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

title
Roman Numeral Converter

Remember to use Read-Search-Ask if you get stuck. Try to pair program and write your own code

Problem Explanation:

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

Relevant Links

• Roman Numerals
• Array.splice()
• Array.indexOf()
• Array.join()

Hint: 1

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

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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.

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Hint: 3

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

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Spoiler Alert!

Solution ahead!

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);

Run Code

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

• For Loops
• While Loops

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);

Run Code

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

• For Loops
• While Loops
• Math

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);

Run Code

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

• For Loops
• .split()
• .reverse()
• .map()
• .toString()
• parseInt()
• .replace()
• .repeat()

NOTES FOR CONTRIBUTIONS:

• 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.
• Please add your username only if you have added any relevant main contents. ( DO NOT remove any existing usernames)

See Wiki Challenge Solution Template for reference.