Uniformity of random numbers taken modulo N

Question

One common way of choosing a random number in [0, n) is to take the result of rand() modulo n: rand() % n. However, even if the r

Answer 1

One approach you can do is the following:

Knowing the value of N, you make R_MAX = ((RAND_MAX + 1) / N) * N; for uniformity.

So you can do your custom rand() function:

int custom_rand(int mod) {
    int x = rand();
    const int R_MAX = ((RAND_MAX + 1) / mod) * mod;    

    while (x > R_MAX) { // discard the result if it is bigger
        x = rand();
    }

    return (x % mod);
}