VBA CODE CORRECTION - vba

Question

I am using VBA for the first time. I started with a very basic code using Runga Kutta Method. I am not able to run it. Can I know where I am going wrong? Can you also post some solutions for this type of differential equation solvers?
Option Explicit
Dim K1 As Double
Dim K2 As Double
Dim K3 As Double
Dim K4 As Double
Dim y As Double
Dim x As Double
Dim dx As Double
Dim x0 As Integer
Dim y0 As Integer
Function f(x, y) As Double
f = 0.5 * x - 0.5 * y
End Function
Sub RK(x, y, dx)
Dim i As Integer
Dim ynew As Double
dx = 0.5
x0 = 0
y0 = 1
K1 = dx * f(x, y)
K2 = dx * f(x + dx / 2, y + K1 / 2)
K3 = dx * f(x + dx / 2, y + K2 / 2)
K4 = dx * f(x + dx, y + K3)
RK = y + ((K1 + 2 * (K2 + K3 + K4)) / 6)
For i = 0 To 6
ynew = RK(x, y, dx)
x = x + dx
y = ynew
Next i
For j = 1 To 6
Call RK(x, y, dx, ynew)
ActiveCell.Offset(i - 1, 1).Value = x
ActiveCell.Offset(i - 1, 2).Value = y
x = x + dx
y = ynew
Next
End Sub

Answer 1

You need to execute the RK procedure and give it the starting values of x, y and dx.
To do this from within the VBE (Visual Basic Editor) open the Immediate window. If it's not visible press Ctrl+G and it should appear at the bottom of the VBE.
In the Immediate window type something like RK 1, 3, 2 and the results will appear in the two columns to the right of the active cell and take up 6 rows.
If you want the procedure to execute on the press of a button, or when a value is updated you'll need to add an extra bit of code for that.
Edit:
Just noticed these lines - RK = y + ((K1 + 2 * (K2 + K3 + K4)) / 6) and ynew = RK(x, y, dx).
The RK procedure won't return a value and you can't set it as a value either.
This link may help explain it: http://what-when-how.com/excel-vba/vba-sub-and-function-procedures/