Bug with large fractions

Bug with large fractions

[Ludovic Courtès]

Hi,

Before I forget about it, I noticed a bug with large fractions.  It
shows up in the following example:

  guile> 123123123123123123.12
  123123123123123000.0

A quick look at numbers.c:mem2decimal_from_point, and then trying to
re-execute the algorithm in there by hand shows that this is not a
reader bug:

  guile> (define x (* 123123123123123123 100))
  guile> x
  12312312312312312300
  guile> (set! x (+ x 12))
  guile> x
  12312312312312312312
  guile> (set! x (/ x 100))
  guile> x
  3078078078078078078/25 ;; this is wrong!
  guile> (exact->inexact x)
  123123123123123000.0

This is executed on a Guile before Han-Wen's fix, FWIW.  Perhaps people
more knowledgeable about this will have an idea of what's going wrong.

Thanks,
Ludovic.



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Re: Bug with large fractions

[Kevin Ryde]

ludo@chbouib.org (Ludovic Courtès) writes:
>
>   guile> 123123123123123123.12
>   123123123123123000.0

I think it's been truncated to 53 bits for the mantissa of a double.
(Unless you mean the rounding went the wrong way or something.)


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Re: Bug with large fractions

[Ludovic Courtès]

Hi,

Kevin Ryde <user42@zip.com.au> writes:

> ludo@chbouib.org (Ludovic Courtès) writes:
>>
>>   guile> 123123123123123123.12
>>   123123123123123000.0
>
> I think it's been truncated to 53 bits for the mantissa of a double.

Right, the value can't actually fit into a double and reals in Guile
_are_ double (it took me a while to get it...  ;-)).

Thanks,
Ludovic.




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Re: Bug with large fractions

[Ludovic Courtès]

Hi,

ludo@chbouib.org (Ludovic Courtès) writes:

>   guile> (define x (* 123123123123123123 100))
>   guile> x
>   12312312312312312300
>   guile> (set! x (+ x 12))
>   guile> x
>   12312312312312312312
>   guile> (set! x (/ x 100))
>   guile> x
>   3078078078078078078/25 ;; this is wrong!

The fraction here is actually correct, only the result of
`exact->inexact' is rounded.

>   guile> (exact->inexact x)
>   123123123123123000.0

Thanks,
Ludovic.



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