还有一个问题：如何将一个table的第四行(<tr>………………</tr>)删去

<html>
<head>
<title>table</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head>
<script language="javascript">
function show(strOperator)
{
        var objChecked = document.getElementsByName("checkboxbutton");
        for(var nLoop = 0; nLoop < objChecked.length; nLoop++)
        {
                if(!objChecked[nLoop].checked)continue;
                switch(strOperator)
                {
                        case "add" :
                                mytable.rows[mytable.rows.length - 1].insertAdjacentElement("afterEnd", mytable.rows[nLoop].cloneNode(true));
                                break;
                        case "hide":
                                mytable.rows[nLoop].style.visibility="hidden";
                                break;
                        case "delete":
                                mytable.deleteRow(nLoop);
                                nLoop--;
                                break;
                }
        }
}
document.onkeydown=moveFocus;
var nPos = 0;
function moveFocus()
{
        var objInput = document.getElementsByName("textfield");
        var nKeycode = event.keyCode;
        if(nKeycode == 38 || nKeycode == 40)
        {
                if(nKeycode == 40)nPos++;
                else nPos--;
                if(nPos < 0) nPos = 0;
                else if(nPos > objInput.length - 1)nPos = objInput.length - 1;
                objInput[nPos].focus();
        }  
}
</script>
<body bgcolor="#FFFFFF">
<table width="90%" border="1" id="mytable">
  <tr> 
    <td> 
      <input type="text" name="textfield">
    </td>
    <td> 
      <input type="checkbox" name="checkboxbutton" value="radiobutton">
    </td>
  </tr>
  <tr> 
    <td> 
      <input type="text" name="textfield">
    </td>
    <td> 
      <input type="checkbox" name="checkboxbutton" value="radiobutton">
    </td>
  </tr>
</table>
<p> 
  <input type="button" name="Button" value=" add " onClick="show('add');">
  <input type="button" name="Button2" value=" delete " onClick="show('delete');">
  <input type="button" name="Button3" value=" hide " onClick="show('hide');">
</p>
</body>
</html>