[Python-ideas] Fwd: Trigonometry in degrees

[Stephan Houben]

Op di 12 jun. 2018 12:41 schreef Nathaniel Smith <njs at pobox.com>:

> On Tue, Jun 12, 2018, 00:03 Stephan Houben <stephanh42 at gmail.com> wrote:
>
>> Hi all,
>>
>> I wrote a possible implementation of sindg:
>>
>> https://gist.github.com/stephanh42/336d54a53b31104b97e46156c7deacdd
>>
>> This code first reduces the angle to the [0,90] interval.
>> After doing so, it can be observed that the simple implementation
>>   math.sin(math.radians(angle))
>> produces exact results for 0 and 90, and a result already rounded to
>> nearest for
>> 60.
>>
>
> You observed this on your system, but math.sin uses the platform libm,
> which might do different things on other people's systems.
>


Ok, I updated the code to treat all the values 0, 30, 45, 60 and 90
specially.

Stephan


>
>> For 30 and 45, this simple implementation is one ulp too low.
>> So I special-case those to return the correct/correctly-rounded value
>> instead.
>> Note that this does not affect monotonicity around those values.
>>
>
> Again, monotonicity is preserved on your system, but it might not be on
> others. It's not clear that this matters, but then it's not clear that any
> of this matters...
>
> -n
>
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