c++ Boost random ints from multiple ranges using single generator

Question

Using boost::random I'm trying to sample sets of uniformly distributed integers from different ranges, but using the same underlying random number generator. For each range I am defining a different function returning the the random numbers. However, it seems that each function returns the same number.

An example illustrating the approach:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/random.hpp>

struct RandomNumberGenerator {
  boost::mt19937 generator;

  RandomNumberGenerator(long seed) {
    generator.seed(seed);
  }

  boost::function<int()> getRandomFunctionInt(int min, int max) {
    boost::uniform_int<> uni_dist(min, max);
    boost::variate_generator<boost::mt19937&, boost::uniform_int<> > uni(generator, uni_dist);

    boost::function<int()> f;
    f = boost::bind(uni_dist, generator);
    return f;
  }
};

int main (int argc, char* argv[]) {
  RandomNumberGenerator rng(1729);
  boost::function<int()> runif1 = rng.getRandomFunctionInt(0,  1000);
  boost::function<int()> runif2 = rng.getRandomFunctionInt(0, 10000);

  for (int i=0; i<10; ++i) {
    std::cout << runif1() << ", " << runif2() << std::endl;
  }
}

The output was:

212, 2121
623, 6226
259, 2590
[...]

Is there any way to un-correlate the functions? For the sake of reproducibility of my experiment I would like to work with a single seed.

Xirema · Accepted Answer · 2016-06-22T22:08:33.907

0

f = boost::bind(uni_dist, generator);

You need to pass a reference to the generator. Right now, you're passing a copy of the generator, which means that each f object has a copy of the generator in its initial state.

f = boost::bind(uni_dist, boost::ref(generator));

A different version using Lambdas would make the ambiguity more visible (and easier to detect/resolve):

f = [uni_dist, this] {return uni_dist(generator);};

Also, c++11 introduced the <random> header, which provides all the same functions and more that boost's random headers provide, so there's not really any reason to keep using the boost version anymore.

edited Jun 22 '16 at 22:08

answered Jun 22 '16 at 16:13

Xirema

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Thanks for pointing out my rookie mistake... answer accepted and appreciated! Switching to the standard `` header is good advice as well. +1 for the solution using lambda functions - one small catch, though: in C++11 member vars (like `generator`) cannot be captured directly, had to resort to a local variable and capture that one: `boost::mt19937 &g = generator` – mrhd Jun 22 '16 at 21:54
@mrhd I think member variables can be captured if you capture `this`. I've revised the answer to account for that. – Xirema Jun 22 '16 at 22:08
Thanks for the update, your solution is more concise. I realize now that it was really the boost::bind() that made a copy since you did not need to pass by reference in `uni_dist(generator)`. – mrhd Jun 23 '16 at 07:56

c++ Boost random ints from multiple ranges using single generator

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