How do I add the results to the list? - vb.net

Question

I am trying to make a program that includes all the presidents and lets you search any letter or name to show the results of presidents associated with that letter or name. However, I am having trouble with actually adding them to the list. This is a class project, so the code is designed to meet requirements for that. Specifically, the part where it says
'Show the match
addPresidentsToList(presidents(idxReturned), idxReturned)
This is saying it isn't declared, but I'm not sure how to go about it.
Any tips or advice would be appreciated, thanks!
Public Class frmLab32
Const IDX_START As Integer = 0
Const IDX_END As Integer = 1
Const IDX_NAME As Integer = 2
Dim presidents() As String
Dim headerRowNeeded As Boolean
Private Sub frmLab32_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Populate array from file
presidents = IO.File.ReadAllLines("USPresWithDates.txt")
End Sub
Private Sub btnSearch_Click(sender As Object, e As EventArgs) Handles btnSearch.Click
Dim idxReturned As Integer = -1
Dim idxStart As Integer = 0
'remove names
lstResults.Items.Clear()
'Header row?
headerRowNeeded = True
'Search for matches
Do
idxReturned = Array.FindIndex(presidents,
idxStart,
Function(el) el.Split(","c)(IDX_NAME).ToLower.Contains(txtSearch.Text.ToLower))
'Match Found?
If idxReturned > -1 Then
'Show the match
addPresidentsToList(presidents(idxReturned), idxReturned)
'Prepare for next search
idxStart = idxReturned + 1
End If
Loop Until idxReturned = -1
'Any names found?
If lstResults.Items.Count = 0 Then
'Update counter
lblCount.Text = "0"
'Tell User
MessageBox.Show(text:="No match found",
caption:="Search Results",
buttons:=MessageBoxButtons.OK,
icon:=MessageBoxIcon.Information)
End If
End Sub
End Class

Answer 1

My text file looks like this.
Washington, George
Adams, John
Jefferson, Thomas
Madison, James
Monroe, James
etc.
I use default names for controls in my text program but it is much better to use descriptive name like you did.
In the Form.Load I set the MaxLength property of the TextBox to 1. This will only allow a single character to be typed in the TextBox. You could set this property in the form designer and skip this line of code. Next fill the presidents array exactly as you did.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
TextBox5.MaxLength = 1
presidents = IO.File.ReadAllLines("C:\Users\maryo\Desktop\Presidents.txt")
End Sub
Now for the search button. We need to validate the user input. A user could enter a number or punctuation character. Char https://learn.microsoft.com/en-us/dotnet/api/system.char?view=netcore-3.1#:~:text=Remarks-,.,bit%20numeric%20(ordinal)%20value. is a .net structure representing a character. It provides some interesting methods that can be helpful from time to time. Char.IsLetter() tells us (true or false) if a character is a letter. Just what we want to know. The only problem is a String is not a Char and .Text property of a TextBox is a String. Since we limited the entry to a single character we can convert this String to a Char with CChar(). If it is a letter we make the assignment to SelectedLetter otherwise we correct the user and exit the method.
Instead of all the index stuff we can use a For Each loop. It checks each element in presidents. String has a method called StartsWith. We can check each string in presidents to see if it .StartsWith the SelectedLetter. If it does, add it to the ListBox.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
ListBox1.Items.Clear()
'Validate user input
Dim SelectedLetter As String = ""
If Char.IsLetter(CChar(TextBox5.Text)) Then
SelectedLetter = TextBox5.Text.ToUpper
Else
MessageBox.Show("Please enter a letter A - Z.", "Enter Letter")
Exit Sub
End If
For Each pres In presidents
If pres.StartsWith(SelectedLetter) Then
ListBox1.Items.Add(pres)
End If
Next
If ListBox1.Items.Count = 0 Then
'Tell User
MessageBox.Show("No match found", "Search Results")
End If
End Sub