动态的表格求合计，为什么数值是null？
谁帮我看看是那里的问题，这个动态表格怎么数值取不到。高手指点一下。最好给一个实例。以下是代码。谢谢！
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=gb2312" />
<title>test</title>
<style type="text/css">
body{font-size:12pt}
td{font-size:11pt}
</style>
</head>
<body>
<form name="form1" action="" method="post" onsubmit="return check(this)">
<input type="hidden" name="num" />
<table width="500px" border="0" align="center" cellpadding="0" cellspacing="1" bordercolor="#FFFFFF" bgcolor="#86c2fe">
<thead>
<tr bgcolor="#C4DDF6">
<td width="35%" height="22" align="center">姓名</td>
<td width="35%" align="center">学号</td>
<td width="20%" align="center" >成绩</td>
<td width='10%' align="center">操作</td>
</tr>
</thead>
<tbody id="tbody">
</tbody>
</table>
<div align="center"><p><input type="button" class="form1" value="增加学生" onClick="addRow()"> <input type="submit" value="提交" /></p>
</div>
<table id ="table2" class="gridtable" >
<tr>
<td colspan="8"> 合计:
<input type="text" id="total_num" class="underline" value="" name="text">
</td>
</tr>
</table>
<p> </p>
</form>
<script type="text/javascript">
var Num=0;
var Input1="<input type='text' name='name' width='100%'/>";
var Input2="<input type='text' name='number' width='100%'/>";
var Input3="<input type='text' name='score' width='100%' onkeyup='sum(this);'/>";
var op="<span style='cursor:hand' onclick='if(confirm(\"确认删除？\"))this.parentNode.parentNode.removeNode(true);'>删除</span>";
function addRow()
{
var tbody=document.getElementById("tbody");
var tr=tbody.insertRow();
var reg=/name='(name|number|score)'/gi
var td=tr.insertCell();
td.innerHTML=Input1.replace(reg,"name='$1"+Num+"'") ;
td=tr.insertCell();
td.innerHTML=Input2.replace(reg,"name='$1"+Num+"'");
td=tr.insertCell();
td.innerHTML=Input3.replace(reg,"name='$1"+Num+"'" );
td=tr.insertCell();
td.innerHTML=op;
Num++;
}
function sum(obj) {
var sum1 = 0;
var obj = document.getElementById("tbody"); //取mybody对象
var length = obj.rows.length ;
var score=document.getElementById("score"+i);
alert("当前值=="+document.getElementById("score"+i));
for(var i=0; i<length; i++ ) {
if (score.value!=""){
sum1 =parseFloat(sum1) + parseFloat(("score"+i).value);
}
}
document.getElementById ("total_num").value = sum1;
}
</script>
</body>
</html>
