Generate random double number in range [0, 1] in C

Question

I have to generate random double number in between 0 and 1. How can i do this using C? I\'ve tried to generate an integer number between 0 and one million and then divide th

Answer 1

generate random double number in between 0 and 1

Some things to consider:

Include 1.0??

I'd expect a goal of "in range [0, 1)" rather than "in range [0, 1]". That inclusion of 1.0 creates complexities as it unbalances the distribution. Let us assume [0, 1).

rand() Quality

rand() is of unspecified quality and and its range [0...RAND_MAX] might not end 1 less than a power of two. Let us assume rand() is good enough for now and RAND_MAX is a Mersenne Number (which is very common) to facilitate joining multiple rand() calls with a simple ^.

Small numbers

About half of all positive double are less than 1.0. Should all of them have a chance?

Typicality DBL_MANT_DIG is 53 and so:

There are 2⁵² double values in the [0.5...1.0) range.
There are 2⁵² double values in the [0.25...0.5) range.
There are 2⁵² double values in the [0.125...0.25) range.
...

Do we want a 50% chance of a value from the first group, 25% from the next, 12.5% from the next .... ?

Or satisfied with:

Form 2⁵³ double values in the [0.0...1.0) range evenly distributed?

---

Let us go for the second goal for now - easier to code.

Generate an whole number double in the range [0...2⁵³) and then divide by 2⁵³.

RAND_MAX >= 0x7FFF by definition, so we get at least 15 bits of random bits.

Below is some illustrative code that, not so efficiently, forms a double [0.0 ... 1.0).

// Illustrative code
double rand_double_01(void) {
  unsigned long long r = 0;
  #define RANDOM_BITS 15
  for (int i = 0; i < DBL_MANT_DIG; i += RANDOM_BITS) {
    r <<= RANDOM_BITS;
    r ^= rand();
  }
  r %= 1uLL << DBL_MANT_DIG;   // Mask off lower 53 bits
  double dr = r; // expected conversion is exact
  // scale [0 ... 1.0)
  dr /= 1uLL << DBL_MANT_DIG;  // expected conversion/quotient exact 
  return double dr; 
}

Note: Above code can fail when DBL_MANT_DIG >= 64 (not common) or FLT_RADIX != 2 (very uncommon).