On 27 Oct 2008, at 11:00 pm, Henning Thielemann wrote:
On Mon, 27 Oct 2008, L.Guo wrote:
I think this is unresonable. then try it in GHC 6.8.3.
Prelude> round 3.5 4 Prelude> round 2.5 2
Is there any explanation about that ?
It's the definition we learnt in school ...
Check http://en.wikipedia.org/wiki/Rounding where you will find that the version of rounding "generally taught in elementary schools" is the one used in Pascal, namely round(X) = truncate(X + 0.5*sign(X))
(The Wikipedia entry says that Pascal uses a different algorithm, but ISO 7185 says "If x is positive or zero, round(x) shall be equivalent to trunc(x +0.5); otherwise, round(x) shall be equivalent to trunc(x-0.5).")
I think one reason is that repeated rounding should not be worse than rounding in one go.
That would be nice, but while round-to-even _reduces_ the harm from double rounding, it does not eliminate it.