Find primes using "naive" algorithm
A quick detail

You've written inc == 2 ? inc = 4 : inc = 2; : this is not the ""right"" way to use the "ternary" operator. Don't get me wrong, it will do what you expect it to do. However, the whole point of the operator is to return a value. In your case, it is no different that : if (inc == 2) inc = 4; else inc = 2;.

This calls for a ternary operator but to write it in a simple way : inc = (inc == 2) ? 4 : 2;