Convert integers to written numbers *

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

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);
        }

        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 make it considerably more concise.)

< br > via < a class="StackLink" href=" http://stackoverflow.com/questions/3213/" >Convert integers to written numbers< /a>

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