# # Recursion

A function that calls itself is said to be recursive. Recursive logic can often be implemented as a loop, too. Recursion must be controlled with a parameter, so that the function knows when to stop recursing and deepening the call stack. Infinite recursion eventually causes a run-time error '28': "Out of stack space".

## # Factorials

``````Function Factorial(Value As Long) As Long
If Value = 0 Or Value = 1 Then
Factorial = 1
Else
Factorial = Factorial(Value - 1) * Value
End If
End Function

``````

## # Folder Recursion

Early Bound (with a reference to `Microsoft Scripting Runtime`)

``````
Sub EnumerateFilesAndFolders( _
FolderPath As String, _
Optional MaxDepth As Long = -1, _
Optional CurrentDepth As Long = 0, _
Optional Indentation As Long = 2)

Dim FSO As Scripting.FileSystemObject
Set FSO = New Scripting.FileSystemObject

'Check the folder exists
If FSO.FolderExists(FolderPath) Then
Dim fldr As Scripting.Folder
Set fldr = FSO.GetFolder(FolderPath)

'Output the starting directory path
If CurrentDepth = 0 Then
Debug.Print fldr.Path
End If

'Enumerate the subfolders
Dim subFldr As Scripting.Folder
For Each subFldr In fldr.SubFolders
Debug.Print Space\$((CurrentDepth + 1) * Indentation) & subFldr.Name
If CurrentDepth < MaxDepth Or MaxDepth = -1 Then
'Recursively call EnumerateFilesAndFolders
EnumerateFilesAndFolders subFldr.Path, MaxDepth, CurrentDepth + 1, Indentation
End If
Next subFldr

'Enumerate the files
Dim fil As Scripting.File
For Each fil In fldr.Files
Debug.Print Space\$((CurrentDepth + 1) * Indentation) & fil.Name
Next fil
End If
End Sub

``````

#### # Remarks

Recursion allows for repeated, self-referencing calls of a procedure.