# Re: (default) Real->Rat precision should match what compiler uses for literals

```I want an epsilon that doesn't confuse newbies and which also is efficient.
epsilon=1/2**(mantissa bits-1) fits the bill.```
```
Why I want this-  It would be great to have numbers survive round-trip
conversions, when feasible.

Specifically I have no need to compare Rats and Nums for equality, but I do
often deal with "flat" text files full of metrics, and remotely sourced
json, and XML. The data types are sometimes unexpected.

-y

On Wed, Mar 7, 2018 at 5:16 PM, Solomon Foster <colo...@gmail.com> wrote:

> On Sun, Mar 4, 2018 at 8:49 AM, yary <not....@gmail.com> wrote:
>
>> In that spirit, I'd expect numeric comparison in general, and epsilon
>> specifically, to be set so these return True:
>>
>> > pi == pi.Rat # Does Num to Rat conversion keep its precision?
>> False
>> > pi.Str.Num == pi # Does Num survive string round-trip? - Nothing to do
>> with epsilon
>> False
>>
>>
> Why on earth would you want to do this?
>
> I mean that quite literally.  The only reason I can see for directly
> comparing a Num and a Rat for equality is to check and see if the Rat has
> the same precision as the Num.  In practice, it's well-known you generally
> shouldn't use equality tests on floating point numbers.  Converting one
> side of the equation to a Rat just makes it make even less sense.
>
>
> I've just been playing around with Num to Rat conversion, and here are
> some quick notes.
>
> 1) You can pass 0 as the epsilon for the Rat constructor, which seems to
> be equivalent to very very small values of epsilon.
>
> 2)  pi.Rat(0) + exp(1).Rat(0) is a Rat, but pi.Rat(0) + exp(1).Rat(0) +
> sin(.2).Rat(0) is a Num.  (On the other hand, pi.Rat() + exp(1).Rat() +
> sin(.2).Rat() is still a Rat.)
>
> 3) Remember (I had forgotten!) that Nums can represent numbers much
> smaller than a Rat can.  1e-100 is a perfectly reasonable Num, but (were
> Rat behaving properly) the closest possible Rat value is 0.
>
> 4) That said, if you actually do (1e-100).Rat(0), it gives you (1
> 100000000000000001590289110975991804683608085639452813897813
> 27557747838772170381060813469985856815104).  Needless to say, that's not
> actually a legal Rat.  Surprisingly (to me, anyway) it is accurate to
> better than 1e-110.
>
> 5) Somewhat more distressingly, (1e+100).Rat gives you (
> 100000000000000001590289110975991804683608085639452813897813
> 27557747838772170381060813469985856815104 1).  That's only accurate to
> 10**83.  Which is to say, it's as accurate as a double gets -- 16-17
> digits.   (BTW, that is a legal Rat.)
>
> I admit don't really know what to do with this.
>
> --
> Solomon Foster: colo...@gmail.com
> HarmonyWare, Inc: http://www.harmonyware.com
>
```