How do I make the sum of numbers equal to 1

Question

I work with probabilities and I need to check that the sum of the values is 1. I was writing unit-tests and one of the tests was failing. This is why it was failing:

double[] probabilities = new[] { 0.4, 0.3, 0.2, 0.1 };
double sum = probabilities.Sum();
//On my PC gives sum of 0.99999999999999989

if (sum != 1)
{
    throw new ArgumentException(
        "Sum of the probabilities does not equal to 1. " +
        "Computed value was: " + sum);
}

What can I change to make this true: 0.4 + 0.3 + 0.2 + 0.1 = 1?

Answer 1

double isn't going to give you the precision you need for that kind of math.

Switch to decimal and all will be fine.