arc4random Random Number Generator

Question

int randomNumber = (arc4random() % 83) + 1;

Is this the best way to generate "the most random" number? Or is there a better way to generate a random number?

You can refer this question: http://stackoverflow.com/questions/3724526/how-to-generate-random-number-from-0-5-to-1-0/3724668#3724668 — Girish Kolari, Dec 14 '10 at 05:19

score 13 · Answer 1 · answered Dec 14 '10 at 04:48

13

When you use arc4random you avoid one pitfall of using % with linear congruential generators (which is the usual algorithm used by rand): the low-order bits aren't less random.

However, you still have truncation issues: i.e., because (1 << 32) % 83 is 77, that means that numbers between 0 and 76 appear (slightly) more frequently than numbers between 77 and 82. To avoid this, you should throw away the incoming value (i.e., call arc4random again) if it's above (1 << 32) / 83 * 83.

(I assume the range of arc4random is from 0 to 2³²-1. Adjust the above explanation accordingly.)

answered Dec 14 '10 at 04:48

C. K. Young

219,335
46
382
435

1

@Joe Blow: The OP asked for the "best" random number generation system ordinarily available, so, I was answering to that. Also, `arc4random` is awesome: it uses kernel-collected entropy and all that good stuff. – C. K. Young Dec 14 '10 at 16:32
1

@Joe Blow: Actually, `arc4random` is crypto-grade randomness. It's equivalent to reading from `/dev/arandom`. So you could use it for crypto-grade stuff, if you handle the randomness correctly (and don't unwittingly skew the results by not doing the check I mentioned). – C. K. Young Dec 16 '10 at 01:40
There are now a number of well-known biases in arc4. For example, http://www.worldlingo.com/ma/enwiki/en/RC4 "The keystream generated by the RC4 is biased in varying degrees towards certain sequences." This is why you can't use arc4 for, for example, scientifically important MC sims. – Fattie Dec 16 '10 at 07:03

score 9 · Accepted Answer · answered Dec 14 '10 at 04:36

9

arc4random has a superior algorithm for generating random numbers based on the current time. There are other rand functions but they are not as good and require seeding.

answered Dec 14 '10 at 04:36

prgmast3r

413
5
8

3

`arc4random` also requires seeding, it just seeds itself before first use. – Joey Dec 14 '10 at 11:18
Rand and similar functions that dont produce as wide of a range as arc4random – prgmast3r Dec 15 '10 at 00:26
1

`arc4random_uniform()` does not suffer from modulo bias for max values that are not powers of 2, which is a problem that straight `arc4random()` has. – Zev Eisenberg May 20 '14 at 17:31

score 2 · Answer 3 · answered Jul 19 '12 at 20:13

The best random number generator I've ever seen (as well as a very clear definition of what random means) can be found in Stephen Wolfram's A New Kind of Science. He's been using a very simple cellular automata as his random number generator for decades in his Mathematica software program so it's been extremely well tested.

arc4random Random Number Generator

3 Answers3

Linked