This Roman Numerals Converter challenge is a great way to show you how to manipulate elements based on a particular number/value algorithm. In this post I show you how to solve this bonfire using the forEach method.

I must admit, this challenge could not have been done without a bit of help from the awesome Free Code Camp forum :) Like all challenges, it's always really, really, really hard to know how to begin. I just find it difficult to truly visualise the problem and understand the logic to unlock it.

In this post I break things down and take a much more deliberate approach to problem solving. So with that being said, lets jump into the challenge and unlock the logic!

How to solve the problem:

Step 1: Map out both the numbers and the numerals.

So first things first, what do we know about Roman Numerals? Well of course most people know that V = 5 and I = 1... But what about the bigger, more crazy numbers? If you're not sure about how to begin coding, I would definitely recommend getting out a pen and paper to simply scribble down what you know. In this case, here's a list of numbers and their RN translations:

// Write out what you know...
const numbers = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
const roman   = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];

In the challenge we are given the following tips to help guide us to the solution:

Step 2: Nut out the logic.

I'm sure there are a million different ways to solve this problem, but nonetheless, here's my pseudo logic:

  1. Create a numbers array that has the same number values as roman numeral values
    (eg) 5 = V, 10 = X
  2. Create a map or forEach loop to iterate over each number in the numbers array and compare the index against the roman numerals array.
  3. While the number to translate is greater tha or equal ton the individual number we are iterating over...
  4. Store the current roman numeral index in a holding variable (ie). tempValue = 'V'
  5. Subtract the number from the original number to convert (the num passed into the function as an argument).
  6. With the remaning number, if the value is greater than 1, keep looping and continue steps 3 - 5

In other words, Imagine the number was 10.

The first thing the function will do is check if 10 matches a roman numeral. In this case it does, so 10 will convert to X.

Imagine the number was 11.

The function will check if 11 matches, which it doesn't. The next highest roman numera after X is XL which is 40. Because 11 sits in between XL and X the function will add X to the result because it's greater than or equal to 10.

The function removes 10 from the original number which leaves 1 remaining. Again, we loop over the numbers and see if there is a match against the roman numerals. In this case, 1 is equal to I. The final result is XI.

You could easily use a for loop for this problem. However, the more I learn about ES6, the more I hear how awesome (and much more efficient) the new array helper methods like map, forEach and filter are. Therefore I decided to take this new approach and test my newly acquired ES6 knowledge. If you would like to learn more about ES6 then I highly recommend ES6 the course on Udemy.

Step 3: Test it out

Now to add the finishing touches: I realise that the result array could be an empty string or array. This code will work either way - as long as you have somewhere to store the result! The plan here is to find the matching numeral to the iterating number. If you can't find an exact numeral to match, then find the next closest. For example, if the number is 55. Begin with iterating until you find 50. Then you subtract the original number from the found number (ie) 55 - 50.

The result is 5. So, then we iterate again until we find the matching numeral, which in this case is "V". Here's what the resulting looks like from start to finish:

function convertToRoman(numToConvert) {
  // list all relevant numbers and numerals
  const numbers = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
  const roman = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];

  // Create an empty string for the result
  let result = '';

  // Loop through the numbers while the numToConvert is greater than the number, keep looping
  numbers.forEach(function(number, i) {
     while (numToConvert >= number) {
       // add numerals as you go eg. 55 --> loop numbers, find L, loop again, find V
       result += roman[i];
       numToConvert -= number;

  return result;

convertToRoman(52); // LII
convertToRoman(1120); // MCXX
convertToRoman(09); // IX

Yay! That's it :)

If you found this lesson helpful or have any questions please feel free to leave a comment below.

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