VB - Substitution of letters - vb.net

Question

so I am making a decryption software that allows the user to input some text and then they can swap out letters in the program. For example, there is a drop down box that allows you to swap all the "O"'s in a user input to "W". So in the input "Stack overflow" the output would be "Stack wverflww".
However, my problem is is that when the user chooses a second letter to change, that has already been swapped, it causes a problem. For example, after the first above example has occurred, if the user then wanted to then change all the "W"'s in their input to "A"'s the output would be "stack averflaa". However, what I'm looking for the code to do is give an output of "Stack wverflwa". So only the original "W"'s of the user input are changed to the letter "A".
I hope the above makes sense.
Someone suggested using a two dimensional array to reassign the letters new letters and I am able to do this, but I have no idea how to then put this into my code and get it working. Below is my code and thank you to anyone who can help me.
Dim chooseLetter, replaceLetter, words2
chooseLetter = selectLetterCombo.Text
replaceLetter = replaceLetterCombo.Text
words2 = UCase(textInputBox.Text)
Dim replaceList As New List(Of String)
For Each z In words2
If z = chooseLetter Then
replaceList.Add(replaceLetter)
Else
replaceList.Add(z)
End If
Next
letterReplaceBox.Text = ""
For Each f In replaceList
letterReplaceBox.Text = letterReplaceBox.Text & f
Next
note: selectLetterCombo.Text is the letter chosen by the user that they want to replace and replaceLetterCombo.Text is the letter chosen by the user that they want to swap the first chosen letter with. Also, textInputBox.text is the text the user has inputted.
Thank you!

Answer 1

You should be able to keep a list of the index of the character that changed and check that before making another change.
'List to keep track of changed character index
Dim replacedCharsList As New List(Of Integer)'member variable
Dim chooseLetter, replaceLetter, words2
chooseLetter = selectLetterCombo.Text
replaceLetter = replaceLetterCombo.Text
words2 = UCase(textInputBox.Text)
Dim replaceList As New List(Of String)
Dim i As Integer
For i = 1 To Len(words2)
'remove the for each and go with a straight for loop to keep track if the index
If Mid(words2, i, 1) = chooseLetter Then
'check to see if we have already replaced this character via the index position
If replacedCharsList.Contains(i) = False Then
'we have not changed this so add the replacement letter and update our index list
replaceList.Add(replaceLetter)
replacedCharsList.Add(i)
Else
'we have already changed this character so just add it as is
replaceList.Add(Mid(words2, i, 1))
End If
Else
replaceList.Add(Mid(words2, i, 1))
End If
Next
letterReplaceBox.Text = ""
For Each f In replaceList
letterReplaceBox.Text = letterReplaceBox.Text & f
Next

Answer 2

I have an answer, but you're really not going to like it:
Option Infer On
Option Strict On
Imports System.Text.RegularExpressions
Module Module1
Dim swaps As New Dictionary(Of Char, Char)
Function DoSwaps(originalText As String, swapLetters As Dictionary(Of Char, Char)) As String
Dim newText As String = ""
For Each c In originalText
If swapLetters.ContainsKey(c) Then
newText &= swapLetters(c)
Else
newText &= c
End If
Next
Return newText
End Function
Sub Main()
Console.Write("Enter the text to be altered: ")
Dim t = Console.ReadLine()
Dim exitNow = False
Do
Console.Write("Enter the letter to swap from and the letter to swap to, or a blank line to quit: ")
Dim s = Console.ReadLine()
If s.Trim().Length = 0 Then
exitNow = True
Else
Dim parts = Regex.Matches(s, "([A-Za-z])")
If parts.Count >= 2 Then
Dim letter1 = CChar(parts.Item(0).Value)
Dim letter2 = CChar(parts.Item(1).Value)
If swaps.ContainsKey(letter1) Then
swaps.Item(letter1) = letter2
Else
swaps.Add(letter1, letter2)
End If
Console.WriteLine(DoSwaps(t, swaps))
End If
End If
Loop Until exitNow
End Sub
End Module
... unless you'd like to learn about the Dictionary class to understand how it works. I used a simple regular expression to parse the user input, but if you're using dropdowns to select the letters then that would just be bonus learning if you explore it.
The essential feature is that you keep the original string (t in the above code) and apply the transformation (I named it DoSwaps) to that each time, not to the previously transformed string.

Answer 3

These two functions will do the job, although there is no allowance for punctuation, just spaces.
Private Function EncryptText(str As String) As String
Dim swapletters() As String = {"l", "s", "d", "f", "g", "h", "j", "k", "a", "q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "z", "x", "c", "v", "b", "n", "m"}
Dim encryptedText As String = ""
For Each letter As Char In str
If letter = " "c Then
encryptedText = encryptedText & " "
Else
Dim charactercode As Integer = Asc(letter) - 97
encryptedText = encryptedText & swapletters(charactercode)
End If
Next
Return encryptedText
End Function
Private Function DecryptText(str As String) As String
Dim swapletters As New List(Of String) From {"l", "s", "d", "f", "g", "h", "j", "k", "a", "q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "z", "x", "c", "v", "b", "n", "m"}
Dim decryptedText As String = ""
For Each letter As Char In str
If letter = " "c Then
decryptedText = decryptedText & " "
Else
Dim character As String = Chr(swapletters.IndexOf(letter) + 97)
decryptedText = decryptedText & character
End If
Next
Return decryptedText
End Function
To use them, declare a string to hold the return value of each function
Dim etext As String
etext = EncryptText("disambiguation is the root of all evil")
results in etext being "faplrsajxlzayt ap zkg oyyz yh lee gcae"
and
Dim dtext As String
dtext = DecryptText("faplrsajxlzayt ap zkg oyyz yh lee gcae")
results in "disambiguation is the root of all evil"