Hi everyone,

This is probably one that's staring me in the face, but I cannot get it to
work.  I have a Sub with string passed ByRef, but changes made to the string
inside the Sub don't affect the string passed, which they definitely should.

Calling code is:

Option Explicit
Public Const Mutation_Rate As Double = 0.001

  a$ =
"000000000000000000000000000000000000000000000000000000000000000000000000000
00000"
  For i = 1 To 10000
    DoEvents
    mutate (a$): Debug.Print a$
  Next i

Sub is:
Public Sub mutate(ByRef bits As String)
  Dim i As Integer
  For i = 1 To Len(bits)
    If Rnd < Mutation_Rate Then
      If Mid(bits, i, 1) = "1" Then
        Mid(bits, i, 1) = "0"
      Else
        Mid(bits, i, 1) = "1"
      End If
    End If
  Next i
End Sub

Running in Debug with a breakpoint set on the assignment to "1":
        Mid(bits, i, 1) = "1"
shows that the 'bits' string is being modified, but a$ in the main procedure
just stays at 80 zeros.

Any hints?

Thanks,

Andrew
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