A function to convert a number to a roman numeral

0-README.md

Roman numerals

1..toRoman() // "I"
18..toRoman() // "XVIII"
1910..toRoman() // "MCMX"
9999..toRoman() // "MMMMMMMMMCMXCIX"

Inspiration

1-toRoman.js

const romanNumerals = [
  ["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"], // ones
  ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"], // tens
  ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"], // hundreds
];

Number.prototype.toRoman = function() {
  return this
    .toString()
    .split("")
    .reverse()
    .reduce((acc, x, i) => (i < 3 ? romanNumerals[i][x] : "M".repeat(x)) + acc, "");
};

String.prototype.toRomanNumeral = function() {
  let str = this;
  const sortLength = (a, b) => b.length - a.length;
  const numeral = romanNumerals.reduce((acc, x) => {
    const check = x.filter(y => ~str.indexOf(y)).sort(sortLength)[0];
    if (check) str = str.slice(0, -check.length);
    return x.indexOf(check) + acc;
  }, '');
  return parseInt(numeral, 10) + str.match(/^M*/)[0].length * 1000;
}

2-ToRoman.cs

using System;
using System.Collections.Generic;
using Linq;

// This function needs testing!
public static string ToRoman (int n) {
    var romanNumerals = new string[][] {
        new string[] { "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" }, // ones
        new string[] { "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" }, // tens
        new string[] { "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" }, // hundreds
    };
    // This needs more chaining
    var chars = new List<char>();
    chars.AddRange(n.ToString());
    chars.Reverse();
    chars.Aggregate((acc, x, i) => (i < 3 ? romanNumerals[i][Int32.Parse(x)] : new String ('M', Int32.Parse(x))) + acc, "");
    return String.Join("", chars);
}