这是javascript自由添加行，删行!　如何在指定的位置插入新的一行
<!--自由添加行-->

<table id= "t1 " border= "0 " cellpadding= "0 " cellspacing= "0 " width= "95% " >
<tr>
<td width= "80 "> <center>
<input type= "radio " name= "tt "> </center> </td>
<td width= "125 "> <center> <input name= "model " class= "selBoxText " id= "model " value= " " size= "15 " onFocus= "watermarkEnter(this, ' ');eventRelation(this, 'UpLoadFile1_ShowTags '); " > </center> </td>
<td width= "107 "> <center> <input name= "xinghao " type= "text " id= "xinghao " size= "12 "> </center> </td>
<td width= "123 "> <center>
<input name= "Pack " type= "text " id= "Pack " size= "12 " onKeyPress= "if (event.keyCode!=46 && event.keyCode!=45 && (event.keyCode <48 || event.keyCode> 57)) event.returnValue=false ">
</center> </td>
<td width= "123 "> <center>
<input name= "Costprice " type= "text " id= "Costprice " size= "12 " onKeyPress= "if (event.keyCode!=46 && event.keyCode!=45 && (event.keyCode <48 || event.keyCode> 57)) event.returnValue=false ">
</center> </td>
<td width= "130 " align= "center "> <input name= "bak " type= "text " id= "bak " size= "15 "> </td>
<td>   </td>
</tr>
</table>

<!--自由添加行-->

<input type= "button " onClick= "add() " value= "添加一行 ">
   
<input type= "button " onClick= "del() " value= "删除一行 ">


<script language= "javascript ">

//添加一行和删除一行
function add(){
var tr =t1.insertRow();
tr.insertCell(0).innerHTML = ' <center> <input type= "radio " name= "tt "> </center> ';
tr.insertCell(1).innerHTML = ' <center> <input name= "model " class= "selBoxText " id= "model " value= " " size= "15 "> </center> ';
tr.insertCell(2).innerHTML = ' <center> <input name= "xinghao " type= "text " id= "xinghao " size= "12 "> </center> ';
tr.insertCell(3).innerHTML = ' <center> <input name= "Pack " type= "text " id= "Pack " size= "12 " onKeyPress= "if (event.keyCode!=46 && event.keyCode!=45 && (event.keyCode <48 || event.keyCode> 57)) event.returnValue=false "> </center> ';

tr.insertCell(4).innerHTML = ' <center> <input name= "Costprice " type= "text " id= "Costprice " size= "12 " onKeyPress= "if (event.keyCode!=46 && event.keyCode!=45 && (event.keyCode <48 || event.keyCode> 57)) event.returnValue=false "> </center> ';
tr.insertCell(5).innerHTML = ' <center> <input name= "bak " type= "text " id= "bak " size= "15 "> </center> ';
tr.insertCell(6).innerHTML = '  ';
//tr.insertCell(7).innerHTML = t1.rows.length;
}

function del(){
var c = document.getElementsByName( 'tt ');
for(var i=0; i <c.length; i++){
if(c[i].checked)
t1.deleteRow(i);
}
}

</script>



[解决办法]
function add()中的 var tr =t1.insertRow();
改为：

var c = document.getElementsByName( 'tt ');
var j = c.length;
for(var i=0; i <c.length; i++){
if(c[i].checked)
j=i;
}
var tr =t1.insertRow(j);

[解决办法]
if (document.all.YourTableId.rows.length <= 20)
{
// Add new row
}
else
{
alert( "Stop, ha ");
}