How can I turn a number into words?

This is a really fun exercise that involves recursion. I highly recommend attempting to work through this problem on your own before simply using the solution I present below. Take your time. Experiment with it. Have fun!

With that said, I present my solution here...

local stringVals = {
    [0] = "",
    [1] = "one",
    [2] = "two",
    [3] = "three",
    [4] = "four",
    [5] = "five",
    [6] = "six",
    [7] = "seven",
    [8] = "eight",
    [9] = "nine",
    [10] = "ten",
    [11] = "eleven",
    [12] = "twelve",
    [13] = "thirteen",
    [14] = "fourteen",
    [15] = "fifteen",
    [16] = "sixteen",
    [17] = "seventeen",
    [18] = "eighteen",
    [19] = "nineteen",
    [20] = "twenty",
    [30] = "thirty",
    [40] = "forty",
    [50] = "fifty",
    [60] = "sixty",
    [70] = "seventy",
    [80] = "eighty",
    [90] = "ninety",
    [100] = "hundred",
    [1000] = "thousand",
}

local groupNames = {
    [1] = "",
    [2] = " thousand ",
    [3] = " million ",
    [4] = " billion ",
}

local function groupToWord(number)
    local result = stringVals[number]
    if not result then
        result = ""
        local hundreds = math.floor(number/100)
        if hundreds > 0 then
            result = result .. stringVals[hundreds] .. " " .. stringVals[100] .. " "
        end
        number = number - hundreds * 100
        local tens = math.floor(number/10)
        number = number - tens * 10
        if tens == 1 and number > 0 then
            --address funny names eleven thru nineteen
            result = result .. stringVals[number + 10]
        else
            if tens > 0 then
                result = result .. stringVals[tens * 10] .. " "
            end
            if number > 0 then
                result = result .. stringVals[number]
            end
        end
    end
    return result
end

local function numberToWords(number, groupIndex)
    if number then
        groupIndex = groupIndex or 1
        local numStr = tostring(number)
        local endStr = numStr:sub(-3)
        local front = numStr:sub(1,-4)
        return numberToWords(tonumber(front), groupIndex + 1) .. groupToWord(endStr) .. groupNames[groupIndex]
    else 
        return ""
    end 
end

for i = 100, 2000, 13 do
    print(i, numberToWords(i))
end

select output:

178 one hundred seventy eight

217 two hundred seventeen

334 three hundred thirty four

555 five hundred fifty five

685 six hundred eighty five

867 eight hundred sixty seven

1010 one thousand ten

1491 one thousand four hundred ninety one

1816 one thousand eight hundred sixteen

1998 one thousand nine hundred ninety eight

192540520 one hundred ninety two million five hundred forty thousand five hundred twenty