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

Here's the proper way to write what you meant:

int i=2;                     // move declaration out
for(/*int i=2*/;i

But that's not efficient. You only have to check for i's not exceeding of sqrt(num). Plus, if you check num%2, there's no more need to check any other even numbers, so you can use an increment of 2. Or you can even count by 6:

if( num == 2 || num == 3 ) return true;
if( num < 2 || num % 2 == 0 || num % 3 == 0 ) return false;
for( int i=5, j=7, lim=sqrt(num); i<=lim; i+=6, j+=6 ){   
        if( num % i == 0 || num % j == 0 ){ 
            return false;
        } 
}
return true;

_{(notice: this is more efficient than another answer here, which says it's an "optimization" of an initial version of this answer).}