convertreal.lua

local format = string.format
local tonumber = tonumber
local fmt = {
    '%.1f',
    '%.2f',
    '%.3f',
    '%.4f',
    '%.4f',
    '%.5f',
    '%.6f',
    '%.7f',
    '%.8f',
    '%.9f',
    '%.10f',
    '%.11f',
    '%.12f',
    '%.13f',
    '%.14f',
    '%.15f',
    '%.16f',
}
local function convertreal(v)
    for i = 1, #fmt do
        local g = format(fmt[i], v)
        if tonumber(g) == v then
            return g
        end
    end
    return format('%.17f', v)
end

local function convertreal2(v)
    local g = format('%.16g', v)
    if tonumber(g) == v then
        return g
    end
    return format('%.17g', v)
end

local function test(s1)
    local f1 = tonumber(s1)
    local s2 = convertreal(f1)
    print(s1)
    print(s2)
    print('-------------------')
end


test '0.1'
test '0.000001'
test '0.00000001'
test '0.0000000001'
test '0.000000000001'
test '0.00000000000001'
test '0.0000000000000001'
test '1.012345678901'
test '1.0123456789012'
test '1.01234567890123'
test '1.012345678901234'
test '1.0123456789012345'
test '1.01234567890123456'
test '1.012345678901234567'