Numbers to words

convertToWords

using System;
using System.Collections.Generic;
using System.Text;

// Convert a number to its english form
// 123 -> One hundred twenty three
// 123456 - > One hundred twenty three thousand four hundred fifty six
// 1234567 - > One million two hundred thirty four thousand five hundred sixty seven
// 12345678 -> twelve million three hundred forty five thousand six hundred seventy eight
// 123456789 -> one hundred twenty three million four hundred fifty six thousand seven hundred eighty nine
// 1234567890 -> one billion two hundred thirty four million five hundred sixty seven thousand eight hundred ninety

namespace numbersToWords
{
    public class Program
    {   
        static Dictionary<int, string> map; 
         
        public static void Main(string[] args)
        {
            map = new Dictionary<int, string>();
            
            map.Add(0, "");
            map.Add(1, "one");
            map.Add(2, "two");
            map.Add(3, "three");
            map.Add(4, "four");
            map.Add(5, "five");
            map.Add(6, "six");
            map.Add(7, "seven");
            map.Add(8, "eight");
            map.Add(9, "nine");
            map.Add(10, "ten");
            map.Add(11, "eleven");
            map.Add(12, "twelve");
            map.Add(13, "thirteen");
            map.Add(14, "fourteen");
            map.Add(15, "fifteen");
            map.Add(16, "sixteen");
            map.Add(17, "seventeen");
            map.Add(18, "eighteen");
            map.Add(19, "nineteen");
            map.Add(20, "twenty");
            map.Add(30, "thirty");
            map.Add(40, "forty");
            map.Add(50, "fifty");
            map.Add(60, "sixty");
            map.Add(70, "seventy");
            map.Add(80, "eighty");
            map.Add(90, "ninety");
            map.Add(100, "hundred");
            map.Add(1000, "thousand");
            map.Add(1000000, "million");
            map.Add(1000000000, "billion");
            
            test(123);
            test(1234);
            test(12345);
            test(123456);
            test(1234567);
            test(12345678);
            test(123456789);
            test(1234567890);
        }
        
        static void test(int num) {
            Console.WriteLine("Converted form of {0} is {1}", num, convertToWords(num));
        }
        
        static string convertToWords(int number) {
           StringBuilder result = new StringBuilder("");
           
           int position = 1;
           
           while (number > 0) {
               int part = number % 1000;
               result.Insert(0, convertThreeDigit(part) + " ");
               number = number / 1000;
               
               position *= (int)Math.Pow(10, 3);
               
               if (number > 0) {
                   result.Insert(0, map[position] + " ");
               }
           }
           
           return result.ToString();
        } 
        
        static string convertThreeDigit(int num) {
            
            if (map.ContainsKey(num)) {
                return map[num];
            }
            
            StringBuilder result = new StringBuilder("");   
            int position = 0; 
            
            if ((num % 100) >= 11 && (num % 100) <= 19) {
                int part = num % 100;
                result.Insert(0, map[part] + " ");
                num = num / 100;
                position += 2;
            }
            
            while (num > 0) {
            
                if (position == 2) {
                    result.Insert(0, "hundred ");   
                    result.Insert(0, map[num] + " "); 
                } else  {
                    int part = (num % 10) * (int)Math.Pow(10, position);
                    var str = map[part];
                    
                    if (!String.IsNullOrEmpty(str)) {
                        result.Insert(0, str + " ");
                    }
                }
                
                num = num / 10;
                
                position++;
            }
                
            return result.ToString();
        }
    }
}