How to generate a boolean with p probability using C rand() function?

Question

How can I generate a random boolean with a probability of p (where 0 <= p <= 1.0) using the C standard library rand() function?

i.e.

bool nextBool(double probability)
{
    return ...
}

Solution 1

bool nextBool(double probability)
{
    return (rand() / (double)RAND_MAX) < probability;
}

or (after seeing other responses)

bool nextBool(double probability)
{
    return rand() <  probability * ((double)RAND_MAX + 1.0);
}

Solution 2

Do you mean generate a random variable so that p(1) = p and p(0) = (1-p)?

If so, compare the output of rand() to p*RAND_MAX.

Comments

[Colin]

The second one might actually be faster. They're mathematically identical.

[caf]

That needs to be probability * (RAND_MAX + 1) - otherwise, passing a value of 1.0 as probability will result in the function returning 0 sometimes.

[leemes]

Thank you very much. I fixed a minor issue in your second version, as the +1 generated an integer overflow