[Question] Random Number Generate Question

Question

link

Given a RNG r(5) which generates number between 1-5 uniformly, make r(7) which generates random number between 1-7.

Solution

int rand7() {
    int r = 0;
    do {
        int a = rand(5) - 1;    //uniformly at random from 0 to 4
        int b = rand(5) - 1;
        r = 5 * b + a;            //uniformly at random from 0 to 24
    }
    while (r >= 21);            // in this event, we have to roll again
    return r % 7 + 1;
}