Android Code Snippet Solving a system of linear equations using Gaussian Elimination

Some will remember Tasks like this one from school:

2x + y = 7
x + 4y = 6

The following code will solve this, with 1..n unknown quantities, like x, y, z ...

Sub gauss
    ' flowchart in
    ' https://www.bragitoff.com/2015/10/gaussian-elimination-lab-write-up-with-algorithm-and-flowchart/
    
    ' example with
    ' 2x + 3y + z = 5
    ' x + 4y + 2z = 3
    ' 4x + y + 3z = 17
        
    Private n,i,j,k As Int
    ' number of unknown values, equations and results
    n = 3
    
    Private a (n,n+1) As Double
    Private x (n) As Double
    Private temp,t As Double
    
    ' data
    a (0,0) = 2
    a (0,1) = 3
    a (0,2) = 1
    a (0,3) = 5
    a (1,0) = 1
    a (1,1) = 4
    a (1,2) = 2
    a (1,3) = 3
    a (2,0) = 4
    a (2,1) = 1
    a (2,2) = 3
    a (2,3) = 17
    
    ' pivoting
    For i=0 To n-1
        For k=i+1 To n-1
            If a (i,i) < a (k,i) Then
                For j=0 To n
                    temp = a (i,j)
                    a (i,j) = a (k,j)
                    a (k,j) = temp
                Next
            End If
        Next
    Next
    ' Gaussian Eliminatio
    For i=0 To n-2
        For k=i+1 To n-1
            t=a (k,i)/a(i,i)
            For j=0 To n
                a (k,j)=a (k,j)-t*a (i,j)
            Next
        Next
    Next
    ' Back-substitution:
    For i=n-1 To 0 Step -1
        x (i) = a (i,n)
        For j=0 To n-1
            If j <> i Then
                x (i)  = x (i) - a (i,j) * x (j)
            End If
        Next
        x (i) = x (i) / a (i,i)
    Next
    For i=0 To n-1
        Log (x (i))
        ' shows the values for x, y and z = 3, -1, 2
    Next
End Sub

 

[derez]

See also a very old Matrix library https://www.b4x.com/android/forum/threads/matrix-operations-library.15949/

 

[grafsoft]

If I had found this library (I didn't look for matrices, stupid me) I would have given it a try