As most of you might know already, a prime number is an integer n with the following properties:
it must be greater than 1 it must be divisible only by itself and 1 And that's it: -15 or 8 are not primes, 5 or 97 are; pretty easy, isn't it?
Well, turns out that primes are not just a mere mathematical curiosity and are very important, for example, to allow a lot of cryptographic algorithms.
Being able to tell if a number is a prime or not is thus not such a trivial matter and doing it with some efficient algo is thus crucial.
There are already more or less efficient (or sloppy) katas asking you to find primes, but here I try to be even more zealous than other authors.
You will be given a preset array/list with the first few primes. And you must write a function that checks if a given number n is a prime looping through it and, possibly, expanding the array/list of known primes only if/when necessary (ie: as soon as you check for a potential prime which is greater than a given threshold for each n, stop).
Storing precomputed results for later re-use is a very popular programming technique that you would better master soon and that is called memoization; while you have your wiki open, you might also wish to get some info about the sieve of Eratosthenes (one of the few good things I learnt as extra-curricular activity in middle grade) and, to be even more efficient, you might wish to consider an interesting reading on searching from prime from a friend of mine [she thought about an explanation all on her own after an evening event in which I discussed primality tests with my guys].
Yes, you will be checked even on that part. And you better be very efficient in your code if you hope to pass all the tests ;)