I don't get what's wrong with my Javascript code

Question

I'm trying to solve Project Euler's second problem which goes:

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.

And this is how I tried to solve it in Javascript + Why I think it's logical:

var max = 4000000; //We create a variable with the maximum amount allowed, which is 4 million.
var result = 0; //Here we create a variable to store our result.

for (var i = 0; i < max; i++) //Now let's loop through the 4 million.
{
    if (i % 2 === 0) //We're checking for even numbers here because as I understood it, we have to only use the even numbers in this problem.
    {
        result = result + i; //Here we're adding "result" (Which is 0) to itself plus *i* (Which we're looping through!). We should have the result at the end of this loop.
    }
}

console.log(result); //Print our result.

I know that Fibonacci numbers add the previous number in the row to themselves, so 1 + 2 would be 3.

According to my logic, that's what I'm doing with result = result + i, yet I'm getting the wrong answer.

I can't see the logic here.

You're adding all the even numbers. You've left out the bit where you're advancing through the Fibonacci sequence. — brycem
– brycem, Commented Nov 13, 2014 at 22:48
You're looking for even values of i. Will that give you even numbers in the Fibonacci sequence? (Hint: No) — Oliver Shaw
– Oliver Shaw, Commented Nov 13, 2014 at 22:48
Like brycem and Oliver said: you are adding together all the even number between 1 and 4 million. You need to first, find the Fibonacci numbers, and then second, add together all the even ones. — the_prole
– the_prole, Commented Nov 15, 2014 at 2:02

j-da · Accepted Answer · 2014-11-16 16:55:59Z

1

Not every even number below 4,000,000 is in the fibonacci sequence. Try the following:

var max = 4000000, first = 1, second = 1, next = 0, result = 0;
while (second < max) {      // until we pass our limit
  next = first + second;    // find the next number in the fibonacci sequence by adding the previous two
  first = second;           // move on to the next numbers in the sequence
  second = next;
  if (second % 2 === 0)     // if the second number is even
    result += second;       // add it to our result
}
return result;              // result holds the sum of all the even fibonacci numbers below our limit!

edited Nov 16, 2014 at 16:55

answered Nov 13, 2014 at 22:58

j-da

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2 Comments

SstrykerR Over a year ago

Can you explain the logic in this? I don't understand why this works and how it's adding the fibonacci numbers.

SstrykerR Over a year ago

That is absolutely beautiful. Thank you!

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