In Lua, what is #INF and #IND?

Question

I'm fairly new to Lua. While testing I discovered #INF/#IND. However, I can't find a good reference that explains it.

What are #INF, #IND, and similar (such as negatives) and how do you generate and use them?

Answer 1

#INF is infinite, #IND is NaN. Give it a test:

print(1/0)
print(0/0)

Output on my Windows machine:

1.#INF
-1.#IND

As there's no standard representation for these in ANSI C, you may get different result. For instance:

inf
-nan

Answer 2

Expanding @YuHao already good answer.

Lua does little when converting a number to a string, since it heavily relies on the underlying C library implementation. In fact Lua print implementation calls Lua tostring which in turn (after a series of other calls) uses the lua_number2str macro, which is defined in terms of C sprintf. Thus in the end you see whatever representation for infinities and NaNs the C implementation uses (this may vary according to which compiler was used to compile Lua and which C runtime your application is linked to).

Answer 3

@YuHao has already answered what the OP has effectively asked: what does +/-1.#INF (+-inf) and -1.#IND (nan) mean. What I want to do here is just to add some value to the question/answer by expanding on to deal with -- to check -- them (which I just needed and learned to):

• inf (+/-1.#INF) are the highest number(+/-) that Lua can represent, and the Language provides such value(s) for you through math.huge. So you can test if a number is +/-INF:

local function isINF(value)
  return value == math.huge or value == -math.huge
end

• nan (-1.#IND) is something that can not be handled numerically, and the result of any operation involving it is also Not-a-number(*). Long-story-short... if a number is a NaN, comparing it against itself will (always) be False. The function below implements the simplest way for checking if a NaN:

local function isNAN(value)
  return value ~= value
end

(*): This (NaN) is formally defined some where in the IEEE754 standard (floating-point numbers).