Re: float output precision questions

Tom Lane wrote:
>
> I was misremembering these papers to claim DBL_DIG+1 is enough, but
> actually they prove that DBL_DIG+2 is necessary and sufficient (and
> give code to do it correctly, too).

Yeahh! If there's a proof its safe to implement.
I also Googled a bit and found another paper saying that 17 is the
minimum number of significant digits guaranteed to distinguish among
IEEE double-precision floating point numbers:

"Robert G. Burger and R. Kent Dybvig. Printing floating-point numbers
quickly and accurately. In Proceedings of the ACM SIGPLAN '96 Conference
on Programming Language Design and Implementation, pages 108--116"
http://citeseer.nj.nec.com/28233.html

>
> Printing DBL_DIG+3 is just producing an extra garbage digit; it won't
> help matters. Any reasonably well-written C library is going to be
> able to reproduce a double value with DBL_DIG+2 digits of I/O; and if
> it's not well-written, I would have no confidence in its ability to do
> so with DBL_DIG+3 digits...

Off course. This is also good in terms of dump storage for big float8
databases. Its one byte less for every float8.

>
> regards, tom lane
>
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