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My question is straightforward. I know abour srand and rand, I know how to seed my random generator and so on. I am specifically interested in the mathematics behind srand. How does the computer use let's say 3333 as a seed and calculates a "random" number?

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    There are quite a few examples of simple pseudo-random number generators on the Internet, if you just search a little. But to be able to understand and write a *good* generator takes a lot of studying and experience, including plenty of math and statistics. – Some programmer dude Sep 15 '20 at 11:44
  • i dont understand the question, when you call `srand` then you pass the seed as parameter. Do you want to know how the first random number is generated from that seed? – 463035818_is_not_an_ai Sep 15 '20 at 11:46
  • Let's take the example with srand(time). Here the first number ALWAYS follows the UNIX time which makes sense. The other numbers are generated by using the previous number as a seed. But question is just how does srand take a seed and calculates the next "random" number? Computers never do "random" things but can use matemathics to perfom what would seem like "randomness". –  Sep 15 '20 at 22:31
  • Ps. Sorry about my formalia on Stack, I am completly new here, and need to study a bit to find out how to highlight things and so on((: –  Sep 15 '20 at 22:32

2 Answers2

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The c standard library's and therefore the c++ standard library's std::rand implementation is up to the library implementation. It is not mandated by the standard. The following function

int getRandomNumber() { return 4; }

(source: https://xkcd.com/221/)

would be a perfectly valid implementation for it.

That is the reason why it should under no circumstances be used any more. Instead defer to the well-defined, implementation independent and portable mersenne twister.

Martin Ba
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bitmask
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    you beat me by 1 second to find the pic :P – 463035818_is_not_an_ai Sep 15 '20 at 11:47
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    @idclev463035818 Well if it's any consolation, that usually happens to me :) – bitmask Sep 15 '20 at 11:49
  • Yes but such an implementation would be a terrible mess. Indeed random but nothing is more deterministic than a constant value. So I'm not sure anyone would call such an implementation "valid" in practice (even if it does not violate the standard). – Fareanor Sep 15 '20 at 11:49
  • [Another related comic](https://dilbert.com/strip/2001-10-25) – Some programmer dude Sep 15 '20 at 11:52
  • @Fareanor Of course not. But the point is that given that the standard *allows* this, means that pretty much any implementation (deterministic depending on seed) would conform. Which makes `rand` pretty much useless for any practical application. – bitmask Sep 15 '20 at 11:52
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    @Fareanor I think part of the joke is that unless you know how it is implemented you cannot tell the difference between this and a real prng – 463035818_is_not_an_ai Sep 15 '20 at 11:53
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    @Someprogrammerdude Perhaps, but I wouldn't be thrilled to be associated with Scott Adams or his work in any way. – bitmask Sep 15 '20 at 11:56
  • @bitmask I fully agree with that, I posted my comment before your edit _"That is the reason..."_ :) – Fareanor Sep 15 '20 at 11:57
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It depends on the library implementation but in general it would be enough to calculate a well mixed hash of the argument.

Christopher Yeleighton
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