Secure Programming Cookbook for C and C++

11.12. Getting a Random Floating-Point Value with Uniform Distribution

Problem

When looking for a random floating-point number, we usually want a value between 0 and 1 that is just as likely to be between 0 and 0.1 as it is to be between 0.9 and 1.

Solution

Because of the way that floating-point numbers are stored, simply casting bits to a float will make the distribution nonuniform. Instead, get a random unsigned integer, and divide.

Discussion

Because integer values are uniformly distributed, you can get a random integer and divide so that it is a value between 0 and 1:

#include <limits.h>
   
double spc_rand_real(void) {
  return ((double)spc_rand_uint(  )) / (double)UINT_MAX;
}

Note that to get a random number between 0 and n, you can multiply the result of spc_rand_real( ) by n. To get a real number within a range inclusive of the range’s bounds, do this:

#include <stdlib.h>
   
double spc_rand_real_range(double min, double max) {
  if (max < min) abort(  );
  return spc_rand_real(  ) * (max - min) + min;
}