Simple rot13 encoder in vb.net - vb.net

Question

I am looking for a simple way to encode an inputted text into Rot13. I am hitting a brick wall at the stage of being able to separate out words into individual characters and integers so that I can change each one and output the result. I can do it with single letters using a simple if statement listed bellow but if anyone can help with a way of doing it for whole words I would be very appreciative.
If kInput = "a" then kOutput = "n"
Thanks, Kai

Answer 1

Looks like people are giving good answers to this but here's my try at it.
Dim input As String = "This is a Test!! Guvf vf n Grfg!!"
Dim result As StringBuilder = New StringBuilder()
For Each ch As Char In input
If (Not Char.IsLetter(ch)) Then
result.Append(ch)
Continue For
End If
Dim checkIndex As Integer = Asc("a") - (Char.IsUpper(ch) * -32)
Dim index As Integer = ((Asc(ch) - checkIndex) + 13) Mod 26
result.Append(Chr(index + checkIndex))
Next
Console.WriteLine(result.ToString())
EDIT: improved to remove need for uppercase check. This will properly handle case and special characters with only 1 if statement inside the loop.

Answer 2

It seems like you're making this way harder than it has to be. No need to separate words, etc, and definitely no need for a large If/Else block:
Public Function Rot13(ByVal input As String) As String
Dim result As Char() = input.ToCharArray()
For i As Integer = 0 To result.Length - 1
Dim temp As Integer = Asc(result(i))
Select Case temp
Case 65 to 77, 97 To 109 'A - M
result(i) = Chr(temp + 13)
Case 78 to 90, 110 To 122 'N - Z
result(i) = Chr(temp - 13)
End Select
Next i
Return New String(result)
End Function
Note that this was entered directly into the browser window and is completely untested.

Answer 3

Just call it once to encode, call it again to decode.
Private Function ROT13_Encode(ByVal Input As String) As String
Dim chrs As Char() = Input.ToCharArray()
Dim ReturnString As String = ""
Dim CharInt As Integer
For Each Chr As Char In chrs
CharInt = Asc(Chr)
If CharInt >= 65 And CharInt <= 77 Then 'A-M
CharInt += 13
ElseIf CharInt >= 78 And CharInt <= 90 Then 'M-Z
CharInt -= 13
ElseIf CharInt >= 97 And CharInt <= 109 Then 'a-m
CharInt += 13
ElseIf CharInt >= 110 And CharInt <= 122 Then 'm-z
CharInt -= 13
End If
ReturnString &= ChrW(CharInt)
Next
Return ReturnString
End Function