How does calling srand more than once affect the quality of randomness?

by

Look at the source of srand() from this question: Rand Implementation

Also, example implementation from this thread:

static unsigned long int next = 1;

int rand(void) // RAND_MAX assumed to be 32767
{
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768;
}

void srand(unsigned int seed)
{
    next = seed;
}

As you can see, when you calling srand(time(0)) you will got new numbers on rand() depends on seed. Numbers will repeat after some milions, but calling srand again will make it other. Anyway, it must repeat after some cycles – but order depends on argument for srand. This is why C rand isn’t good for cryptography – you can predict next number when you know seed.

If you have fast loop, calling srand every iteration is without sense – you can got same number while your time() (1 second is very big time for modern CPUs) give another seed.

There is no reason in simple app to call srand multiple times – this generator are weak by design and if you want real random numbers, you must use other (the best I know is Blum Blum Shub)

For me, there is no more or less random numbers – it always depends on seed, and they repeat if you use same seed. Using time is good solution because it’s easy to implement, but you must use only one (at beginning of main()) or when you sure that you calling srand(time(0)) in another second.

More Related Contents:

  1. Recommended way to initialize srand?
  2. Rand() % 14 only generates the values 6 or 13
  3. rand() generating the same number – even with srand(time(NULL)) in my main!
  4. How often should I call srand() in a C++ application?
  5. srand() — why call it only once?
  6. Why do people say there is modulo bias when using a random number generator?
  7. How to generate a random number in C++?
  8. Generate random numbers using C++11 random library
  9. How to generate different random numbers in a loop in C++?
  10. Create Random Number Sequence with No Repeats
  11. Random number generation in C++11: how to generate, how does it work? [closed]
  12. How to succinctly, portably, and thoroughly seed the mt19937 PRNG?
  13. Generate random numbers following a normal distribution in C/C++
  14. Random array generation with no duplicates
  15. How does modulus and rand() work?
  16. Random number c++ in some range [duplicate]
  17. How do I generate thread-safe uniform random numbers?
  18. Random number generator that produces a power-law distribution?
  19. How to generate random variable names in C++ using macros?
  20. Generating a random integer from a range
  21. Random output different between implementations
  22. Is 1.0 a valid output from std::generate_canonical?
  23. Same random numbers every time I run the program
  24. What is a seed in terms of generating a random number? [duplicate]
  25. The implementation of random_device in VS2010?
  26. How to use function srand() with time.h? [duplicate]
  27. stl random distributions and portability
  28. How to make sure that std::random_shuffle always produces a different result?
  29. C++ function for picking from a list where each element has a distinct probability
  30. What difference between rand() and random() functions?

Leave a Comment