怎样才能得到单选框的值？

[xiejava1018]

我有一组单选框
<input type="radio" name="subject1" value="A">
<input type="radio" name="subject1" value="B">
<input type="radio" name="subject1" value="C">
<input type="radio" name="subject1" value="D">
如何得到所选中的单选框的值？
subject1.value读出来是undefined;该如何读到value的值？

[zhangguagua]

function getvalue()
{
var a = document.getElementsByTagName("input");
for (var i=0; i<a.length; i++){
if ((a[i].type == "radio")&&( a[i].checked ))
alert(a[i].value);
}

}

[心云意水]

<script>
var ys=document.body.getElementsByName("subject1")
for(i=0;i<ys.length;i++)
if(ys[i].checked)
alert("subject1'value is"+ys[i].value)
</script>

[无爱大叔]

<input type="radio" name="subject1" id="subject1" value="A">
<input type="radio" name="subject1" id="subject1" value="B">
<input type="radio" name="subject1" id="subject1" value="C">
<input type="radio" name="subject1" id="subject1" value="D">
<button onclick="a()">hehe</button>
<SCRIPT LANGUAGE="JavaScript">
<!--
function a(){
el=document.all.tags("input")
for(i=0;i<el.length;i++){
if(el(i).checked){
alert(el(i).value)
}
}
}
//-->
</SCRIPT>