default random object?

[Don]

Nick Sabalausky wrote:
> "Don" <nospam at nospam.com> wrote in message 
> news:gnb7mt$2osv$1 at digitalmars.com...
>> Andrei Alexandrescu wrote:
>>> Benji Smith wrote:
>>>> Benji Smith wrote:
>>>>> Maybe a NumericInterval struct would be a good idea. It could be 
>>>>> specialized to any numeric type (float, double, int, etc), it would 
>>>>> know its own boundaries, and it'd keep track of whether those 
>>>>> boundaries were open or closed.
>>>>>
>>>>> The random functions would take an RND and an interval (with some 
>>>>> reasonable default intervals for common tasks like choosing elements 
>>>>> from arrays and random-access ranges).
>>>>>
>>>>> I have a Java implementation around here somewhere that I could port to 
>>>>> D if anyone is interested.
>>>>>
>>>>> --benji
>>>> Incidentally, the NumericInterval has lots of other interesting 
>>>> applications. For example
>>>>
>>>>    auto i = NumericInterval.UBYTE.intersect(NumericInterval.SBYTE);
>>>>    bool safelyPolysemious = i.contains(someByteValue);
>>>>
>>>>    auto array = new double[123];
>>>>    auto i = NumericInterval.indexInterval(array);
>>>>    bool indexIsLegal = i.contains(someIndex);
>>>>
>>>> Using a numeric interval for generating random numbers would be, in my 
>>>> opinion, totally ideal.
>>>>
>>>>    double d = uniform(NumericInterval.DOUBLE); // Any double value
>>> I've never been in a situation in my life where I thought, hey, a random 
>>> double is exactly what I'd need right now. It's a ginormous interval!
>>>
>>> Andrei
>> It's worse than that. Since the range of double includes infinity, a 
>> uniform distribution must return +-infinity with probability 1. It's 
>> nonsense.
> 
> I would think that, with the possible exception of mathematicians, most of 
> the time anyone would want a random floating-point number they would 
> consider +/-infinity a special case. Most non-math-oriented code isn't 
> really designed to handle infinity anyway. Besides, technically, double also 
> includes NaN ;) 

You're asking for a random *number* and NaN is not a number :).
If you have a double x, which could be double.max, -double.max, 0.0, 
-double.min*double.epsilon, or +double.min*double.epsilon, there's 
hardly any operations you can do on it which won't generate an infinity 
or NaN.
But cutting out infinity doesn't help much -- anything from the range of 
double will be really close to double.max. If the code is correct, it's 
still going to have to deal with infinity.


double d = uniform(NumericInterval.DOUBLE);
d *=2;
// BUG: There's a 50% chance that d is now +- infinity.