How to use <random> to replace rand()?

Question

C++11 introduced the header <random> with declarations for random number engines and random distributions. That's great - time to replace those uses of rand() which is often problematic in various ways. However, it seems far from obvious how to replace

srand(n); // ... int r = rand(); 

Based on the declarations it seems a uniform distribution can be built something like this:

std::default_random_engine engine; engine.seed(n); std::uniform_int_distribution<> distribution; auto rand = [&](){ return distribution(engine); } 

This approach seems rather involved and is surely something I won't remember unlike the use of srand() and rand(). I'm aware of N4531 but even that still seems to be quite involved.

Is there a reasonably simple way to replace srand() and rand()?

Answer 1

Is there a reasonably simple way to replace srand() and rand()?

Full disclosure: I don't like rand(). It's bad, and it's very easily abused.

The C++11 random library fills in a void that has been lacking for a long, long time. The problem with high quality random libraries is that they're oftentimes hard to use. The C++11 <random> library represents a huge step forward in this regard. A few lines of code and I have a very nice generator that behaves very nicely and that easily generates random variates from many different distributions.

Given the above, my answer to you is a bit heretical. If rand() is good enough for your needs, use it. As bad as rand() is (and it is bad), removing it would represent a huge break with the C language. Just make sure that the badness of rand() truly is good enough for your needs.

C++14 didn't deprecate rand(); it only deprecated functions in the C++ library that use rand(). While C++17 might deprecate rand(), it won't delete it. That means you have several more years before rand() disappears. The odds are high that you will have retired or switched to a different language by the time the C++ committee finally does delete rand() from the C++ standard library.

I'm creating random inputs to benchmark different implementations of std::sort() using something along the lines of std::vector<int> v(size); std::generate(v.begin(), v.end(), std::rand);

You don't need a cryptographically secure PRNG for that. You don't even need Mersenne Twister. In this particular case, rand() probably is good enough for your needs.

Update
There is a nice simple replacement for rand() and srand() in the C++11 random library: std::minstd_rand.

#include <random> #include <iostream>  int main () {     std:: minstd_rand simple_rand;      // Use simple_rand.seed() instead of srand():     simple_rand.seed(42);      // Use simple_rand() instead of rand():     for (int ii = 0; ii < 10; ++ii)     {         std::cout << simple_rand() << '\n';     } } 

The function std::minstd_rand::operator()() returns a std::uint_fast32_t. However, the algorithm restricts the result to between 1 and 2³¹-2, inclusive. This means the result will always convert safely to a std::int_fast32_t (or to an int if int is at least 32 bits long).

Answer 2

How about randutils by Melissa O'Neill of pcg-random.org?

From the introductory blog post:

randutils::mt19937_rng rng;  std::cout << "Greetings from Office #" << rng.uniform(1,17)           << " (where we think PI = "  << rng.uniform(3.1,3.2) << ")\n\n"           << "Our office morale is "   << rng.uniform('A','D') << " grade\n";