is_prime.c

// n must have exactly 2 distinct divisors to be prime
int is_prime(int n) {
    if (n < 0) {
        n = -n;
    }

    if (n <= 100) { // first 25 primes (add more?)
        switch (n) {
            case  2: case  3: case  5: case  7: case 11: case 13: case 17:
            case 19: case 23: case 29: case 31: case 37: case 41: case 43:
            case 47: case 53: case 59: case 61: case 67: case 71: case 73:
            case 79: case 83: case 89: case 97:
                return 1; // prime
            default:
                return 0; // not prime
        }
    }

    for (int i = 2; i < n; ++i) { // exclude 1 and n
        if (n % i == 0) {
            return 0; // not prime: divisible by i
        }
    }

    return 1; // prime
}