Convert decimal to hex in Lua 4?

Question

I found this formula to convert decimal numbers to hexadecimal color values in Lua:

http://lua-users.org/lists/lua-l/2004-09/msg00054.html

However, I have a few questions about the formula:

1. My input needs to be normalized between 0 and 1 instead of 0 and 255. Is this a potential problem?
2. I am stuck with Lua 4.01 instead of whatever the latest version is. I can't upgrade. Is this a problem?

Thanks!!

Answer 1

In Lua 5.x you can use the string.format function with the %x format specifier to convert integers to their hexadecimal representation. In your case it would look like this:

local input = 0.5
local output = string.format("%x", input * 255) -- "7F"

I don't know Lua 4.0.1 well so I can't tell you if this function is available (perhaps under a different name). That said, if its not, then you might be able to workaround by turning this into a C function that uses sscanf .

Answer 2

The example function demonstrated at http://lua-users.org/lists/lua-l/2004-09/msg00054.html does not converts negative numbers. Here is an example of conversion for both, negative and positive numbers:

function decimalToHex(num)
    if num == 0 then
        return '0'
    end
    local neg = false
    if num < 0 then
        neg = true
        num = num * -1
    end
    local hexstr = "0123456789ABCDEF"
    local result = ""
    while num > 0 do
        local n = math.mod(num, 16)
        result = string.sub(hexstr, n + 1, n + 1) .. result
        num = math.floor(num / 16)
    end
    if neg then
        result = '-' .. result
    end
    return result
end