freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/javascript-algorithms-and-data-structures-projects/roman-numeral-converter/index.md

---
title: Roman Numeral Converter
---
![:triangular_flag_on_post:](https://forum.freecodecamp.com/images/emoji/emoji_one/triangular_flag_on_post.png?v=3 ":triangular_flag_on_post:") Remember to use <a>**`Read-Search-Ask`**</a> if you get stuck. Try to pair program ![:busts_in_silhouette:](https://forum.freecodecamp.com/images/emoji/emoji_one/busts_in_silhouette.png?v=3 ":busts_in_silhouette:") and write your own code ![:pencil:](https://forum.freecodecamp.com/images/emoji/emoji_one/pencil.png?v=3 ":pencil:")

### ![:checkered_flag:](https://forum.freecodecamp.com/images/emoji/emoji_one/checkered_flag.png?v=3 ":checkered_flag:") Problem Explanation:

You will create a program that converts an integer to a Roman Numeral.

#### Relevant Links

*   <a href='http://www.mathsisfun.com/roman-numerals.html' target='_blank' rel='nofollow'>Roman Numerals</a>
*   <a href='http://forum.freecodecamp.com/t/javascript-array-prototype-splice/14307' target='_blank' rel='nofollow'>Array.splice()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf' target='_blank' rel='nofollow'>Array.indexOf()</a>
*   <a href='http://forum.freecodecamp.com/t/javascript-array-prototype-join/14292' target='_blank' rel='nofollow'>Array.join()</a>

## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:") Hint: 1

Creating two arrays, one with the Roman Numerals and one with the decimal equivalent for the new forms will be very helpful.

> _try to solve the problem now_

## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:") Hint: 2

If you add the numbers to the arrays that go before the new letter is introduced, like values for 4, 9, and 40, it will save you plenty of code.

> _try to solve the problem now_

## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:") Hint: 3

You can't have more than three consecutive Roman numerals together.

> _try to solve the problem now_

## Spoiler Alert!

![warning sign](//discourse-user-assets.s3.amazonaws.com/original/2X/2/2d6c412a50797771301e7ceabd554cef4edcd74d.gif)

**Solution ahead!**

## ![:beginner:](https://forum.freecodecamp.com/images/emoji/emoji_one/beginner.png?v=3 ":beginner:") Basic Code Solution:

    var convertToRoman = function(num) {

      var decimalValue = [ 1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1 ];
      var romanNumeral = [ 'M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I' ];

      var romanized = '';

      for (var index = 0; index < decimalValue.length; index++) {
        while (decimalValue[index] <= num) {
          romanized += romanNumeral[index];
          num -= decimalValue[index];
        }
      }

      return romanized;
    }

    // test here
    convertToRoman(36);

![:rocket:](https://forum.freecodecamp.com/images/emoji/emoji_one/rocket.png?v=3 ":rocket:") <a href='https://repl.it/CLmf/0' target='_blank' rel='nofollow'>Run Code</a>

### Code Explanation:

*   We start off by creating two arrays with default conversion with matching indices. These are called `decimalValue` and `romanNumeral`. We also create an empty string variable, `romanized`, which will house the final roman number.
*   Using a for loop, we loop through the indicies of the `decimalValue` array. We continue to loop until while the value at the current `index` will fit into `num`.
*   Next, we add the roman numeral and decrease `num` by the decimal equivalent.
*   Finally, we return the value of `romanized`.

#### Relevant Links

*   <a href='http://forum.freecodecamp.com/t/javascript-for-loop/14666' target='_blank' rel='nofollow'>For Loops</a>
*   <a>While Loops</a>

## ![:sunflower:](https://forum.freecodecamp.com/images/emoji/emoji_one/sunflower.png?v=3 ":sunflower:") Intermediate Code Solution:

    function convertToRoman(num) {
     var romans = ["I", "V", "X", "L", "C", "D", "M"],
         ints = [],
         romanNumber = [],
         numeral = "";
      while (num) {
        ints.push(num % 10);
        num = Math.floor(num/10);
      }
      for (i=0; i<ints.length; i++){
          units(ints[i]);
      }
      function units(){
        numeral = romans[i*2];
        switch(ints[i]) {
          case 1:
            romanNumber.push(numeral);
            break;
          case 2:
            romanNumber.push(numeral.concat(numeral));
            break;
          case 3:
            romanNumber.push(numeral.concat(numeral).concat(numeral));
            break;
          case 4:
            romanNumber.push(numeral.concat(romans[(i*2)+1]));
            break;
          case 5:
            romanNumber.push(romans[(i*2)+1]);
            break;
          case 6:
            romanNumber.push(romans[(i*2)+1].concat(numeral));
            break;
          case 7:
            romanNumber.push(romans[(i*2)+1].concat(numeral).concat(numeral));
            break;
          case 8:
            romanNumber.push(romans[(i*2)+1].concat(numeral).concat(numeral).concat(numeral));
            break;
          case 9:
            romanNumber.push(romans[i*2].concat(romans[(i*2)+2]));
          }
        }
      return romanNumber.reverse().join("").toString();
    }

    // test here
    convertToRoman(97);

![:rocket:](https://forum.freecodecamp.com/images/emoji/emoji_one/rocket.png?v=3 ":rocket:") <a href='https://repl.it/C1YV' target='_blank' rel='nofollow'>Run Code</a>

### Code Explanation:

*   Create an array of Roman Numerals (`romans`).
*   Use a for loop to create an array of the digits (`ints`) in the number.
*   Loop through the array of digits (base 10) and as you do, increment the Roman Numeral (base 5) index by 2 (`numeral = romans[i*2]`).
*   Within the loop, use Switch Case to push the proper Roman Numerals (backwards) onto that array.
*   Reverse the Roman Numerals array and turn it into a string.

#### Relevant Links

*   <a href='http://forum.freecodecamp.com/t/javascript-for-loop/14666' target='_blank' rel='nofollow'>For Loops</a>
*   <a>While Loops</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math' target='_blank' rel='nofollow'>Math</a>

## ![:sunflower:](https://forum.freecodecamp.com/images/emoji/emoji_one/sunflower.png?v=3 ":sunflower:") Intermediate Code Solution:

    function convertToRoman(num) {
      var romans = 
      // 10^i 10^i*5
        ["I", "V"], // 10^0
        ["X", "L"], // 10^1
        ["C", "D"], // 10^2
        ["M"]       // 10^3
      ],
          digits = num.toString()
            .split('')
            .reverse()
            .map(function(item, index) {
              return parseInt(item);
            }),
          numeral = "";

      // Loop through each digit, starting with the ones place
      for (var i=0; i<digits.length; i++) {
        // Make a Roman numeral that ignores 5-multiples and shortening rules
        numeral = romans[i][0].repeat(digits[i]) + numeral;
        // Check for a Roman numeral 5-multiple version
        if (romans[i][1]) {
          numeral = numeral
            // Change occurrences of 5 * 10^i to the corresponding 5-multiple Roman numeral
            .replace(romans[i][0].repeat(5), romans[i][1])
            // Shorten occurrences of 9 * 10^i
            .replace(romans[i][1] + romans[i][0].repeat(4), romans[i][0] + romans[i+1][0])
            // Shorten occurrences of 4 * 10^i
            .replace(romans[i][0].repeat(4), romans[i][0] + romans[i][1]);
        }
      }

      return numeral;
    }

    // test here
    convertToRoman(36);

![:rocket:](https://forum.freecodecamp.com/images/emoji/emoji_one/rocket.png?v=3 ":rocket:") <a href='https://repl.it/C1YV' target='_blank' rel='nofollow'>Run Code</a>

### Code Explanation:

*   Use an array (`romans`) to create a matrix containing the Roman numeral for a given power of 10 and, if available, the Roman numeral for that power of 10 times 5.
*   Convert the input number (`num`) to a reversed array of digits (`digits`) so that we can loop through those digits starting with the ones position and going up.
*   Loop through each digit, starting with the ones place, and create a Roman numeral string by adding each higher-power Roman numeral to the start of the `numeral` string a number of times equal to `digit`. This initial string ignores the Roman numerals that are a power of 10 times 5 and also ignores shortening rules.
*   If the relevant power of 10 has a 5-multiple Roman numeral, in `numeral`, replace 5-in-a-row occurrences with the relevant 5-multiple Roman numeral (i.e., V, L, or D) and shorten occurrences of 9 * 10^i (e.g., VIIII to VIX) and 4 * 10^i (e.g., XXXX to XL). Order is important here!
*   Finally, return `numeral`.

#### Relevant Links

*   <a href='http://forum.freecodecamp.com/t/javascript-for-loop/14666' target='_blank' rel='nofollow'>For Loops</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split' target='_blank' rel='nofollow'>.split()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reverse' target='_blank' rel='nofollow'>.reverse()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map' target='_blank' rel='nofollow'>.map()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString' target='_blank' rel='nofollow'>.toString()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt' target='_blank' rel='nofollow'>parseInt()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace' target='_blank' rel='nofollow'>.replace()</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat' target='_blank' rel='nofollow'>.repeat()</a>

## ![:clipboard:](https://forum.freecodecamp.com/images/emoji/emoji_one/clipboard.png?v=3 ":clipboard:") NOTES FOR CONTRIBUTIONS:

*   ![:warning:](https://forum.freecodecamp.com/images/emoji/emoji_one/warning.png?v=3 ":warning:") **DO NOT** add solutions that are similar to any existing solutions. If you think it is **_similar but better_**, then try to merge (or replace) the existing similar solution.
*   Add an explanation of your solution.
*   Categorize the solution in one of the following categories — **Basic**, **Intermediate** and **Advanced**. ![:traffic_light:](https://forum.freecodecamp.com/images/emoji/emoji_one/traffic_light.png?v=3 ":traffic_light:")
*   Please add your username only if you have added any **relevant main contents**. (![:warning:](https://forum.freecodecamp.com/images/emoji/emoji_one/warning.png?v=3 ":warning:") **_DO NOT_** _remove any existing usernames_)

> See ![:point_right:](https://forum.freecodecamp.com/images/emoji/emoji_one/point_right.png?v=3 ":point_right:") <a href='http://forum.freecodecamp.com/t/algorithm-article-template/14272' target='_blank' rel='nofollow'>**`Wiki Challenge Solution Template`**</a> for reference.