VB Weighted Least Squares Example

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Imports System

Imports CenterSpace.NMath.Core
Imports CenterSpace.NMath.Matrix

Namespace CenterSpace.NMath.Matrix.Examples.VisualBasic

  ' A .NET example in Visual Basic demonstrating the features of the classes for solving weighted least squares problems.
  Module WeightedLeastSquaresExample

    Sub Main()

      ' Construct some test data.
      Dim A As New DoubleMatrix("5x2[1 2  1 3  1 6  1 10  1 7]")
      Dim B As New DoubleVector("[ 3 6 8 10 11]")

      ' Constructs some arbitrary weights. Observations are given 
      ' increasingly higher weights.
      Dim Weights As New DoubleVector(A.Rows, 0.2, 0.2)

      Console.WriteLine("Solving weighted least squares problem, Ax = b ")
      Console.WriteLine("with A = ")
      Console.WriteLine("and b = " & B.ToString())
      Console.WriteLine("with weights = " & Weights.ToString())

      ' Construct a weighted least squares object and solve the problem. The third
      ' parameter says do not prepend a column of ones to the input matrix A (the
      ' column of ones represents a constant term in the model) as the first column
      ' of A is already ones.
      Dim WLS As New DoubleCOWeightedLeastSq(A, Weights, False)
      Dim Solution As DoubleVector = WLS.Solve(B)
      Console.WriteLine("Solution = " & Solution.ToString("G5"))
      Console.WriteLine("Residuals = " & WLS.ResidualVector(B).ToString("G5"))

      ' Construct a random problem, obtain an ordinary least squares solution,
      ' then use the resulting residuals to solve the same problem using a
      ' weighted least squares - downweighting the outliers.
      Dim RNG As New RandGenUniform(-1, 2, 24)
      Dim Rows As Integer = 10
      Dim Cols As Integer = 3
      A = New DoubleMatrix(Rows, Cols, RNG)
      B = New DoubleVector(Rows, RNG)

      ' Ordinary Least Squares (OLS):
      Dim Ols As New DoubleQRLeastSq()
      Dim OlsSolution As DoubleVector = Ols.Solve(B)
      Dim Residuals As DoubleVector = Ols.ResidualVector(B)
      Console.WriteLine("OLS solution = " & OlsSolution.ToString("G5"))

      ' Now, construct a Bisquare weighting function object
      '// and use it to compute weights:
      Dim WeightingFunction As New DoubleBisquareWeightingFunction(A)
      WeightingFunction.GetWeights(Residuals, Weights)

      ' Perform the factorization with the input matrix and weights. Once the factorization is
      ' done we can use the weighted least squares to quickly solve for different right hand
      ' sides. The third parameter to the Factor method indicates that we want a column of 
      ' ones prepended to the input matrix A representing a constant term in the model.
      WLS.Factor(A, Weights, True)

      ' Now solve the weighted least squares problem.
      Dim WlsSolution1 As DoubleVector = WLS.Solve(B)
      Console.WriteLine("WLS solution = " & WlsSolution1.ToString("G5"))

      ' Solve with a different right hand side.
      Dim C As New DoubleVector(Rows, RNG)
      Dim WlsSolution2 As DoubleVector = WLS.Solve(C)
      Console.WriteLine("WLS solution 2 = " & WlsSolution2.ToString("G5"))

      Console.WriteLine("Press Enter Key")
    End Sub
  End Module
End Namespace

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