Uniform random number generator in closed interval

Question

I need a unbiased uniform random number generator (of double) for c or c++ on a closed interval, say [0,1].

Many random number generators, such as the GSL gsl_rng_uniform(r),works only in open intervals, as [0,1), but this does not fit into my case.

Al.

Answer 1

There is something wrong with the concept of a closed interval unless you have integer values - since the probability of hitting the "end" of the interval is infinitesimally small. If you have an integer random number generator with a maximum value RAND_MAX, then you get a closed interval by computing

rand() / double(RAND_MAX)

since it will go exactly from 0 to 1 (inclusive)

If this does not give you sufficient resolution, you could consider "concatenating" multiple random numbers. Given that a double actually has a mantissa of 53 bits (plus one bit that is "always 1"), you could do this (this code assumes that RAND_MAX = 0xFFFFFFFF for readability; you can improve on it...):

#include <stdio.h>
#include <stdlib.h>

double goodRand() {
  unsigned long long r;
  unsigned long long int r1, r2;
  r1 = rand();
  r2 = rand();
  r = (r1 << 22) + (r2 & 0x003FFFFF);
  return (double) r/ (double)(0x001FFFFFFFFFFFFF);
}

int main(void) {
  int i;
  double rMax = 0;
  for (i =0; i < 10000; i++) {
    double temp;
    temp = goodRand();
    rMax = (temp>rMax)?temp:rMax;
  }
  printf("max value is %lf\n", rMax);
}

Now you have very finely sampled random numbers with a maximum value of 1.0 (inclusive). You can make the code a bit more compact but this is the general idea...