How do I call a function/return a value in VBA?

Question

I am trying to code a method, and for every step I have to call another method (which I coded as a separate Sub).

Sub implicit()

'Setting limits and stepsize
x0 = 0
xfinal = 100
h = 5
Dim y2 As Double
Dim y As Double

'Calculating number of intervals
n = (xfinal - x0) / h

'Setting initial value
y = 100000

'Loop for implicit method
For i = 1 To n
    Call methodN(y)
    y2 = y + h * (0.1 * y2 - 0.0000008 * y2 ^ 2)
    Cells(2 + i, 3) = y2
    y = y2
Next i

End Sub


'N method
Sub methodN(y)
y2 = y - (y * (1 - 0.1 * h + 0.0000008 * h * y) - y) / (1 - 0.1 * h + 0.0000016 * h * y)
End Sub

When I put Return y2 in the second sub as the last line I get a compiler error. I know I'm not calling the Sub properly and I'm not returning the y2 value.

Answer 1

In VBA a Sub is a non returning method. You need to use Function instead, and then can Return a value

Answer 2

In short, it should look like this:

Function methodNewton(y)

y2 = y - (y * (1 - 0.1 * h + 0.0000008 * h * y) - y) / (1 - 0.1 * h + 0.0000016 * h * y)

methodNewton = y2

End Function

or more simply:

Function methodNewton(y)
  methodNewton = y - (y * (1 - 0.1 * h + 0.0000008 * h * y) - y) / (1 - 0.1 * h + 0.0000016 * h * y)
End Function

and then call it this way instead:

y2 = methodNewton(y)

but you are missing an h variable, so maybe like this:

Function methodNewton(y, h)
  methodNewton = y - (y * (1 - 0.1 * h + 0.0000008 * h * y) - y) / (1 - 0.1 * h + 0.0000016 * h * y)
End Function

and then call it like this

y2 = methodNewton(y, h)