Thread: Overflow and time-efficient in a for-loop?

Project Euler, problem 2: Determine the sum of the even numbers in the Fibonacci sequence up to 4 000 000. First I tried to
use a recursive algorithm for the sequence but I realized I dont have all the time in the world so I now use an iterative. It is still
extremely slow. Can I improve my code?

 
 
 
public class Euler2Correct {
 
    public static int Fibonacci(int j){ 
 
        /**
         * Metod for returnerning number [I]j[/I] in the sequence.
         * 
         */
 
        if(j<=1){
            return 1;
        }
        else if(j==2){
            return 2;
        }
 
        int tmp;
        int a=2;
        int b=1;
 
        for(int k=3; k<=j; k++){
 
            tmp=a+b;
            b=a;
            a=tmp;
 
        }
 
        return a;
 
    }
 
    public static void main(String[]args){
 
       int s=0;
 
        for(int i=2; i<4000000; i=i+3){ //Every three number is even
 
            s = s + Fibonacci(i);
 
        }
        System.out.println(s);
 
    }  
}

You could read up on dynamic programming if you want to get it much faster, but I am not going to explain it in this post, it would be a little bit too much.

Originally Posted by Cornix

You could read up on dynamic programming if you want to get it much faster, but I am not going to explain it in this post, it would be a little bit too much.

I was more concerned about obvious flaws and time-consuming operations(like the recursive way, although simpler). I went through the Wiki page and it looks interesting but I doubt that it requires that kind of knowledge. Just something easy and neat.

Yes, this was a good observation. So I better keep the running total in the Fibonacci method, and use the previous results to prevent doing all that extra work. I'll modify it.