So, I'm beggining to learn programming and trying project euler with java. The problem 10 seems pretty straight forward and I thought I could use a method I used to get primes before in another problem. The thing is, the method works except when I put it in the for loop and I can't manage to see the diference between this and the other.
So this is my code
package euler10;
public class Primesum {
public static void main(String[] args) {
int suma=0;
for (int i=0; i<2000000; i=i+2){
if (isPrime(i) == true){
System.out.println(i);
suma=suma+i;
}
}
System.out.println(suma);
}
public static boolean isPrime(int num) {
boolean prime = false;
long i;
for (i=2; i < Math.sqrt(num) ; i++){
long n = num%i;
if (n == 0){
prime = false;
} else {
prime = true;
}
}
return prime;
}
}
The isPrime method works fine out of the loop but in it it's always true. Even with even numbers it will return true, and I think those aren't very primey :)
I don't really think that there is anything to do with the loop...
However, there is a logic flaw in the code...
public static boolean isPrime(int num) {
long i;
for (i=2; i <= Math.sqrt(num) ; i++){
long n = num%i;
if (n == 0){
return false;//found a divisor : not prime
}
}
//went through all the way to sqrt(num), and found no divisor: prime!
return true;
}
We can stop whenever the first divisor is found, there is no need to find all of them -- that is another excercise...
Also, logically, if one wanted to use the boolean variable this way, it would have been initialised with true, and put to false, and kept at that, when a divisor is found...
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