[Numpy-discussion] What is the sign of nan?

Charles R Harris charlesr.harris@gmail....
Mon Sep 29 18:44:18 CDT 2008


On Mon, Sep 29, 2008 at 5:16 PM, Robert Kern <robert.kern@gmail.com> wrote:

> On Mon, Sep 29, 2008 at 18:10, Charles R Harris
> <charlesr.harris@gmail.com> wrote:
> >
> > On Mon, Sep 29, 2008 at 4:54 PM, Charles R Harris
> > <charlesr.harris@gmail.com> wrote:
> >>
> >> On Mon, Sep 29, 2008 at 4:40 PM, Charles R Harris
> >> <charlesr.harris@gmail.com> wrote:
> >>>
> >>> On Mon, Sep 29, 2008 at 4:28 PM, Robert Kern <robert.kern@gmail.com>
> >>> wrote:
> >>>>
> >>>> On Mon, Sep 29, 2008 at 17:13, Charles R Harris
> >>>> <charlesr.harris@gmail.com> wrote:
> >>>> >
> >>>> > On Mon, Sep 29, 2008 at 3:52 PM, Charles R Harris
> >>>> > <charlesr.harris@gmail.com> wrote:
> >>>> >>
> >>>> >> Hi All,
> >>>> >>
> >>>> >> I've been cleaning up the ufunc loops and the sign function
> currently
> >>>> >> doesn't have a defined behavior for nans. This makes the results
> >>>> >> depend on
> >>>> >> the order/type of comparisons in the code, which looks fragile to
> me.
> >>>> >> So
> >>>> >> what should it return? I vote for nan but am open for suggestions.
> >>>> >
> >>>> > And while we're at it, lets decide how to treat max/min when nans
> are
> >>>> > involved. Or should we just say the behavior is undefined.
> >>>>
> >>>> When feasible, I would like float(s)->float functions to return NaN
> >>>> when given a NaN as an argument. At least as the main versions of the
> >>>> function. Specific NaN-ignoring functions can also be introduced, but
> >>>> as separate functions. I don't know what exactly to do about
> >>>> float->int functions (e.g. argmin). I also don't know how these should
> >>>> interact with the current seterr() state.
> >>>
> >>> So the proposition is, sign, max, min return nan when any of the
> >>> arguments is nan.
> >>>
> >>> +1
> >>
> >> I also propose that all logical operators involving nan return false,
> >> i.e., ==, !=, <, <=, >, >=, and, or, xor, not.
> >>
> >
> > Currently this is so except for !=. On my machine nan != nan is true.
> Looks
> > like it is being computed in C as !(nan == nan). Hmm, anyone know of a C
> > standard on this?
>
> C99 Annex F:
>
>  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
>
> In particular:
>
> """
> F.8.3 Relational operators
>  x != x -> false       The statement x != x is true if x is a NaN.
> """
>

Thanks, that was very helpful. I wonder how widespread the less, lessequal,
etc. macros are?

Chuck
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