Random Number Generator not working

Question

I use this function to create random numbers between 100000000 and 999999999

int irand(int start, int stop) {
  double range = stop - start + 1;
  return start + (int)(range * rand()/(RAND_MAX+1.0));
}

When I use it like this, it's properly working

while(1) {
    DWORD dw = irand(100000000, 999999999);
    printf("dynamic key: %d\n", dw);
    Sleep(100);
}

But when I use this function without the while(1), I always get the same number. What do I need to fix?

Answer 1

The random number generator must be seeded with some source of entropy for you to see different sequences each time, otherwise an initial seed value of 1 is used by the rand function. In your case, calling srand (time(NULL)); once before the irand function is first called should do the trick. You might find this question useful: why do i always get the same sequence of random numbers with rand() ?

Answer 2

Not a direct answer to your question, but something you should probably read if you're struggling with random numbers. This recent article by our very own Jon Skeet is a good intro to random numbers and the trubles one might run into: http://csharpindepth.com/Articles/Chapter12/Random.aspx