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Master your primes: sieve with memoization
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).
Memoization
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 ;)