How to exclude numbers in arc4random?

Question

I am using arc4random to generate a random number. I would like to generate a number between 1-9. How can I exclude 0?

int r = arc4random() % (9);
    NSLog(@"Random Number: %d",r);

Answer 1

int r = (arc4random() % 8) + 1

Answer 2

You can use arc4random_uniform(), as in

arc4random_uniform(9) + 1

Generally, to generate a number between lo and hi (inclusive), you use:

arc4random_uniform(hi - lo + 1) + lo

If you don't want to use arc4random_uniform() and want to stick with arc4random(), noting that the resulting value from using modulus formula is not uniformly distributed, use

(arc4random() % (hi - lo + 1)) + lo

Answer 3

int r = arc4random() % 8 + 1;

See other answers (e.g., one from me) for why you probably don't want to use % for this task, and what you should use instead.

Answer 4

You could simply try repeatedly until you get a number in the range you want, throwing out numbers you don't want. This has the fancy name "acceptance-rejection method" in math. It's simple, and it works.

In case you're worried that this could take a long time, in theory it could. But this approach is used all the time in statistical applications. The probability of going through a while-loop N times decreases rapidly as N increases, so the average number of times through the loop is small.