default random object?

[Bill Baxter]

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