Call function recursively without additional parameter - recursion

Question

I wrote vbscript function which search the list of free ports on Unix. So my code looks like this:
Function FindFreePorts(ByVal intPortToStart, ByVal intCountOfPortToSearch, ByVal arrFreePorts)
Dim i, arrCommand, arrTmp, blnFindAllPort, j, strCommand
Dim blnFree, intCountOfFreePorts
strCommand = "lsof -i | awk '{print $8}' | sed -n '/"
For i = intPortToStart To intPortToStart+intCountOfPortToSearch - 1
If i = intPortToStart+intCountOfPortToSearch - 1 Then
strCommand = strCommand & ".*"& i & "$/s/.*://p'"
Else
strCommand = strCommand & ".*:" & i &"\|"
End If
Next
Push arrCommand, strCommand
arrTmp = Array()
arrTmp = ExecuteCommandOnUnix(arrCommand, Null, _
Environment.Value("UNIXHOST_NAME"), _
Environment.Value("UNIXHOST_USER"), _
Environment.Value("UNIXHOST_PWD"))
' Count of busy port is UBound(arrTmp) - the other ports are free
' Find free ports
intCountOfFreePorts = intCountOfPortToSearch
For i = intPortToStart To intPortToStart+intCountOfPortToSearch - 1
blnFree = True
For j = 0 To UBound(arrTmp)
If arrTmp(j) = CStr(i) Then
blnFree = False
j = UBound(arrTmp)
End If
Next
If blnFree Then
Push arrFreePorts, i
intCountOfFreePorts = intCountOfFreePorts - 1
End If
Next
'
If intCountOfFreePorts = 0 Then
blnFindAllPort = True
Else
blnFindAllPort = False
End If
If Not blnFindAllPort Then
' We found UBound(arrFreePorts), we need to find intCountOfPortToSearch - UBound(arrFreePorts)
Do While intCountOfPortToSearch - UBound(arrFreePorts) - 1 <> 0
arrFreePorts = FindFreePorts(intPortToStart + intCountOfPortToSearch + 1, intCountOfPortToSearch - UBound(arrFreePorts) - 1, arrFreePorts)
If intCountOfPortToSearch - UBound(arrFreePorts) - 1 = 0 Then
Exit Do
End If
Loop
End If
FindFreePorts = arrFreePorts
End Function
As you can see I call this function recursively in Do While cycle. Everything works ok but I don't like arrFreePorts parameter here. So I should write this code to execute my function:
arrPort = FindFreePorts(intStartFromPort, intCountToSearch, arrPort)
But I have no idea how to rewrite this function without this parameter. Then I could call it in more simple way:
arrPort = FindFreePorts(intStartFromPort, intCountToSearch)
Thanks in advance.

Answer 1

To keep things (experiments, illustration of #Bond's contribution) simple, let's consider the task of putting the chars of a string into an array.
A function that shall return an array (without getting it via parameter or global) needs to create the array:
Function f1(s) ' default ByRef is ok, because Left and Mid return new (sub)strings
If "" = s Then
f1 = Array() ' return empty array for empty string
Else
Dim t : t = f1(Mid(s, 2)) ' recurse for tail - sorry no tail recursion
f1 = cons(Left(s, 1), t) ' copy! result of cons to f1/function's return value
End If
End Function
The growing of the array could be done inline, but for clarity I use a helper function cons():
Function cons(e, a) ' default ByRef is ok; neither e nor a are changed
ReDim aTmp(UBound(a) + 1) ' create larger array
Dim i : i = 0
aTmp(i) = e ' 'new' head
For i = 1 To UBound(aTmp)
aTmp(i) = a(i - 1) ' copy 'old' elms
Next
cons = aTmp ' copy! aTmp to cons/function's return value
End Function
Calling the function is nice:
WScript.Echo 3, "[" & Join(f1("12456789"), ",") & "]"
The price to pay for avoiding the passing of 'the' array:
a lot of copies (array assingment copies in VBScript)
no tail recursion (I doubt, however, that VBScript exploits it)
about factor 10 slower than the Sub approach ((c) Bond) below
As in this case the recursive calls can/should work on the 'same' array, a Sub does the task more efficiently (and is less complex):
Sub s1(s, a) ' default ByRef is ok; a should be modified, s isn't touched
If "" <> s Then
ReDim Preserve a(UBound(a) + 1) ' grow! a
a(UBound(a)) = Left(s, 1)
s1 Mid(s, 2), a ' tail recursion for tail
End If
End Sub
Calling it looks nasty:
ReDim a(-1) : s1 "123456789", a : WScript.Echo 3, "[" & Join(a, ",") & "]"
But that can be avoided by a wrapper:
Function f2(s)
ReDim aTmp(-1)
s1 s, aTmp
f2 = aTmp
End Function