VB Matrix Slice Example

Imports System

Imports CenterSpace.NMath.Core

Namespace CenterSpace.NMath.Examples.VisualBasic

  A .NET example in Visual Basic showing how to use the indexing class Slice with the matrix classes. 
  Module MatrixSliceExample

    Sub Main()

      Dim rows As Integer = 5
      Dim cols As Integer = 5
      Dim A As New DoubleMatrix(rows, cols, 0, 1)

          | 0  5  10  15  20 |
          | 1  6  11  16  21 |
      A = | 2  7  12  17  22 |
          | 3  8  13  18  23 |
          | 4  9  14  19  24 |

      Console.WriteLine()
      Console.WriteLine("A...")
      Console.WriteLine(A.ToTabDelimited())
      Console.WriteLine()

      Last two columns of the matrix A:
      Dim colSlice As New Slice(3, 2)
      Dim rowSlice As Slice = Slice.All
      Dim ALast2Col As DoubleMatrix = A(rowSlice, colSlice)

                  | 15  20 |
                  | 16  21 |
      ALast2Col = | 17  22 |
                  | 18  23 |
                  | 19  24 |
      Console.WriteLine("ALast2Col...")
      Console.WriteLine(ALast2Col.ToTabDelimited)
      Console.WriteLine()

      You can use negative strides too. Here is the matrix A, reversed.
      We re-use the Slice objects, setting their start, stop, and stride
      fields using the Set method.
      rowSlice = New Slice(rows - 1, rows, -1)
      colSlice.Set(cols - 1, cols, -1)
      Dim ARev As DoubleMatrix = A(rowSlice, colSlice)

             | 24  19  14  9  4 |
             | 23  18  13  8  3 |
      ARev = | 22  17  12  7  2 |
             | 21  16  11  6  1 |
             | 20  15  10  5  0 |
      Console.WriteLine("ARev...")
      Console.WriteLine(ARev.ToTabDelimited)
      Console.WriteLine()

      Notice that when you create a new DoubleMatrix using the Slice or Slice class,
      you are creating a different "view" of the Matrix data. That is, the 
      DoubleMatrix instance returned by the indexing operator taking a Slice object,
      and the DoubleMatrix instance being indexed share the same data:
      ARev(0, 0) = 100
      Console.WriteLine("ARev[0,0] = {0}", ARev(0, 0))   uRev = [0 8 7 6 5 4 3 2 1 0]
      Console.WriteLine()
      Console.WriteLine("A[{0},{1}] = {2}", rows - 1, cols - 1, A(rows - 1, cols - 1)) A[4,4] 
      Console.WriteLine()

      We now use the DoubleMatrix method Set(Slice,Slice,value) to change the 
      contents of A to alternate values 0 and 1 - like a checker board.
      Dim numEvenElts As Integer = CInt(rows / 2)
      Dim numOddElts As Integer = rows / 2
      Dim evenElts As New Slice(0, numEvenElts, 2)
      Dim oddElts As New Slice(1, numOddElts, 2)
      A.Set(evenElts, oddElts, 0)
      A.Set(evenElts, evenElts, 1)
      A.Set(oddElts, evenElts, 0)
      A.Set(oddElts, oddElts, 1)

          | 1  0  1  0  1 |
          | 0  1  0  1  0 |
      A = | 1  0  1  0  1 |
          | 0  1  0  1  0 |
          | 1  0  1  0  1 |
      Console.WriteLine("A...")
      Console.WriteLine(A.ToTabDelimited())
      Console.WriteLine()

      Console.WriteLine()
      Console.WriteLine("Press Enter Key")
      Console.Read()

    End Sub

  End Module

End Namespace