Boost library- not generating distinct random numbers

Question

I am using this following piece of code and calling it in a loop so that i can get multiple random numbers for my perfect hash function. But it is giving me same number, not distinct. "min" receives 0 or 1 and max receives "p-1". Where p is a prime number greater then hash table size.

long long int generateSingleRandomNo2(long long int min, long long int max) 
{  
typedef boost::uniform_int<long long int> NumberDistribution; 
typedef boost::mt19937 RandomNumberGenerator; 
typedef boost::variate_generator<RandomNumberGenerator&,NumberDistribution> Generator; 

NumberDistribution distribution(min,max); 
RandomNumberGenerator generator; 
Generator numberGenerator(generator, distribution); 
generator.seed(time(0)); // seed with the current time 
long long int A=numberGenerator();
return A;
/*if(A>min)
  return A ;
else 
generateSingleRandomNo1(min,max);*/

}

You should inspect your seed values; are *they* distinct? Why are you re-seeding each time? — Oliver Charlesworth, Sep 07 '14 at 14:25
I am using boost for the first time ! Where i should place it @OliCharlesworth — rayan, Sep 07 '14 at 14:29
Another similar question that might help: http://stackoverflow.com/questions/1845456/boostrandom-generate-the-same-number-every-time — Ryan Bemrose, Sep 07 '14 at 15:00

Rimas · Accepted Answer · 2014-09-07T16:18:10.407

To get distinct random numbers you need call seed(...) only once. One of possible solutions is to use static RandomNumberGenerator instance:

long long int generateSingleRandomNo2(long long int min, long long int max) 
{  
  struct RandomNumberGenerator : public boost::mt19937 {
    RandomNumberGenerator() {
      seed(time(NULL)); // seed with the current time
    }        
  };
  typedef boost::uniform_int<long long int> NumberDistribution; 
  typedef boost::variate_generator<RandomNumberGenerator&,NumberDistribution> Generator; 

  // static generator instance
  static RandomNumberGenerator generator;

  // generate random number
  NumberDistribution distribution(min, max); 
  Generator numberGenerator(generator, distribution); 
  long long int A = numberGenerator();
  return A;
  /*if(A>min)
    return A ;
    else 
    generateSingleRandomNo1(min,max);
  */
}

Boost library- not generating distinct random numbers

1 Answers1