单选按钮用法, 选择的值

如果没有选择的话，返回的值为 undefined

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>xxx</title>
<script>
/**
*state 的值为单选中 value 的值
*/
function test(form){
   var i = form.ok.length;
   var state;
   for(var j=0; j<i; j++){
       if(form.ok[j].checked){
           state = form.ok[j].value;
       }
   }
   if(state == undefined){
       alert("你没有选择！");
       return false;
   }
   alert(state);
   return false;
}
</script>
</head>
<body>
<table border="1" width="150">
<form name="myform" id="myform" method="post" action="test.php">
<input type="radio" name="ok" value="1" />aaaa<br />
<input type="radio" name="ok" value="2" />bbbb<br />
<input type="radio" name="ok" value="3" />cccc<br />
<button name="sub" onclick="return test(myform)">Button</button>
</form>
</table>
</body>
</html>