On Mon, 16 Feb 2009 12:20:34 +0900, Bill Baxter wrote: > On Mon, Feb 16, 2009 at 12:07 PM, Steve Schveighoffer > <schvei...@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
