C++ code for checking for prime numbers not working

Question

I\'m having trouble with this C++ code. The integer num is a number that I want to check if it is prime. However this program is always returning false. It\'s proba

Answer 1

You don't have to check every number, as a lot of them can easily be ruled out. For example, after checking that num is not divisible by 2, you can skip all other even numbers. That saves you half the tests.

We also definitely know that any other factor must be less than num/2 (or really sqrt(num), but that is harder to compute). That knowledge can save us another half of the tests.

So now we have:

if (num % 2 == 0)
    return false;

for(int i = 3; i < num / 2; i += 2){ 
     if(num % i == 0){ //can be divided by a number other than itself or 1 so we trip out
         return false;
     }
}

// arriving here we have found no factors, so it must be a prime
return true;