fastest way to randomize case on string

Question

For some protocol testing I need to randomize the case of each character in a lot of strings. The strings are commands, created by my application, which will be sent via a winsock control to a client.

As it involves a lot of strings I want each part to be as fast as possible.

Right now I have:

Private Function RandomCaps(strText As String) As String
  Dim lngChar As Long
  Dim strLower As String, strUpper As String
  Dim strRandom As String
  strRandom = ""
  strLower = LCase$(strText)
  strUpper = UCase$(strText)
  For lngChar = 1 To Len(strText)
    If Int(2 * Rnd) = 0 Then
      strRandom = strRandom & Mid$(strLower, lngChar, 1)
    Else
      strRandom = strRandom & Mid$(strUpper, lngChar, 1)
    End If
  Next lngChar
  RandomCaps = strRandom
End Function

This is pretty straightforward, but probably not the fastest way.

What could I do to improve its speed?

Answer 1

Instead of concatenating strings together, use Mid to change the string in-place:

Private Function RandomCaps(s As String) As String
    Dim uc As String
    Dim i As Long

    RandomCaps = LCase$(s)
    uc = UCase$(s)
    For i = 1 To Len(s) 
        If Rnd < 0.5 Then
            Mid(RandomCaps, i, 1) = Mid(uc, i, 1)
        End If
    Next i
End Function

You can try using MidB, but it hardly makes any difference – and since it works with individual bytes, you're in for some nasty surprises if you don't know how VB6 stores strings.