Question

Is there an efficient method of converting an integer into the written numbers, for example:

string Written = IntegerToWritten(21); 

would return ‘Twenty One’.

Is there any way of doing this that doesn’t involve a massive look-up table?

Answer 1

This should work reasonably well:

public static class HumanFriendlyInteger {     static string[] ones = new string[] { '', 'One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine' };     static string[] teens = new string[] { 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen' };     static string[] tens = new string[] { 'Twenty', 'Thirty', 'Forty', 'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety' };     static string[] thousandsGroups = { '', ' Thousand', ' Million', ' Billion' };      private static string FriendlyInteger(int n, string leftDigits, int thousands)     {         if (n == 0)         {             return leftDigits;         }          string friendlyInt = leftDigits;          if (friendlyInt.Length > 0)         {             friendlyInt += ' ';         }          if (n < 10)         {             friendlyInt += ones[n];         }         else if (n < 20)         {             friendlyInt += teens[n - 10];         }         else if (n < 100)         {             friendlyInt += FriendlyInteger(n % 10, tens[n / 10 - 2], 0);         }         else if (n < 1000)         {             friendlyInt += FriendlyInteger(n % 100, (ones[n / 100] + ' Hundred'), 0);         }         else         {             friendlyInt += FriendlyInteger(n % 1000, FriendlyInteger(n / 1000, '', thousands+1), 0);             if (n % 1000 == 0)             {                 return friendlyInt;             }         }          return friendlyInt + thousandsGroups[thousands];     }      public static string IntegerToWritten(int n)     {         if (n == 0)         {             return 'Zero';         }         else if (n < 0)         {             return 'Negative ' + IntegerToWritten(-n);         }          return FriendlyInteger(n, '', 0);     } } 

(Edited to fix a bug w/ million, billion, etc.)