mgroves

Project Euler is an ongoing series of math/programming problems. I’m going to try to go through as many of these as I can this year, post the code for them, and talk about anything interesting that I learn in the process.

You can follow my progress by checking out a CodePlex repository for Project Euler that I’ve set up. If you have a Project Euler account, you should be able to see my progress on the mgroves Project Euler profile page.

The first problem was pretty easy: “Find the sum of all the multiples of 3 or 5 below 1000.” To do this, I just loop through each number from 3 to 999, and take the modulus 3 and 5 of that number. If either modulus is 0, add it to a list. When done, sum up the list:

            for (int i = 3; i < x; i++)
            {
                if (((i%3) == 0) || ((i%5 == 0)))
                {
                    list.Add(i);
                }
            }

The second problem is slightly more difficult: Find the sum of all even numbers in the Fibbonaci sequence which do not exceed 4,000,000. My code takes a straightforward approach; I'm sure the math majors out there can think of a better way:

        public static IList FibonacciAllEvenValueTermsLessThan(int x)
        {
            var list = new List();
            int term1 = 1;
            int term2 = 2;
            list.Add(term2);
            while(term2 < x)
            {
                var newterm1 = term2;
                var newterm2 = term1 + term2;
                if((newterm2 % 2) == 0)
                {
                    list.Add(newterm2);
                }
                term1 = newterm1;
                term2 = newterm2;
            }
            return list;
        }

For both problems, I take the result and use the Linq Sum() extension method, because I'm a total Linq junky these days. I also started out using longs instead of ints, before I realized that, hey, 32-bit numbers go up to around 4 billion (2³²), not 4 million.

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