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;
   }
}

This function can be used as follows:

int main()
{
   int limit = 0;
   std::cout << "Upper limit:";
   std::cin >> limit;

   for (int i = limit; i > 1; i--)
   {
      if (is_prime(i))
      {
         std::cout << "Largest prime:" << i << std::endl;
         return 0;
      }
   }
}