Re: table sort: IEEE floating-point number NaN?

On Sun, 26 Apr 2026 at 00:49, 'Lars Müller' via lua-l <
[email protected]> wrote:

> If passing NaNs may cause a sorting algorithm to produce an error, that's
> a feature. You must not try to sort the unsortable (and if you do, you
> should be explicit about how you want it sorted). -Lars
>

<kidding for="political correctness">
If you can sort by being explicit then it is nor unsortable, just "hard to
sort".
</kidding>

The sorter goes through a comparator, it is easy to define one which sorts
nan in a defined place, like some SQL engines do with nulls and NULLS
FIRST/LAST clauses:

-- Sort NANs first, even before -Inf.
function nan_first_less(a,b)
   if is_nan(b) then
      return false -- NaN is not less than nothing, even other NaN
   elseif is_nan(a) then
      return true -- B is not a Nan, so a is less then it.
   else
      return a<b -- no NaNs, so this works right.
   end
end

is_nan is trivial and has been done here before. This can be used to sort
NAN first or last. It can be optimized a bit by unrolling is_nan. Also,
knowing floating point numbers are enumerable, you can place NaN anywhere
in the FP range, but it is a bit harder, I do not recall if Lua sports
signeds zeros, but with a proper comparator you can also do things like
-0,NaN,+0, or signaling before non signaling nans.

IEE754 numbers are just a data chunk for a sorter, 64 bits in RAM, with a
proper comparator they can always be sorted in any way you want, you just
have to be a little careful writing it, as soon as you enter Nan (there are
many representations for them, although I think lua just generates
one)/Inf/unnormalized/signed zero territory things get a bit bug prone.

Francisco Olarte.

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