程序运行时，在Text1中输入字符串，单击“开始统计”按钮，在List1中显示统计结果。函数IsLetter(x)功能，若x是字母，则返回值为字母x在字母序列“A”~“Z”中的序号，如字母“A”的序号为0，字母“D”的序号为3。若x不是字母，则返回值为-1。

Function IsLetter(x As String) As Integer

        If "A" <= x And x <= "Z" Then

            IsLetter = Asc(x) - Asc("A")

        ElseIf "a" <= x And x <= "z" Then

            IsLetter = Asc(x) - Asc("a")

        Else

                       ①        

        End If

    End Function

    Private Sub Command1_Click()

        Dim d(25) As Integer, m As Integer, n As Integer

        Dim s As String, c As String

        List1.Clear

        For m = 0 To 25                  '初始化数组d

            d(m) = 0

        Next m

        s = Text1.Text

        For m = 1 To Len(s)              '读取字符串中的字符进行判断

            c = Mid(s, m, 1)

            n = IsLetter(c)

            If n <> -1 Then    ②  

        Next m

        For m = 0 To 25                  '输出结果

            If    ③   Then

               List1.AddItem (Chr(m + Asc("a")) & ":" & d(m))

            End If

        Next m

   End Sub

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