if statements

using System;

public static class Darts
{
    public static int Score(double x, double y)
    {
        var distanceFromCenter = Math.Sqrt(x * x + y * y);

        if (distanceFromCenter > 10.0)
            return 0;

        if (distanceFromCenter > 5.0)
            return 1;

        if (distanceFromCenter > 1.0)
            return 5;

        return 10;
    }
}

The first step is to calculate the distance from the center of the board, which we can with the cartesian coordinates distance formula

var distanceFromCenter = Math.Sqrt(x * x + y * y);

Before we'll look at the score calculation, let's re-iterate the games rules:

Directly translating this to code gives us:

if (distanceFromCenter > 10.0)
    return 0;

if (distanceFromCenter > 5.0 && distanceFromCenter <= 10.0)
    return 1;

if (distanceFromCenter > 1.0 && distanceFromCenter <= 5.0)
    return 5;

return 10;

However, due to the order of evaluation, we know in our second condition, distanceFromCenter <= 10.0 must always true as otherwise distanceFromCenter > 10.0 would have been true. The same reasoning applies to the distanceFromCenter <= 5.0 condition. We can thus shorten our code to:

if (distanceFromCenter > 10.0)
    return 0;

if (distanceFromCenter > 5.0)
    return 1;

if (distanceFromCenter > 1.0)
    return 5;

return 10;