// returns the nearest center to the input vector, using the L2 norm
        public static double[] NearestCenter(double[] vector, double[][] centers)
        {
            // written in LINQ to "prove it can be done". easier to see that it is functional, too
            return centers.Aggregate(centers[0], (old, cur) => (vector.Select((v, i) => v - old[i])
                                                                      .Select(diff => diff * diff)
                                                                      .Sum() >
                                                                vector.Select((v, i) => v - cur[i])
                                                                      .Select(diff => diff * diff)
                                                                      .Sum()) ? cur : old);
        }

T Aggregate<T>(this IEnumerable<T> collection, T first, Func<T, T, T> foo)
{
    T old = first;
    T r;
    foreach (var cur in collection)
    {
        r = foo(old, cur);
        old = cur;
    }
    return r;
}