请教各位高手，如何才能将vba过程代码修改为函数

请教各位高手，如何才能将以下过程代码修改为函数，使其在任何工作表和任意单元格均能方便使用，特别是A、B、C等列不连续的情况下。谢谢！

Sub tt()
    With Sheet1
        arr = .[a1].CurrentRegion
        r = .[c65536].End(3).Row
        .Range("d2:d" & r).ClearContents
        For i = 2 To r
            c = arr(i, 3): c1 = arr(i + 1, 3): xstr = ""
            For j = 2 To UBound(arr)
                If InStr(c, "<") > 0 Then
                    jdg = Val(Replace(c, "<", ""))
                    If arr(j, 2) < jdg Then xstr = xstr & arr(j, 1) & "(" & arr(j, 2) & ") "
                End If
               
                If InStr(c, "-") > 0 And InStr(c1, "-") > 0 Then
                    jdg = Val(c): jdg1 = Val(c1)
                    If jdg <= arr(j, 2) And arr(j, 2) < jdg1 Then xstr = xstr & arr(j, 1) & "(" & arr(j, 2) & ") "
                End If
                点击下载此附件
                If InStr(c, "-") > 0 And InStr(c1, "-") = 0 Then
                    jdg = Val(c)
                    If jdg <= arr(j, 2) Then xstr = xstr & arr(j, 1) & "(" & arr(j, 2) & ") "
                End If
               
            Next
            .Cells(i, 4) = Trim(Replace(xstr, ".", "0."))
        Next
    End With
End Sub