Question

I have read here — without understanding much — that it’s bad to use mod range. So this typical recommendation for Objective-C

int r = arc4random() % 45;

might be a bad idea to get a number from 0 to 45 (something about the distribution and this formula having a preference for low bits). What should one use in Objective-C?

^{<sarcasm>
I am so glad to be able to finally learn this stuff after using only high-level languages (Java et. al) all this time. Tomorrow I will try to make fire with two twigs. </sarcasm>}

Answer 1

Java is just as high level as Objecive C here – in this case Java’ Random.getInt() is the same as arc4random in that they both return a 32-bit pseudo-random number.

The issue raised in the URL (and I have seen elsewhere) is that rand()

could be repeating itself every 32768
values.

Whilst OSX’s arc4random could have (2**1700) states.

But as in all uses of pseudo-random generators you need to be aware of their weaknesses before using them e.g. a preference for low bits in some generators and also the comment in the OpenBSD arc4random man page where it says

arc4random_uniform() is recommended
over constructions like “arc4random()
% upper_bound” as it avoids “modulo
bias” when the upper bound is not a
power of two.