How can I properly use the for...next statement? - vb.net

Question

I'm new to vb.net and English is not my first language, Please note this :)
I've try nested for...next loop but there's a problem which that value does not increase :(
Please help me!
Dim h As Integer = 8
Dim i As Integer = 5
Dim time As String = Format(Now, "yyyy-MM-dd HH:mm:ss")
Dim pno As String = PatNo.Text
Dim itname() As String =
{"<R>SPH", "<R>CYL", "<R>AX", "<R>AVE", "<L>SPH", "<L>CYL", "<L>AX", "<L>AVE"}
Dim list As List(Of String) = New List(Of String)()
For a As Integer = 8 To 18
list.Add(ReceiveBox.Lines(a))
Next
Dim arrlist() As String = list.ToArray()
For linerepeat As Integer = 1 To 3
If linerepeat = 3 Then
i -= 3
Else i = 5
End If
For g As Integer = 0 To 10
For f As Integer = 0 To 7
For number As Integer = 1 To 3
cmd.Connection = conn
Dim itvalue As String = Mid(arrlist(g), h, i)
Dim query As String = "INSERT INTO EYE_TEST VALUES ('" & time & "','" & pno & "','" & txt.Text & "','" & itname(f) & number & "','" & itvalue & "')"
cmd.CommandText = query
cmd.CommandType = CommandType.Text
cmd.ExecuteNonQuery()
h += 7
Next
If h = 29 Then h = 8
Next
'If g = 3 And 5 And 9 Then Exit For
Next
Next

Answer 1

To reproduce your error I copied your code block and commented out lines of code that I cannot reproduce (for instance, your SQL command).
Dim h As Integer = 8
Dim i As Integer = 5
' Dim time As String = Format(Now, "yyyy-MM-dd HH:mm:ss")
' Dim pno As String = PatNo.Text
' Dim itname() As String =
'{"<R>SPH", "<R>CYL", "<R>AX", "<R>AVE", "<L>SPH", "<L>CYL", "<L>AX", "<L>AVE"}
' Dim list As List(Of String) = New List(Of String)()
' For a As Integer = 8 To 18
' list.Add(ReceiveBox.Lines(a))
' Next
' Dim arrlist() As String = list.ToArray()
For linerepeat As Integer = 1 To 3
If linerepeat = 3 Then
i -= 3
Else : i = 5
End If
For g As Integer = 0 To 10
For f As Integer = 0 To 7
For number As Integer = 1 To 3
'cmd.Connection = conn
'Dim itvalue As String = Mid(arrlist(g), h, i)
'Dim query As String = "INSERT INTO EYE_TEST VALUES ('" & time & "','" & pno & "','" & txt.Text & "','" & itname(f) & number & "','" & itvalue & "')"
'cmd.CommandText = query
'cmd.CommandType = CommandType.Text
'cmd.ExecuteNonQuery()
h += 7
Next
If h = 29 Then h = 8
Next
'If g = 3 And 5 And 9 Then Exit For
Debug.Print(" g:" & g)
Next
Next
I then added Debug.Print(" g:" & g) at the end of your "g" for loop to see if it was iterating. When I debug this I get the following output:
g:0
g:1
g:2
g:3
g:4
g:5
g:6
g:7
g:8
g:9
g:10
g:0
g:1
g:2
g:3
g:4
g:5
g:6
g:7
g:8
g:9
g:10
g:0
g:1
g:2
g:3
g:4
g:5
g:6
g:7
g:8
g:9
g:10
This shows that the "g" for loop is iterating.
Try adding Debug.Print(" g:" & g) at the end of your "g" for loop. When you run the program the results will show in the Output window (if you do not see this window, click View -> Output). If you do not get the same results as me then I would expect the problem to be caused by your SQLConnection object or the query you try to execute. Maybe during one of the iterations a value does not get set properly and an error is caused which might end your for loop unexpectedly.