Parsing the Parameters of a Function

Question

I am trying to create a UDF within VBA which go through some function syntax and treat it as Text.

The function will look like :

FunctionA( Param1 , Param2 , Param3 , Param 4 )

I am trying to develop a UDF which will pull out the value of the Param based on the position I input into my UDF function.

GetN( FunctionA , 3 ) = "Param3"

GetN FunctionA , 1 ) = "Param1"  

Here's my function so far but it's off....

It's behaving like :

GetN( FunctionA , 0 ) = Param2 

Here's my function:

Function GetN(sInputString As String, n As Integer) As String
     Dim sFindWhat As String
     Dim j, FindA, FindB As Integer
     Application.Volatile
     sFindWhat = ","

     FindA = 0
     For j = 0 To n
         FindA = InStr(FindA + 1, sInputString, sFindWhat)
         FindB = InStr(FindA + 1, sInputString, sFindWhat)
         If FindB = 0 Then FindB = InStr(FindA + 1, sInputString, ")")
         If FindA = 0 Then Exit For
     Next
     GetN = Trim(Mid(sInputString, FindA + 1, FindB - FindA - 1))

 End Function

Thank you for help

Answer 1

Split should work, though to correctly handle the case of nested functions, a preliminary hack is to first replace commas at the top level by a safe delimiter (e.g. [[,]]) and then splitting on that delimiter:

Function GetParameterN(func As String, n As Long) As String
    Dim args As Variant
    Dim safeArgs As String
    Dim c As String
    Dim i As Long, pdepth As Long

    func = Trim(func)
    i = InStr(func, "(")
    args = Mid(func, i + 1)
    args = Mid(args, 1, Len(args) - 1)

    For i = 1 To Len(args)
        c = Mid(args, i, 1)
        If c = "(" Then
            pdepth = pdepth + 1
        ElseIf c = ")" Then
            pdepth = pdepth - 1
        ElseIf c = "," And pdepth = 0 Then
            c = "[[,]]"
        End If
        safeArgs = safeArgs & c
    Next i
    args = Split(safeArgs, "[[,]]")
    GetParameterN = Trim(args(n - 1))
End Function

For example,

Sub test()
    Dim i As Long
    For i = 1 To 3
        Debug.Print GetParameterN("f(x,g(x,y,z),z)", i)
    Next i
End Sub

Produces:

x
g(x,y,z)
z

I see no good reason to make this function volatile.