Fibonacci Sequence Problem

[Zigster]

The challenge:

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

My code:

public class Problem2 {
	public static void main (String args[]){
		
int fibonacci, fibonacci2, sum, evens;
fibonacci=1;
fibonacci2=1;
sum=2;
evens=0;

	while (fibonacci<=4000000 || fibonacci2<=4000000 || sum<=4000000)
		{
		if (fibonacci<=4000000 && fibonacci%2==0){evens= evens + fibonacci;
		}
		if (fibonacci2<=4000000 && fibonacci2%2==0){
			evens= evens + fibonacci2;
		}
		if (sum<=4000000 && sum%2==0){
			evens= evens + sum;
{sum=fibonacci + fibonacci2;
fibonacci= sum + fibonacci2;
fibonacci2= sum + fibonacci;
	}
System.out.println(fibonacci);
System.out.println(fibonacci2);
System.out.println(sum);

	if (fibonacci>=4000000 ||
fibonacci2>=4000000 ||
sum>=4000000){
		break;
	}
	}
}

System.out.println("Sum of the evens under 4 million: ");
System.out.println(evens);
	}
}

This is what it prints out:

3
5
2
13
21
8
55
89
34
233
377
144
987
1597
610
4181
6765
2584
17711
28657
10946
75025
121393
46368
317811
514229
196418
1346269
2178309
832040
5702887
9227465
3524578
Sum of the evens under 4 million:
1089156

Problems:

The answer is completely wrong, my "evens" interger totally fails, although I don't know why. The fibonacci numbers are correct, but not in order, and it's giving me numbers over 4 million which I don't want. I'm trying to tell it (but obviously failing) at the beginning and end of the loop to stop if any number is equal or greater to 4 million. But, since I'm adding numbers together, 3.9 mil + 3.9 mil is gonna be more than 4 mil, but my code isn't going to stop that. Although how it gets a 9mil number I have no idea.

What I want to do is for "evens" to check every number under 4 million to see if %2==0, and then add that number to evens. I'd prefer if numbers over 4 million weren't generated by fibonacci, fibonacci2 or sum as well.

Cheers!

[Zigster]

Would an array be better for this? How can I put the numbers I've printed into an array? And is there some way to get Phi and Pi in java (So I can use the Fibonacci forumla)? Thanks!

[kosmos890]

Ι use recursion to find fibonacci sequence. I wish this help you.

public class Main{
	
	public static void main(String[] args){

		long N=0,sum=0,y; 

		while(fib(N)<4000000){
		
			y=fib(N);

			if(y%2==0){
				sum+=y;
				System.out.println("Even F"+N+"\t "+y);
			}

			else{
				System.out.println("F"+N+"\t "+y);
			}
			
			N++;
		}

		System.out.println("SUM :"+sum);
	
	}

	public static long fib(long N){
		
		if(N==1){   //f1=1
			return 1;
		}

		if(N==0){   //f0=0
			return 0;
		}
		//fn=fn-1 + fn-2
		return fib(N-1)+fib(N-2);
	}
}

[JosAH]

I don't understand all that juggling; no matter how you generate the Fibonacci numbers, somewhere in the body of a loop (where the current Fibonacci number < 4000000) I'd like to see something like this:

if (fib%2 == 0) // even?
   sum+= fib; // add it to the sum

kind regards,

Jos

[Zigster]

Thanks guys. Yes my code is very awkward. I couldn't remember how to solve the sequence with a formula (been a fair while since I did this kind of maths back in high school). Brand new to coding and java.

So is: fib(N-1)+fib(N-2) the formula? I still don't really understand how it's being resolved in Kosmos' code, mostly the fib method I don't get. And why do you use long instead of int? And when fib (or y) is in the main method, what exactly does it do? Executes the fib method?

I tried to copy and paste kosmos's code into eclipse to try it, but I get this error: Error: Could not find or load main class Main?

Cheers!

[kosmos890]

I use vim and terminal to write and run code.
My public class (top level class) is named Main, thus the source file should be named Main.java
The source file must have the same name as the public class.

The function fib() returns the Nth term. fib(3)=2, fib(4)=3
The next term is the sum of the two previous terms.
Therefore fib(5)=fib(4)+fib(3)=3+2=5 , fib(N)=fib(N-1)+fib(N-2)
The function is recursive Java Recursion with examples

Even F0	 0
F1	 1
F2	 1
Even F3	 2
F4	 3
F5	 5
Even F6	 8
F7	 13
F8	 21
Even F9	 34
F10	 55
F11	 89
Even F12	 144
F13	 233
F14	 377
Even F15	 610
F16	 987
F17	 1597
Even F18	 2584
F19	 4181
F20	 6765
Even F21	 10946
F22	 17711
F23	 28657
Even F24	 46368
F25	 75025
F26	 121393
Even F27	 196418
F28	 317811
F29	 514229
Even F30	 832040
F31	 1346269
F32	 2178309
Even F33	 3524578
SUM :4613732

[Zigster]

Thanks Kosmos, it works now! Your explanation is great, I understand what is happening! That is what I wanted to do, but didn't know how. I'm going to bank the knowledge and move on to the next challenge.

Actually I went back and fixed up this code:

public class Problem2 {
	public static void main (String args[]){
		
		int fibonacci, fibonacci2, sum, evens;
		fibonacci=0;
		fibonacci2=1;
		sum=0;
		evens=0;
		System.out.println("Fibonacci Sequence, evens only:");
			while (sum<4000000)
			{
				sum += fibonacci;
				fibonacci = fibonacci2;
				fibonacci2 = sum;
				
				if (sum%2==0)
				{
				System.out.println(sum);
				evens+=sum;
				}
			}
		System.out.println("Sum of the evens under 4 million: " + evens);
	}
}

[oxana]

public class Fibonacci {
public static void main (String[] args)
int sum=0, b=0, c=1;

for(int i = 0; i<=20; i++){
sum = b+c;
b=c+sum;
c=sum+b;
System.out.println(b+c+sum);

}



}
}

[oxana]

this is wrong

[oxana]

//this works

public class Fibonacci {
public static void main (String[] args) {
int sum=0, b=0, c=1;

for(int i = 0; i<=200; i++){
sum = b+c;
b=c;
c=sum;
System.out.println(sum);
}



}
}
out put 1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765
10946
17711
28657
46368
75025
121393
196418
317811
514229
832040
1346269
2178309
3524578
5702887
9227465
14930352
24157817
39088169
63245986
102334155
165580141
267914296

[JosAH]

Are you out of scribbling paper? Please, don't abuse this forum for it.

kind regards,

Jos