Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-9-13
Source: WG14
Reference Document:
N1633
Related:
N1497
Version: 1.1
Date: April 2013
Subject: Possible defect report: fmod(0.,NaN) and
fmod(NaN,infinity)
First question. When Annex F is in effect, what should the value of fmod(0.,NaN) be? The two choices are 0. or NaN.
Annex F.10.7.1 The fmod functions has:
- fmod(+/-0, y) returns +/-0 for y not zero.
- fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero (and neither is a NaN).
So, that first bullet item says fmod(0.,NaN) is 0.
Elsewhere in that annex (F.10 Mathematics, paragraph 11), we have:
Functions with a NaN argument return a NaN result and raise no floating-point exception, except where stated otherwise.
That says that fmod(0.,NaN) is NaN.
One idea is to explicitly add words about a NaN to the first bullet item in F.10.7.1, such as:
- fmod(+/-0, y) returns +/-0 for y not zero nor NaN.
However, if F.10#11 covers NaN arguments before any other arguments are considered, then words about NaN could be removed from the second case in F.10.7.1, such as:
- fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero.
I believe that takes us back to before N1497 was done.
Second question: what should fmod(NaN,infinity) be? Must it be the same NaN argument, or may it be any NaN?
Annex F.10.7.1 The fmod functions has:
- fmod(x, +/-infinity) returns x for x not infinite.
Which says fmod(NaN,infinity) must be the same NaN argument.
But, if F.10#11 covers this NaN argument, then this case is just some NaN.
It appears that the third bullet should either be left alone or changed to:
- fmod(x, +/-infinity) returns x for finite x.
Some other functions that discuss NaN arguments in Annex F are: frexp, ilogb, modf, hypot, pow, fmax, fmin, and fma. Of those, only hypot, pow, fmax, and fmin have exceptions on NaN in implies NaN out.
Committee Discussion
- Given that, unless stated otherwise, if the argument is a NaN, the result is a NaN, and there is no floating point exception.
- The parenthetical comment might be causing confusion, and removing it seems to make it more confusing.
- Adding more explicit wording is possible but seems unnecessary.
Proposed Committee Response
The consensus was to do nothing and the author agrees.