default random object?

[Bill Baxter]

On Mon, Feb 16, 2009 at 2:33 AM, Don Clugston <nospam at nospam.com> wrote:
> Andrei Alexandrescu wrote:
>>
>> Don wrote:
>>>
>>> Andrei Alexandrescu wrote:
>>>>
>>>> auto rng = Random(unpredictableSeed);
>>>> auto a = 0.0, b = 1.0;
>>>> auto x1 = uniform!("[]")(rng, a, b);
>>>> auto x2 = uniform!("[)")(rng, a, b);
>>>> auto x3 = uniform!("(]")(rng, a, b);
>>>> auto x4 = uniform!("()")(rng, a, b);
>>>
>>> This is a general issue applying to any numeric range. I've been giving
>>> the issue of numeric ranges some thought, and I have begun an implementation
>>> of a general abstraction.
>>> Any open range can be converted into a closed range, but the converse
>>> does not apply. So any implementation will be using "[]" internally.
>>>
>>> -range("[)", a, b) == range("(]", -b, -a)
>>> range("[)", a, b) == range("[]", a, predecessor(b))
>>> range("()", a, b) == range("[]", successor(a), predecessor(b))
>>>
>>>
>>> There's a couple of difficult situations involving floating-point
>>> numbers.
>>> *  "[)" has the uncomfortable property that (-2,-1, rng) includes -2 but
>>> not -1, whereas (1, 2, rng) includes 1 but not 2.
>>>
>>> * any floating point range which includes 0 is difficult, because there
>>> are so many numbers which are almost zero. The probability of getting a zero
>>> for an 80-bit real is so small that you probably wouldn't encounter it in
>>> your lifetime. I think this weakens arguments based on analogy with the
>>> integer case.
>>>
>>> However, it is much easier to make an unbiased rng for [1,2) than for
>>> [1,2] or (1,2) (since the number of members in the range is even).
>>
>> So what would you recommend? [a, b) for floats and [a, b] for ints, or [a,
>> b) for everything?
>>
>> Andrei
>
> I'm leaning towards [a,b) for everything (consistency with arrays), but I'd
> want to know what the reasoning of the boost/c++0x guys was.

How do you create a random uint that can take on any of uint's values
with [a,b)?  That's the main reason I can think of to go with [a,b]
for integral types.  With floats it's never useful to use the entire
value range.

--bb