subtract by adding

function subtract(num1, num2)
    if 0 > num1 or 0 > num2 then return "can't subtract negative numbers sorry lol" end

    local res = ""

    local n1 = num1-num2 > 0 and num1 or num2
    local n2 = tostring( n1 == num1 and num2 or num1 )

    for i=1,#n2-1 do
        --print( n2:sub(i,i) )
        res = res .. tostring( 9 - tonumber(n2:sub(i,i)) )
    end
    res = res .. 10 - n2:sub(#n2, #n2)

    return tonumber( tostring( n1 + tonumber(res) ):sub(2, -1) ) -- where you actually add the numbers you get
end



-------------------------

for i = 0, 100 do
    local x, y = math.random(1000000), math.random(1000000)

    if y>x then x,y = y,x end

    print(("-"):rep(10))
    print(string.format("%s - %s", x, y))
    print( "=> " .. subtract(x, y) )
end