[squeak-dev] a==b "true" but a=b "false"

Trygve Reenskaug trygver at ifi.uio.no
Fri Sep 5 08:27:39 UTC 2014

Old Fortran wisdom from the stone age:
/never test Floats for equality/
A Float is an approximation; there are an infinite number of values that 
yield the same Float. Calculations with Floats are likely to introduce 
rounding errors. Always test Floats for equality within a tolerance.

Float nan = Float nan is just one of the many tests that are meaningless 
in practical programming.


On 04.09.2014 21:33, Eliot Miranda wrote:
> On Wed, Sep 3, 2014 at 4:11 PM, Chris Muller <ma.chris.m at gmail.com 
> <mailto:ma.chris.m at gmail.com>> wrote:
>     When could it happen?
>     A:  With Float nan.
>        Float nan == Float nan   "true"
>        Float nan = Float nan   "false"
>     It violates what I have long considered an invariant that if a==b,
>     then a=b.
>     Real world context:  When two DB clients change the same object in the
>     exact same way, it's not a conflict.  If they change it differently,
>     it is.
>     They've both made the same reference to Float nan, but just asking if
>     they made equivalent changes still reports false.  So I have to put in
>     a complicated special check for NaN first..??
>     Okay, I actually wrote:  (a==b or: [a=b]) but who would ever write
>     that or not "simplify it" in teh future if they saw it?
>     Comments?  Why shouldn't Float nan = Float nan?
> Because non-a-numbers result from overflowing or underflowing 
> computations, and hence, just because two computations produce the 
> same not-a-number doesn't imply that the two computations are 
> equivalent.  This is why the IEEE-754 standard requires that NaN != NaN.
> -- 
> best,
> Eliot


Trygve Reenskaug      mailto: trygver at ifi.uio.no
Morgedalsvn. 5A       http://folk.uio.no/trygver/
N-0378 Oslo             http://fullOO.info
Norway                     Tel: (+47) 22 49 57 27
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