Question

I read about skipList implementation in C++ and I don’t understand this random function :

float frand() {
    return (float) rand() / RAND_MAX;
}
int random_level() {
    static bool first = true;

    if (first) {
        srand( (unsigned)time(NULL) );
        first = false;
    }

    int lvl = (int)(log(frand())/log(1.-P));
    return lvl < MAX_LEVEL ? lvl : MAX_LEVEL;
} 

Thanks for reading and I’m waiting for your answer 🙂

Answer 1

So, the way skiplists work is it makes the new node link to other nodes at levels, randomly choosing to add a level or not. Normally this means flipping a coin once for each level the new node is intended to link to. if it comes up heads, you go up a level and flip again, if tails, you’re done.

What this does is it simulates the flipping of that coin several times, but only calling the random number source once, and applying a function with the same probability distribution as summing consecutive coin flips