Question

Well i got some hard time figuring out what am I doing wrong.
Basically I need to remove Listbox1 items that are less then avarage, but its giving me:

System.ArgumentOutOfRangeException was unhandled
Message=InvalidArgument=Value of ‘9’ is not valid for ‘index’.
Parameter name: index

Private Sub Label3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label3.Click
    Dim Myrand As New Random
    Dim res As Double
    Dim i As Integer
    Dim n As Integer
    Dim tot As Double
    Dim avarage As Double

    ListBox1.Items.Clear()

    For i = 0 To 14 Step 1
        res = Math.Round(Myrand.NextDouble, 3)
        ListBox1.Items.Add(res)
        tot = tot + res
    Next

    avarage = tot / ListBox1.Items.Count
    MsgBox(avarage)

    For i = 0 To ListBox1.Items.Count - 1 Step 1
        If ListBox1.Items(i) < avarage Then
            ListBox1.Items.RemoveAt(i)
            n = n + 1
        End If
    Next

    MsgBox("Removed " & n & " items!")
End Sub

Any suggestions?

Answer 1

It is grabbing the max Count at the start of the For/Next Loop and doesn’t reevaluate it. Try iterating backwards that way you are removing from where you have been not from where you are going.

i.e.

For i = ListBox1.Items.Count - 1 To 0 Step -1
    If ListBox1.Items(i) < avarage Then
        ListBox1.Items.RemoveAt(i)
        n = n + 1
    End If
Next

From above MSDN Link emphasis mine:

When a For…Next loop starts, Visual Basic evaluates start, end, and
step. This is the only time it evaluates these values. It then assigns
start to counter. Before it runs the statement block, it compares
counter to end. If counter is already larger than the end value (or
smaller if step is negative), the For loop ends and control passes to
the statement following the Next statement. Otherwise the statement
block runs.