How to predict rand() function in C?

Depending on the range of values for your random number sequence, you are probably on a loser - rand doesn't just have an algorithm, it also has a seed value which is normally initialized from the system clock at some point in the application. Depending on the seed, where the algorithm "starts" and thus the sequence it produces. Even if the range is just 1, 2, or 3 the number of possible sequences rapidly grows out of any possible prediction unless you know the algorithm, and the starting seed, and how many items have been "taken" from the random sequence already.