CODEWITHSUNDEEP
c++boostseed
Paradox
Paradox

Reputation: 2035

Boost C++: Seeding random numbers in a function

I have a function that should simulate a new random exponential variable every time it is called:

 #include <boost/random.hpp>    

//Simulates a single exponential random variable                                                            
double generateExponential(double lambda) {
      boost::mt19937 rng; //Mersenne Twister Generator                                                          
      rng.seed(time(0));                                                           
      boost::variate_generator< boost::mt19937&, boost::exponential_distribution<> > rndm(rng, boost::exponenti\
    al_distribution<>(lambda));
      return rndm();
  }

for example,

double newExp = generateExponential(10);

However, each time I call the function, it generates the same random number. I want it to generate a different random number each time the function is called. I thought "rng.seed(time(0))" might fix it but it hasn't. How could I get it to do this?

Upvotes: 1

Views: 1270

Answers (2)

jwfearn
jwfearn

Reputation: 29597

If you can't change the signature of your function, then you could use a static instance of your generator. No need to re-seed.

#include <boost/random.hpp>

typedef boost::mt19937 G;
typedef boost::exponential_distribution D;

double generateExponential(double lambda)
{
  static G rng(std::time(0)); // initialized and seeded once
  boost::variate_generator<G &, D<> > rndm(rng, D<>(lambda));
  return rndm();
}

Upvotes: 2

user1084944
user1084944

Reputation:

Generally speaking, a source of random numbers should be a resource whose lifespan is that of your entire program, not that of an individual function call.

Consequently, you need the object representing said source of random numbers to have an appropriate lifespan. Making it a variable local to your function is a Bad Idea. It should be an object passed into your function, or maybe a global object.

(also, frequently reseeding a random number generator is another well known Bad Idea)

Upvotes: 1

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