Question

i’m getting a bit confused with operators and there use with random generation. I guess i’m just asking does this code do what I want it to?

Generate a ‘random’ TRUE or FALSE depending on what probability I assigned the function.

bool randtf(int probability) {
    if ((rand() % 100) < probability)
        return true;
    else
        return false;
}

so if randtf(63) it has a 63% chance of being TRUE?

Any guidance would be much appreciated. Thanks.

Answer 1

Yes, to a first approximation.

No, more accurately. rand() returns a number between 0 and RAND_MAX, which in practice will always be of the form (1 << n) - 1. This isn’t a multiple of 100, so you won’t get a perfectly uniform distribution when you take the modulo.

You can fix that by using rejection sampling. For the sake of argument, let’s assume RAND_MAX == 32767 (i.e. 16-bit). The first step is to keep generating random numbers, rejecting them until you obtain one less than 32700 (the largest multiple of 100 that’s less than RAND_MAX). If you then do the modulo trick on that, you will get a uniform distribution.

Of course, this assumes a sane, statistically robust, implementation of rand(), which is quite an assumption!