On 2016-06-22, Christopher Reimer <christopher_rei...@icloud.com> wrote: >> On Jun 22, 2016, at 7:59 AM, Grant Edwards <grant.b.edwa...@gmail.com> wrote: >> >>> On 2016-06-22, Random832 <random...@fastmail.com> wrote: >>>> On Wed, Jun 22, 2016, at 10:19, Grant Edwards wrote: >>>> >>>> Is that guaranteed by Python, or just a side-effect of the >>>> implementation? Back in the days when Python used native C >>>> integers I think the latter. >>> >>> AIUI, native C integers have never reliably supported signed zero >>> even with representations that naively seem to have it. There's no >>> well-defined way to detect it - no int version of copysign, for >>> instance - and implementations are free to erase the distinction on >>> every load/store or define one of them to be a trap representation. >> >> It's been almost 25 years since I used hardware that supported signed >> zero integers (CDC 6600). I don't recall there being a C compiler >> available. We used Pascal and assembly, though I think FORTRAN was >> what most people used. I don't recall whether the Pascal >> implementation exposed the existence of -0 to the user or not. > > When I took mathematics in college, the following was true: > > -1 * 0 = 0
Well, we're talking about computers and programming languages, not mathematics. > I would probably have gotten rapped on the knuckles by my instructors > if I answered -0. Zero was zero. No plus or minus about that. No > discussion of signed integers ever mentioned signed zero. > > Did I miss something in college? If you took computer architecture courses, then yes, you missed the chapters on machine representation of integers. If you build a CPU that uses either 1's compliment or signed magnitude representations then there is a postive zero and a negative zero. If you design it to use 2's complement, then there is only a positive zero. 1's complement and signed-magnitude are excedingly rare these days, but used to be much more common. -- Grant -- https://mail.python.org/mailman/listinfo/python-list
