RGB to Hex in Lua

gistfile1.lua

-- passing a table like {255, 100, 20}

function application:rgbToHex(rgb)
	local hexadecimal = '0X'

	for key, value in pairs(rgb) do
		local hex = ''

		while(value > 0)do
			local index = math.fmod(value, 16) + 1
			value = math.floor(value / 16)
			hex = string.sub('0123456789ABCDEF', index, index) .. hex			
		end

		if(string.len(hex) == 0)then
			hex = '00'

		elseif(string.len(hex) == 1)then
			hex = '0' .. hex
		end

		hexadecimal = hexadecimal .. hex
	end

	return hexadecimal
end

If you want a string you can just string.format("%X", number). If you want specific formatting you can apply width flags (see man printf).

I.e. to make a HTML color code, string.format("#%02X%02X%02X", r, g, b).

An even better version without loops:

function rgbToHex(r,g,b)
    -- EXPLANATION:
    -- The integer form of RGB is 0xRRGGBB
    -- Hex for red is 0xRR0000
    -- Multiply red value by 0x10000(65536) to get 0xRR0000
    -- Hex for green is 0x00GG00
    -- Multiply green value by 0x100(256) to get 0x00GG00
    -- Blue value does not need multiplication.

    -- Final step is to add them together
    -- (r * 0x10000) + (g * 0x100) + b =
    -- 0xRR0000 +
    -- 0x00GG00 +
    -- 0x0000BB =
    -- 0xRRGGBB
    local rgb = (r * 0x10000) + (g * 0x100) + b
    return string.format("%x", rgb)
end

This is horrible and unreliable. DO NOT USE THIS:

1. pairs(rgb) guarantees absolutely no order. If you pass the RGB values as a list {r, g, b}, it will work though, because pairs traverses the list part in order. There's no guarantee this is the case though. Horrible bugs will happen as soon as you pass {r = r, g = g, b = b} tables. For these, absolutely no order is guaranteed. You could be getting RGB if you're lucky, leading you not to discover this undefined behavior, you could also get BGR, GRB, ... though, scratching your head as to why it's not working. To "fix" this, ipairs could be used. Additionally, this is basically a footgun. You can happily pass less than 3 or more than 4 numbers, leading to invalid hex color codes. It will also happily write numbers > 255, screwing up the hex color codes by displacing the following digits and creating a length > 6.

2. Bad code quality, uneeded complexity. application: which is absolutely useless; self isn't needed. key is unused: _ should be used. Brackets instead of spaces around conditions. The inner while loop basically just does string.format("#%X", value), but a lot slower. The following ifs do the padding, basically string.format("#%02X", value).

3. Bad performance. This is unnecessarily slow.

Use string.format("#%02X%02X%02X", r, g, b) instead.

Regarding the following:

An even better version without loops:

function rgbToHex(r,g,b)
    - ...
    local rgb = (r * 0x10000) + (g * 0x100) + b
    return string.format("%x", rgb)
end

If you want padding, the last line needs to be return string.format("%06x", rgb).

If you want padding, the last line needs to be return string.format("%06x", rgb).

Do you know where can I find a list of formattings and options for C-style string formatting?

If you want padding, the last line needs to be return string.format("%06x", rgb).

Do you know where can I find a list of formattings and options for C-style string formatting?

What about https://en.wikipedia.org/wiki/Printf_format_string?

What about https://en.wikipedia.org/wiki/Printf_format_string?

Thanks

would u let that one slide marcelo codget

im kinda noob at coding but here is my solution:

rgb = {235,123,175}
function convert_to_hex (rgb)
    local hex = ""
    for i = 1 , #rgb do
        local hex_table = {"0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"}
        local num =  rgb[i]/16 
        local whole = math.floor( num )
        local remainder = num - whole
        hex = hex .. hex_table[whole+1] .. hex_table[remainder*16 + 1]
    end
    return hex
end

print(convert_to_hex(rgb))

That's a frightening piece of code you wrote up there