How to generate a random number between 0 and 1?

In your version rand() % 10000 will yield an integer between 0 and 9999. Since RAND_MAX may be as little as 32767, and since this is not exactly divisible by 10000 and not large relative to 10000, there will be significant bias in the ‘randomness’ of the result, moreover, the maximum value will be 0.9999, not 1.0, and you have unnecessarily restricted your values to four decimal places.

It is simple arithmetic, a random number divided by the maximum possible random number will yield a number from 0 to 1 inclusive, while utilising the full resolution and distribution of the RNG

double r2()
{
    return (double)rand() / (double)RAND_MAX ;
}

Use (double)rand() / (double)((unsigned)RAND_MAX + 1) if exclusion of 1.0 was intentional.

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