Re: [Maxima-discuss] SIGN1 strangeness, was: Problem with bfloat

[Raymond T. <toy...@gm...>]

>>>>> "Robert" == Robert Dodier <rob...@gm...> writes:

    Robert> On 2014-09-01, David Billinghurst <dbm...@gm...> wrote:
    >> I:87^611$
    >> M:91^211$
    >> sign(sqrt(I)-M);

    Robert> Thanks for the example. Here's a proposed fix. This causes $FLOAT to
    Robert> always fail (even for Lisps which otherwise allow float infinity)
    Robert> and therefore SIGN1 takes a different route and sign returns 'pos.

    Robert> The patch assumes that (float inf) minus (float inf) != 0.
    Robert> I'm pretty sure that's required by IEEE 754 (in fact I think the
    Robert> result is supposed to be NaN) but I can't quote chapter & verse
    Robert> at the moment. 

Yes, inf - inf is NaN, in IEEE754 arithmetic.  As is inf*0. I would
add a comment to the code saying so for the next user who isn't so
familiar with IEEE754.

On the other hand, didn't you also write some kind of float-infinity-p
function already? If not, we should probably add a float-infinity-p
and float-nan-p functions just for cases like this.

--
Ray