The Modern C++ Challenge

4. Largest prime smaller than given number

A prime number is a number that has only two divisors, 1 and the number itself. To find the largest prime smaller than a given number you should first write a function that determines if a number is prime and then call this function, starting from the given number, towards 1 until the first prime is encountered. There are various algorithms for determining if a number is prime. Common implementations for determining the primality appear as follows:

bool is_prime(int const num) {   if (num <= 3) { return num > 1; }   else if (num % 2 == 0 || num % 3 == 0)    {       return false;    }   else    {      for (int i = 5; i * i <= num; i += 6)       {         if (num % i == 0 || num % (i + 2) == 0)          {            return false;         }      }      return true;   }}