an array of two elements, element[0] will be the result of division(quotient) and element[1] will be the remainder(modulo). It's a very fast way of doing the exact same thing as our hash solution.

Which Is Better?

After reviewing both possible solutions, I believe using a hash is the best solution for converting Integer to Strings. Though a nested array does the exact same thing in a same amount of time (.each will force a hash to loop through all key value pairs), the soution for the hash is more intuitive, especially using the .divmod() method.

Extension: Numbers to Words

If you are comfortable with converting arabic numerals to roman numerals, you can try a different challenge of converting arabic numerals to its correlated word. i.e. 5 would be converted to "five" and 103 would be converted to "one hundred three". This is a very similar problem to Roman Numerals, but you would need more hashes for each digits place in your arabic numeral and use conditionals to select the correct translation. Try it for yourself!