std::srand sets the seed of the random number sequence that can be created by std::rand sequence.

The code below demonstrates that after setting a seed of zero, 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;

  }

}

Example: RandomizeTimer

RandomizeTimer can be used to set a seed based on the system time.


#include <cstdlib>

#include <ctime>

 

//From http://www.richelbilderbeek.nl/CppRandomizeTimer.htm

void RandomizeTimer()

{

  std::srand(std::time(0));

}

Setting the seed of the Boost Random library

The code below demonstrates that after setting a seed of zero, the first 'randomly drawn' number is always the same.


#include <cassert>

#include <ctime>

#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);

  }

}

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(C++) std::srand

Example: RandomizeTimer

Setting the seed of the Boost Random library