// !!!!! see ComputeSumAndSumOfSquares + StandardDeviation
//           + Deconstruction of the returned tuple
//           in https://docs.microsoft.com/en-us/dotnet/csharp/tuples
void Main()
{
	double[] x = new double[] { 0, 1 };
	//StandardDeviation1(x).Dump();
	//StandardDeviation2(x).Dump();
	kumSum();
}
private static (int Count, double Sum, double SumOfSquares)
				ComputeSumAndSumOfSquares(IEnumerable<double> sequence)
{
	double sum = 0;
	double sumOfSquares = 0;
	int count = 0;

	foreach (var item in sequence)
	{
		count++;
		sum += item;
		sumOfSquares += item * item;
	}

	////return (Nb:count, Sum:sum, Sum2: sumOfSquares);
	return (count, sum, sumOfSquares);
}

// x...
//// Better:
// x...

void kumSum()
{
	var X = new double[] { 1, 2, 3, 4 };
	//IEnumerable<double> X = new double[] { 1, 2, 3, 4 };
	double sum = 0;
	var Y = X.Select(w => sum += w);
	Y.ToArray().Dump();
}