Question

http://projecteuler.net/problem=1

If we list all the natural numbers below 10 that are multiples of 3 or
5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the
sum of all the multiples of 3 or 5 below 1000.

If I change “int maxNum” to 10 or any other small number like 20, I’m getting the right answer.

But somehow when I do it with a big number like 1000, it will give me a number which I don’t expect to come, I don’t know why, please help.

Does it do it because it has reach the max value of an int?

class Program
{


    static void Main(string[] args)
    {
        //TASK: If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
        //Find the sum of all the multiples of 3 or 5 below 1000.

        int multiplierA = 3;
        int multiplierB = 5;

        int maxNum = 1000;
        int i = 1;

        int deelEen = MultiplyFactory(multiplierA, i, maxNum);
        int deelTwee = MultiplyFactory(multiplierB, i, maxNum);

        int result = deelEen + deelTwee;

        Console.WriteLine(result);
        Console.Read();
    }

    static int MultiplyFactory(int multiplier, int i, int maxNum)
    {
        List<int> savedNumbers = new List<int>();
        while(multiplier*i < maxNum)
        {
            savedNumbers.Add(multiplier*i);
            i++;
        }

        int answer = 0;
        foreach(int getal in savedNumbers)
        {
            Console.WriteLine(getal);
            answer = answer + getal;
        }
        savedNumbers.Clear();
        return answer;

    }
}

Answer 1

I think the problem is that you need to find sum of all the multiples of 3 OR 5. And what your program is doing is finding sum of all multiplers of 3 + sum of all multipliers of 5.
You can return arrays of int and then sum distinct numbers from arrays.

You can also use Linq to get distinct values from lists

class Program
{
    static void Main(string[] args)
    {
        //TASK: If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
        //Find the sum of all the multiples of 3 or 5 below 1000.

        int multiplierA = 3;
        int multiplierB = 5;

        int maxNum = 1000;
        int i = 1;
        int result = 0;

        List<int> deelEen = MultiplyFactory(multiplierA, i, maxNum);
        List<int> deelTwee = MultiplyFactory(multiplierB, i, maxNum);

        foreach (int val in deelEen)
            result += val;

        foreach (int val in deelTwee)
            if (!deelEen.Contains(val)) result += val;

        Console.WriteLine(result);
        Console.Read();
    }

    static List<int> MultiplyFactory(int multiplier, int i, int maxNum)
    {
        List<int> savedNumbers = new List<int>();
        while (multiplier * i < maxNum)
        {
            savedNumbers.Add(multiplier * i);
            i++;
        }

        return savedNumbers;

    }
}