What's the difference between arc4random and arc4random_uniform?
arc4random, objective-c, random
Solution
`arc4random` returns an integer between 0 and (2^32)-1 while `arc4random_uniform` returns an integer between 0 and the upper bound you pass it.
From `man 3 arc4random`:
arc4random_uniform() will return a uniformly distributed random number less than upper_bound. 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.
For example if you want an integer between 0 and 4 you could use
arc4random() % 5
or
arc4random_uniform(5)
Using the modulus operator in this case introduces modulo bias, so it's better to use arc4random_uniform.
To understand modulo bias assume that `arc4random` had a much smaller range. Instead of 0 to (2^32) -1, it was 0 to (2^4) -1. If you perform % 5 on each number in that range you will get 0 four times, and 1, 2, 3 and 4 three times each making 0 more likely to occur. This difference becomes less significant when the range is much larger, but it's still better to avoid using modulus.
Problem
I've seen old posts about the differences between `random` and `arc4random` in Objective-C, and I've seen answers to this online but I didn't really understand, so I was hoping someone here could explain it in an easier-to-understand manner. What is the difference between using `arc4random` and `arc4random_uniform` to generate random numbers?