VB.net | Finding duplicates in a multidimensional array - vb.net

Question

I have two forms: Act9.vb and List.vb. The code in both forms is below. I'm using vb.net "4.7.2" in visual studio.
I have been very frustrated with this program for a while now. For some reason the program only checks new clients against the first and second clients already in the list. For example, if the following entries are in the list:
╔═════════╦═════════════╗
║ ClientA ║ 32423223343 ║
╠═════════╬═════════════╣
║ ClientB ║ 23422322343 ║
╠═════════╬═════════════╣
║ ClientC ║ 23423423423 ║
╠═════════╬═════════════╣
║ ClientD ║ 43533453333 ║
╠═════════╩═════════════╣
║ etc... ║
╚═══════════════════════╝
Then if I try to modify ClientA or ClientB (pressing btnModify and then typing "ClientA"/"ClientB" in the inputbox), then it works, but if I try the same with ClientC, D, E, etc. it doesn't. I get this message: "This client doesn't exist. Please try again."
Same thing with adding new clients: it won't let me add ClientA or B twice, but if I try to add Client C more then once it doesn't realize that it's already in the multidimensional array and let's me add it a second time.
If someone knows anything that can help, please share.
Thanks in advance!
Public Class Act9
Public Clients(1, 1) As String
Public size As Integer = 0
Sub Add()
Dim tempClient As String
Dim tempTel As String
tempClient = InputBox("Please enter the clients name :", "Name")
If Duplicate(tempClient) Then
MsgBox("This client already exists")
Else
tempTel = InputBox("Please enter the client's phone number:", "Phone number")
Clients(0, size) = tempClient
Clients(1, size) = tempTel
size += 1
ReDim Preserve Clients(1, size)
End If
End Sub
Function Duplicate(ByVal tempClient As String) As Boolean
Dim output As Boolean = False
For i As Integer = LBound(Clients) To UBound(Clients)
If Clients(0, i) = tempClient Then
output = True
End If
Next
Return output
End Function
Private Sub BtnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
Add()
End Sub
Private Sub btnShow_Click(sender As Object, e As EventArgs) Handles btnShow.Click
List.ShowDialog()
End Sub
Private Sub btnErase_Click(sender As Object, e As EventArgs) Handles btnErase.Click
ReDim Clients(1, size)
List.lstClients.Items.Clear()
size = 0
End Sub
Private Sub btnModify_Click(sender As Object, e As EventArgs) Handles btnModify.Click
modify()
End Sub
Sub modify(Optional who As String = Nothing)
Dim change As Boolean = False
If who = Nothing Then who = InputBox("Please enter the name of the client you wish to modify:", "Modify")
For i As Integer = LBound(Clients) To UBound(Clients)
If Clients(0, i) = who Then
Clients(0, i) = InputBox("Please enter the new name for the client:", "Name")
Clients(1, i) = InputBox("Please enter the new phone number for the client", "Phone number")
change = True
Exit For
End If
Next
If change = False Then MsgBox("This client doesn't exist. Please try again.")
End Sub
End Class
Public Class List
Private Sub List_Load(sender As Object, e As EventArgs) Handles Me.Load
lstClients.Items.Clear()
For i As Integer = 0 To Act9.size - 1
lstClients.Items.Add(Act9.Clients(0, i) & vbTab & Act9.Clients(1, i))
Next
End Sub
End Class

Answer 1

#Craig Told me to use breakpoints which are the perfect tools for this. I'm very grateful for the advice.
Here is how I ended up getting it to work properly:
I replaced LBound and Ubound with my variable "size" (which was already taking care of counting the size of my multidimensional array).
The For loop in "Duplicate" becomes:
For i As Integer = 0 To size
If Clients(0, i) = tempClient Then
output = True
End If
Next
The one in "Modify becomes:
For i As Integer = 0 To size
If Clients(0, i) = who Then
Clients(0, i) = InputBox("Please enter the new name for the client:", "Name")
Clients(1, i) = InputBox("Please enter the new phone number for the client", "Phone number")
change = True
Exit For
End If
Next

Answer 2

The reason why your original code didn't work out is because you LBound/UBound on the first dimension but your array extends on the second dimension. The first dimension is only ever 0 to 1 so you only ever checked the second dimension indexes 0 and 1 (the first two items) for the name:
For i As Integer = LBound(Clients) To UBound(Clients)
If you use
LBound(Clients, 2) to UBound(Clients, 2)
It will get the upper limit of the second dimension rather than the first. UBound uses 1-based indexing whereas VB uses 0 based. If you want the same thing in 0 based you can use
Array.GetUpperBound(Clients, 1)
to find the limit of the second dimension
Other tips:
your modify method makes the same bounding mistake
if this were programming 201 you'd probably be using a List(Of Client), Client being a class having a pair of string properties for name and tel, and an overridden equals method that compares the name of an incoming Client, so that list.Contains can be used to prevent duplicates. Eventually you'd probably override GetHashCode too and use a HashSet(Of Client). And this whole thing would actually be a lot easier. Multidimensional arrays are usually a poor storage solution for.. well.. everything
your size variable is public; it should have a capital s
your modify method is a method; it should have a capital M. All methods in .net have capital initial letters
maybe btnErase should set the size before it redims
your duplicate method checks every item in the array. It carries on checking even if it already found a duplicate item. You can skip the part where you create a Boolean and just straight Return True inside the If; your keys are always in the last place you look because you stop looking when you find them :)
perhaps if you implement a FindIndex method that takes a name and returns the index of where that person is, or -1 if it didn't find them then you can use it for both IsDuplicate (call findindex and return true if the result is greater than -1) and for Modify (find the index of the person and change them or put a message Not Found if -1 comes bac). This means you can have just one method that searches the array and you use it twice. It means you Don't Repeat Yourself - a software engineering principle we try to stick to. At the moment your duplicate and modify methods both have the same loop (with the same bug)