Have you ever needed to generate a random number in code? whether it's for rolling a dice, or shuffling a set, this tweet thread is here for you! There's no reason that it should be easy or obvious, very experienced programmers repeat common mistakes. I did, before I learned ...
- Have you ever needed to generate a random number in code? whether it's for rolling a dice, or shuffling a set, this tweet thread is here for you! There's no reason that it should be easy or obvious, very experienced programmers repeat common mistakes. I did, before I learned ...
O.k. let's start with the most common problem, and the most common mistakes: how do we pick a random number between 0 and N inclusive, let's say N = 5, so like a dice that starts at zero because we're nerds.A common solution is to r = rand() % (N + 1). Easy, right? Wrong! This solution is biased. To see how, imagine that RAND_MAX is "15". 0 % 6 == 0, 6 % 6 == 0, and 12 % 6 == 0 , so there are three rand() values each that return 0. Same works for 1, 2, 3 ...But there are only two rand() values that return 4: 4 % 6 and 10 % 6. Same for 5. So 0, 1, 2, 3 are all 50% more likely than 4 or 5. That's bias! Of course with a big RAND_MAX, this bias diminishes, but it's still there. So don't do it this way.O.k. so what if we use a float to avoid this? It's tempting! We can build a float between 0.0 and 1.0 by doing f = (float) rand() / (float) RAND_MAX. So we build a little macro or function for that, and now we can just do r = randFloat() * 5, right?No! This doesn't work other, because FLOATING POINT IS LIES. The distribution between 0.0 and 1.0 is not evenly spread and there's all sorts of rounding fuzziness that will make r non-uniform. No good.The solution here is pretty interesting and totally non-intuitive. What you have to do is to compute the highest multiple of n that is smaller than RAND_MAX. Then you call rand() and if you get a number that's higher, you discard it and go again. E.g. github.com/awslabs/s2n/bl…int random(int max) { while(1) { int r = rand(): if (r < (RAND_MAX - (RAND_MAX % max))) { return r % max; } } }O.k, so there's some probability that this program might never finish! HALTING PROBLEM, etc, etc. The probability is worst when max == (RAND_MAX / 2) + 1. But even then it's a very rapidly diminishing binomial distribution ... so we just shrug our shoulders. It's fine.This is why you should use those APIs that ask for the max, that's why they exist, and this is what's going on under the hood. But if you have to do it yourself. This is how. O.k. next, what if we need to random selection ...jumble = [ A , B , C, D, E, F ] for (i = 0; i < jumble.length; i++) { int r = rand(i) save = jumble[i] jumble[i] = jumble[r] jumble[r] = save It's O(N) and it works every time. Super easy to remember if you code it a few times.Sadly, this one is too long for a tweet until next year when twitter decides that we need more space to offend one another, so for now, bookmark this page keithschwarz.com/darts-dice-coi… , or print it out and frame it in case the Internet ever dies. It's one of the best bits.A normal distribution is a super common statistical distribution, it describes the distribution of lots of phenomena and the central limit theorem says that basically any distribution is secretly just one step removed from the normal distribution.Second: if you find yourself building a whole RNG, it's really very hard, again, get deep in the weeds and learn about DRBGs, fork-safety, thread-safety, /dev/urandom, getrandom() and so on. Avoid if you can!Third: always use a secure RNG, your language or programming environment should have one. *Don't* ever seed an RNG yourself. One exception: for fuzz inputs and other tests, where you may want repeat deterministically for debugging. But DON'T LET IT LEAK INTO PRODUCTION.Last tip: always measure your little random functions with a histogram or whatever. I still code these wrong and have to check. Thanks for reading!
Nice epic thread on random number generation. I came here to point out "rand() Considered Harmful" talk which is an awesome take on this channel9.msdn.com/Events/GoingNa… and slides view.officeapps.live.com/op/view.aspx?s… In C++ we discourage the use of rand() and have <random> stackoverflow.com/a/17798317/170…