How to calculate the Linest of given numbers

Linest function in excel returns a table of statistics for a straight line that best fits a data set.

The C# code for getting the linest of numbers is given below:

public static double CalculateLinest(double[] y, double[] x)

{

   double linest = 0;

   if (y.Length == x.Length)

   {

      double avgY = y.Average();

      double avgX = x.Average();

      double[] dividend = new double[y.Length];

      double[] divisor = new double[y.Length];

      for (int i = 0; i < y.Length; i++)

      {

         dividend[i] = (x[i] - avgX) * (y[i] - avgY);

         divisor[i] = Math.Pow((x[i] - avgX), 2);

      }

      linest = dividend.Sum() / divisor.Sum();

   }

   return linest;

}

How to calculate standard deviation of given numbers

The standard deviation of numbers is calculated as below:

The C# code to calculate the standard deviation is given below:

public double CalculateStandardDeviation(double?[] numbers)

{

   // select only non null numbers from the input array

   numbers = numbers.Where(n => n.HasValue).ToArray(); 

   // square of sum of numbers

   double sqrOfSum = Math.Pow(numbers.Sum().Value,2); 

   // sum of squares of numbers

   double sumOfSqrs = numbers.Sum(n => Math.Pow(n.Value, 2));

   // number of values in set

   double count = (double)numbers.Length; 

   // calculates the dividend part of formula

   double dividend = (count * sumOfSqrs) - sqrOfSum; 

   // returns standard deviation

   return Math.Sqrt(dividend / (count * (count - 1)));

}