It's well known that [rand() is evil](https://codeforces.com/blog/entry/61587). The consensus appears to be to use the STL Mersenne twister, and the STL millisecond(?) precision clock as a seed. This is all fine and dandy, but there's a problem, as appears in the linked blog:↵
↵
~~~~~↵
for (int i = 1; i < N; i++)↵
swap(permutation[i], permutation[uniform_int_distribution<int>(0, i)(rng)]);↵
~~~~~↵
↵
There's so many *words*. `uniform_int_distribution` is like, 2 really big words. And I hate big words. Fortunately, there's a solution:↵
↵
~~~~~↵
#include <bits/stdc++.h>↵
#include <experimental/random>↵
// Note: experimental headers are not included by bits/stdc++.h↵
using namespace std;↵
int main() {↵
experimental::reseed(chrono::steady_clock::now().time_since_epoch().count());↵
for (int i = 0; i < 10; i++)↵
cout << experimental::randint(0, 10) << '\n';↵
}↵
~~~~~↵
[Try it](https://codeforces.com/problemset/customtest)↵
↵
randint uses an STL PRNG to return a number in the range of the 2 arguments,exinclusive. The PRNG used appears to be implementation-defined (meaning it might be the subtract-with-carry or linear congruential generator), but that's ok, because what we may lose in RNG quality is made up for by the fact that a hacker won't know which generator is being used.↵
(Note: the documentation says the state of the "per-thread" RNG is unpredictable at the beginning of execution. This does not appear to be the case on Codeforces after some cursory testing. Calling `reseed()` is still necessary.)↵
↵
Defined in the `experimental/random` header is also `experimental::shuffle()` and `experimental::sample()`, which both work like their non-experimental counterparts, using the implementation-defined RNG instead of having to make one to pass as argument.↵
↵
This does have some drawbacks.↵
↵
- Isn't a function object, can't be passed to one of the couple of STL functions that want one↵
↵
- PRNG used is implementation-defined↵
↵
- Experimental header↵
↵
For these reasons, I see it as a convenient middle ground to a Mersenne twister, when you really just need one number for, say, a hash function.↵
↵
[Documentation](https://en.cppreference.com/w/cpp/experimental/randint)
↵
~~~~~↵
for (int i = 1; i < N; i++)↵
swap(permutation[i], permutation[uniform_int_distribution<int>(0, i)(rng)]);↵
~~~~~↵
↵
There's so many *words*. `uniform_int_distribution` is like, 2 really big words. And I hate big words. Fortunately, there's a solution:↵
↵
~~~~~↵
#include <bits/stdc++.h>↵
#include <experimental/random>↵
// Note: experimental headers are not included by bits/stdc++.h↵
using namespace std;↵
int main() {↵
experimental::reseed(chrono::steady_clock::now().time_since_epoch().count());↵
for (int i = 0; i < 10; i++)↵
cout << experimental::randint(0, 10) << '\n';↵
}↵
~~~~~↵
[Try it](https://codeforces.com/problemset/customtest)↵
↵
randint uses an STL PRNG to return a number in the range of the 2 arguments,
(Note: the documentation says the state of the "per-thread" RNG is unpredictable at the beginning of execution. This does not appear to be the case on Codeforces after some cursory testing. Calling `reseed()` is still necessary.)↵
↵
Defined in the `experimental/random` header is also `experimental::shuffle()` and `experimental::sample()`, which both work like their non-experimental counterparts, using the implementation-defined RNG instead of having to make one to pass as argument.↵
↵
This does have some drawbacks.↵
↵
- Isn't a function object, can't be passed to one of the couple of STL functions that want one↵
↵
- PRNG used is implementation-defined↵
↵
- Experimental header↵
↵
For these reasons, I see it as a convenient middle ground to a Mersenne twister, when you really just need one number for, say, a hash function.↵
↵
[Documentation](https://en.cppreference.com/w/cpp/experimental/randint)
