I was randomly surfing around the internet when I stumbled upon Google Labs Aptitude Test (GLAT)(Found here). I can’t really remember which links I followed from there but I stumbled into a question that asks to find the first 10 consecutive digits of E that are prime. While definitely not a hard task I wrote a small program in C to test whether or not a number is prime.

The core function isPrime() is below

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | bool isPrime(unsigned long long n) { // prime numbers are natural by definition if(n <= 1) { return false; } unsigned long long root = sqrtl(n); // is the number divisible by n, such that n >= 2 and n <= sqrt(number)? // if so then the number is composite for(unsigned long long i = 2; i <= root; i++) { if(n % i == 0) { return false; } } return true; // it is prime! } |

**note:** I use long long just to allow the calculation of bigger numbers. Though if the number gets to large enough this algorithm can still slow down a bit, as it could potentially have to iterate from 3 to the square root of the number being tested. For most cases this algorithm executes plenty fast enough and it is definitely better than iterating over every number between 1 and N, as many sites do.