When the user clicks on the form, the MouseUp event handler calls the FindDot function to see if there is a dot at that location. If there is a dot there, the program removes it. Otherwise the program adds a new dot.

Function FindDot loops through the DotX and DotY collections checking whether the point clicked lies within the dot radius distance of each dot's center.

' The locations of the dots.
Private DotX As New Collection
Private DotY As New Collection

' Make a new dot if there isn't one already here.
Private Sub picMap_MouseUp(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
Dim dot_index As Integer

    dot_index = FindDot(X, Y)
    If dot_index < 0 Then
        ' Make a new dot.
        DotX.Add X
        DotY.Add Y
    Else
        ' Remove the existing dot.
        DotX.Remove dot_index
        DotY.Remove dot_index
    End If

    ' Redraw the map.
    picMap.Refresh
End Sub

' Find the dot at this location. Return its index
' or -1 if there is no dot here.
Private Function FindDot(ByVal X As Single, ByVal Y As _
    Single) As Integer
Dim i As Integer
Dim dx As Single
Dim dy As Single

    For i = 1 To DotX.Count
        ' See how far apart the dot and point are.
        dx = X - DotX(i)
        dy = Y - DotY(i)
        If dx * dx + dy * dy < DOT_R2 Then
            ' We found it.
            FindDot = i
            Exit Function
        End If
    Next i
    FindDot = -1
End Function

' Draw the dots on the map background.
Private Sub picMap_Paint()
Dim i As Integer

    picMap.Cls
    For i = 1 To DotX.Count
        picMap.Circle (DotX(i), DotY(i)), DOT_R
    Next i
End Sub