Question

double get_random(double min, double max) {
  /* Returns a random double between min and max */

  return min * ((double) rand() / (double) RAND_MAX) - max;
}

That’s my function to generate random doubles between a min and a max. However, when I call get_random(-1.0, 1.0);, I get values between -2.0 and -1.0.

Any idea of what I’m doing wrong and how I can fix it?

Answer 1

Shouldn’t the formula be

(max - min) * ( (double)rand() / (double)RAND_MAX ) + min

• (double)rand() / (double)RAND_MAX returns a random number between 0 and 1
• (max - min) * ( (double)rand() / (double)RAND_MAX ) returns a random number between 0 and max - min.
• the whole expression will return a random number between 0 + min and min + (max-min) – i.e. min and max.