Project Euler - Number 2

Question

I'm quite new to programming in C#, and thought that attempting the Euler Problems would be a good idea as a starting foundation. However, I have gotten to a point where I can't seem to get the correct answer for Problem 2.

"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 is this:

int i = 1;
int j = 2;
int sum = 0;

while (i < 4000000) 
{
     if (i < j)
     {
         i += j;

         if (i % 2 == 0)
         {
             sum += i;
         }

     }

     else
     {
         j += i;

         if (j % 2 == 0)
         {
             sum += j;
         }
     }
}

MessageBox.Show("The answer is " + sum);

Basically, I think that I am only getting the last two even numbers of the sequence and adding them - but I don't know how to get all of the even numbers of the sequence and add them. Could someone please help me, whilst trying to progress from my starting point?

P.S. - If there are any really bad layout choices, do say as eliminating these now will help me to become a better programmer in the future :)

Answer 1

I just logged in into my Project Euler account, to see the correct answer. As others say, you forgot to add the initial term 2, but otherwise your code is OK (the correct answer is what your code outputs + 2), so well done!

It is pretty confusing though, I think it would look way clearer if you'd use 3 variables, something like:

int first = 1;
int second = 1;
int newTerm = 0;
int sum = 0;

while (newTerm <= 4000000) 
{
    newTerm = first + second;

    if (newTerm % 2 == 0)
    {
         sum += newTerm;
    }

    first = second;
    second = newTerm;
}

MessageBox.Show("The answer is " + sum);

Answer 2

You need to set the initial value of sum to 2, as you are not including that in your sum with the current code.

Also, although it may be less efficient memory-usage-wise, I would probably write the code something like this because IMO it's much more readable:

var fibonacci = new List<int>();

fibonacci.Add(1);
fibonacci.Add(2);

var curIndex = 1;

while(fibonacci[curIndex] + fibonacci[curIndex - 1] <= 4000000) {
    fibonacci.Add(fibonacci[curIndex] + fibonacci[curIndex - 1]);
    curIndex++;
}

var sum = fibonacci.Where(x => x % 2 == 0).Sum();