[Glass] Negative infinity and positive infinity have the same sign
Martin McClure via Glass
glass at lists.gemtalksystems.com
Thu Jan 26 12:10:32 PST 2017
On 01/25/2017 12:45 PM, monty via Glass wrote:
> On 3.3.0, "-0.0 sign = 0.0 sign". On Pharo, "-1 sign = -1", "-0.0 sign = -1", "0.0 sign = 0", and "1 sign = 1". IEEE Standard 754 mandates that negative zero have the same sign bit as a negative number.
> _______________________________________________
Hi Monty,
The ANSI Smalltalk standard says that #sign should answer 0 if "the
receiver equals zero". This agrees with ISO/IEC 10967 Portable Numerics
standard, which says the same thing. The IEEE 754 spec does not specify
a "sign" operation. The closest equivalent I see is the "isSignMinus"
operation. We don't currently have that message in GemStone, but we
might add it. In GemStone, you can distinguish positive and negative
zero by sending #_sign.
Pharo's implementation seems very odd. It looks like it's been that way
a long time (John Maloney in 1998?) but the comment contradicts itself.
It says
"Answer 1 if the receiver is greater than 0, -1 if less than 0, else 0.
Handle IEEE-754 negative-zero by reporting a sign of -1"
But negative zero *is* equal to 0, so it claims to be answering both 0
and -1 for -0.0. Leaving that aside, it's disturbingly asymmetric to
answer 0 for 0.0 but -1 for -0.0.
Regards,
-Martin
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