[Haskell-cafe] Why 'round' does not just round numbers ?

[Janis Voigtlaender]

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 ...

Hmm, Henning, this is strange. The two of us went to the very same
school, but I know for a fact that I learnt 2.5 to round to 3, not 2 as
above. ;-)

> I think one reason is that repeated rounding should not be worse than 
> rounding in one go. Consider the rule 'use ceiling when the first 
> removed digit is 5'. Then
> 
> 0.45 - (round to one place) -> 0.5 - (round to integer) -> 1
> 
> but
> 
> 0.45 - (round to integer) -> 0

That is of course true (and was the topic of heated discussion with my
fourth grade math teacher), but does not explain 2.5 -> 2.

Ciao, Janis.

-- 
Dr. Janis Voigtlaender
http://wwwtcs.inf.tu-dresden.de/~voigt/
mailto:voigt at tcs.inf.tu-dresden.de