Random integers c++

Question

I'm trying to produce random integers (uniformly distributed). I found this snippet on an other forum but it works in a very weird way..

 srand(time(NULL));
    AB=rand() % 10+1;

Using this method I get values in a cycle so the value increases with every call until it goes down again. I guess this has something to do with using the time as aninitializer? Something like this comes out.

 1 3 5 6 9 1 4 5 7 8 1 2 4 6 7.

I would however like to get totally random numbers like

1 9 1 3 8 2 1 7 6 7 5...

Thanks for any help

Answer 1

You can use the the Park-Miller "minimal standard" Linear Congruential Generator (LCG): (seed * 16807 mod(2^31 - 1)). My implementation is here Random integers with g++ 4.4.5

The C language 'srand()' function is used to set the global variable that 'rand()' uses. When you need a single sequence of random numbers, 'rand()' is more than enough, but oftentimes you need several random number generators. For those cases, my advice would be to use C++ and a class like 'rand31pmc'.

If you want to generate random numbers in a small range, then you can use the Java library implementation available here: http://docs.oracle.com/javase/7/docs/api/java/util/Random.html#nextInt%28int%29

Answer 2

Also check out the Boost Random Number Library:

Boost Random Number Library

Answer 3

• srand() has to be done once per execution, not once for each rand() call,

• some random number generators have a problem with using "low digit", and there is a bias if you don't drop some number, a possible work around for both issues:

  int alea(int n){ 
     assert (0 < n && n <= RAND_MAX); 
     int partSize = 
       n == RAND_MAX ? 1 : 1 + (RAND_MAX-n)/(n+1); 
     int maxUsefull = partSize * n + (partSize-1); 
     int draw; 
     do { 
        draw = rand(); 
     } while (draw > maxUsefull); 
     return draw/partSize; 
  }
  

Answer 4

You should call srand() only once per program.