default random object?

[Steve Schveighoffer]

On Mon, 16 Feb 2009 12:20:34 +0900, Bill Baxter wrote:

> On Mon, Feb 16, 2009 at 12:07 PM, Steve Schveighoffer
> <schveiguy at yahoo.com> wrote:
>> On Sun, 15 Feb 2009 17:27:38 -0800, Andrei Alexandrescu wrote:
>>
>>> Ok. Let me just note that rand()%max is a lousy method of generating
>>> random numbers between 0 and max-1 and everybody should put that in
>>> the bin with Popular Examples That Should Never Be Used, together with
>>> exponential Fibonacci, linear-space factorial, and bubblesort.
>>
>> Do you mean rand()%max specifically in the case of the C function rand
>> ()?  Or using %max on any random number generator in general.  If the
>> first case, I totally agree, and I found out that from experience
>> first, then googling second :)  But if the latter, I wasn't aware that
>> all RNGs were bad at randomizing the lower bits, so could you explain
>> why?
>>
>> It's also possible that Lionello meant the latter case as well.
> 
> It's bad in general.  Say your RNG generates perfectly uniform numbers
> between 0 and 10.
> Now you say  RNG()%7 to get values 0..6   Not all inputs map to outputs
> evenly now.  More inputs map to 0 than to 6, so the distribution is no
> longer uniform.
> Of course if you % by something that divides evenly into the RNG's range
> then it shouldn't be a problem, other than the lack of randomness in
> lower bits of some rand() implementations.
> 
> --bb

Hm... good point, but probably not significant unless you are generating 
numbers in a range close to the maximum range.  For example, the 0..6 
range isn't going to divide evenly into int.max, but the bias towards 
certain values is going to be very slight.  I would guess then that the 
correct method is to add n to the nth random number generated?  Or is 
there a better way?

-Steve