[Ecls-list] Rounding bug in float

[Juan Jose Garcia-Ripoll]

We rely on the GMP library for bignums and rational operations. It
seems not to be as precised as you would wish

> (setq a (/ (1- (ash 1 100)) (ash 1 100)))
1267650600228229401496703205375/1267650600228229401496703205376
> (float a 1.0d0)

Breakpoint 9, cl_float (narg=2, x=0x3491f0) at num_co.d:64
ecl_to_double (x=0x3491f0) at number.c:683
(gdb) call (double)__gmpq_get_d(aux)
$1 = 0.99999999999999989


On Wed, Mar 25, 2009 at 5:41 AM, Raymond Toy <toy.raymond at gmail.com> wrote:
> With ecl 8.12.0 on Linux, I get
>
> (float (/ (1- (ash 1 100)) (ash 1 100)) 1d0) -> .999999...99d0
>
> I think the answer should be 1d0 since the difference between 1 and the
> rational above is far less than double-float-negative-epsilon.  Both
> clisp and cmucl produce 1d0.
>
> Ray
>
>
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