JavaScript Challenge: Cardinal Numbers

posted by Jonah Dempcy

This challenge is to write functions to add and subtract numbers using the names of numbers as strings. More precisely, the challenge is to write add() and subtract() functions that take two strings as input, and return one string. For example:

add("one", "ten"); // should return "eleven"
subtract("twenty", "fifteen"); // should return "five"

For the purposes of this exercise, we will limit the maximum numbers handled to 99. So, you don't have to handle, say, add("three-thousand-five-hundred-and-fifty-two", "ten-million-and-seven"). Though, you certainly get bonus points if you handle this case as well.

Here is my solution below, which is by no means optimal. The following is my first attempt at solving this issue, coded in roughly 30 minutes, so go easy if you find any SNAFUs.

Note: I prefer 4-space indentation but I set it to 2-space for this article so the code example will fit on the page without horizontal scrolling.

var cardinalNumbers = {
    "one": 1,
    "two": 2,
    "three": 3,
    "four": 4,
    "five": 5,
    "six": 6,
    "seven": 7,
    "eight": 8,
    "nine": 9,
    "ten": 10,
    "eleven": 11,
    "twelve": 12,
    "thirteen": 13,
    "fourteen": 14,
    "fifteen": 15,
    "sixteen": 16,
    "seventeen": 17,
    "eighteen": 18,
    "nineteen": 19
}

var multiplesOfTen = {
    "twenty": 20,
    "thirty": 30,
    "forty": 40,
    "fifty": 50,
    "sixty": 60,
    "seventy": 70,
    "eighty": 80,
    "ninety": 90
}
                        
function convertWordToNumber(word) {
  var number = new Number(); // this is where we will store the result              
  
  if (word.indexOf("-") == -1) { // If it is less than 20
    number = cardinalNumbers[word];
  } else {
    var multipleOfTen = word.split("-")[0];   // e.g. "seventy"
    var cardinalNumber = word.split("-")[1];   // e.g. "six"
    number = multiplesOfTen[multipleOfTen] + cardinalNumbers[cardinalNumber]; 
  } 
  
  return number;
}

function convertNumberToWord(number) {
  var word = new String(); // this is where we will store the result
    
  if (number < 20) {
    for (var cardinalNumber in cardinalNumbers) {
      if (number == cardinalNumbers[cardinalNumber]) {
        word = cardinalNumber;
        break;
      }
    }
  } else if (number < 100) {
    if (number % 10 == 0) { // If the number is a multiple of ten
      for (var multipleOfTen in multiplesOfTen) {
        if (number == multiplesOfTen[multipleOfTen]) {
          word = cardinalNumber;
          break;
        }
      }
    } else { // not a multiple of ten
      for (var multipleOfTen in multiplesOfTen) {
        for (var i = 9; i > 0; i--) {
          if (number == multiplesOfTen[multipleOfTen] + i) {
            word = multipleOfTen + "-" + convertNumberToWord(i);
            break;
          }
        }
      }
    }
  } else {
    alert("We don't handle numbers greater than 99 yet.");
  }
    
  return word;
}

// These functions takes words as inputs, e.g. "one" and "two"

function add(x, y) {
  return convertNumberToWord(convertWordToNumber(x) + convertWordToNumber(y));
}

function subtract(x, y) {
  return convertNumberToWord(convertWordToNumber(x) - convertWordToNumber(y));
}


// Test convertWordToNumber()
alert(convertWordToNumber("three"));               // should alert 3
alert(convertWordToNumber("seventy-three"));       // should alert 73   


// Test convertNumberToWord()
alert(convertNumberToWord(8));                     // should alert "eight"
alert(convertNumberToWord(46));                    // should alert "forty-six"


// Test add() 
alert(add("one", "two"));                          // should alert "three"

// Test subtract() 
alert(subtract("fifteen", "eleven"));              // should alert "four"

Feel free to post your solutions in the comments. Particularly good solutions will get an entire article devoted to them.