(C++) std::srand

The seed is the starting point of the std::rand sequence. std::srand() sets this seed. The code below demonstrates that after the same seed (zero in this case), the first 'randomly drawn' number is always the same.

include <iostream>


int main () 
{
  for (int i=0; i<10; ++i)
  {
    std::srand(0);
    std::cout << std::rand() << std::endl;
  }
}

Having a 'random' seed

Use std::clock to set a seed

include <iostream>

include <ctime>


int main()
{
  std::srand(std::clock());
}

Setting the seed of the Boost random library

include <ctime>

include <cassert>

include <boost/random/normal_distribution.hpp>

include <boost/random/lagged_fibonacci.hpp>


int main()
{
  //A random number drawn from a normal distribution
  const double mean  = 0.0;
  const double sigma = 1.0;
  boost::normal_distribution<double> norm_dist(mean, sigma);
  boost::lagged_fibonacci19937 engine;
  //Obtain a random seed
  std::srand(std::clock());
  const unsigned long seed = std::rand(); //Must be of type unsigned long!
  //Set the seed
  engine.seed(seed);
  //Obtain the first value
  const double firstValue = norm_dist.operator () <boost::lagged_fibonacci19937>((engine));
  //Obtain more random numbers
  for (int i=0; i<10; ++i) //Check 10 times
  {
    //Set the seed again
    engine.seed(seed);
    //Get a first temp value (why this is needed I don't know)
    const double temp = norm_dist.operator () <boost::lagged_fibonacci19937>((engine));
    //Obtain the value that should be the same as firstValue
    const double value = norm_dist.operator () <boost::lagged_fibonacci19937>((engine));
    assert(value==firstValue);
  }
}

Code links

last edited (January 3, 2007) by bilderbikkel, Number of views: 5913, Current Rev: 15 (Diff)

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