Dim x1 As Single, x2 As Single
Dim Y As Single
Dim n As Integer
Private Sub Command1_Click(Index As Integer)
    Text1.Text = Text1.Text + CStr(Index)
End Sub
'输入小数点时的处理办法
Private Sub Command2_Click(Index As Integer)
   Text1.Text = Text1.Text + "."
End Sub
'按运算符时的处理方法
Private Sub Command3_Click(Index As Integer)
    x1 = Val(Text1.Text)
    Text1.Text = ""
    n = Index
End Sub
'单击退格按钮时的处理方法
Private Sub Command4_Click()
    Text1.Text = Mid(Text1.Text, 1, Len(Text1.Text) - 1)
End Sub
'单击清除按钮时的处理方法
Private Sub Command5_Click()
    x1 = 0: x2 = 0: Y = 0
    Text1.Text = ""
End Sub
'单击退出按钮时的处理方法
Private Sub Command6_Click()
    Unload Me
End Sub

'单击"="按钮时的处理方法
Private Sub Command7_Click()
    x2 = Val(Text1.Text)
    On Error GoTo err1 '设置捕获错误
    Select Case n
            Case 0
            Y = x1 + x2
            Case 1
            Y = x1 - x2
            Case 2
            Y = x1 * x2
            Case 3
            Y = x1 / x2
        End Select
    Text1.Text = CStr(Y)
    Exit Sub
err1:    '错误处理程序
    MsgBox "您输入的数据有误，请重新输入！", 0 + 16, "错误！"
    Text1.Text = ""
    x1 = 0: x2 = 0: Y = 0
    Exit Sub

End Sub


在这段程序代码中。。
Dim n As Integer
n = Index
为什么要有这2个部分啊。。。。求解释。这个有什么用，是什么意思？