怎么用JS控制RADIO的初始值?少见!

[xinrong]

<form name="form1">
<input type="radio" name="a1" value="0">0<br/>
<input type="radio" name="a1" value="1">1<br/>

<input type="radio" name="a2" value="0">0<br/>
<input type="radio" name="a2" value="1">1<br/>

<input type="radio" name="a3" value="0">0<br/>
<input type="radio" name="a3" value="1">1<br/>
....
....

<input type="button" name="but" value="设置" onClick="set();">
</form>
有N组(name相同的为一组)单选按钮,想写一JS方法,做到以下效果:
点击"设置"按钮时,N组单选按钮全部选中值为"0"的选项,再次点击时,则全部选中值为"1"的选项.如此循环下去.

[surfchen]

<form name="form1">
<input type="radio" name="a1" value="0">0<br/>
<input type="radio" name="a1" value="1">1<br/>

<input type="radio" name="a2" value="0">0<br/>
<input type="radio" name="a2" value="1">1<br/>

<input type="radio" name="a3" value="0">0<br/>
<input type="radio" name="a3" value="1">1<br/>
<input type="button" name="but" value="设置" onClick="set();">
</form>

<script type="text/javascript">
<!--
function set()
{
f_obj1=document.form1.a1;
f_obj2=document.form1.a2;
f_obj3=document.form1.a3;
if (f_obj1.item(0).checked==true)
{
f_obj1.item(1).checked=true;
f_obj2.item(1).checked=true;
f_obj3.item(1).checked=true;
}
else
{
f_obj1.item(0).checked=true;
f_obj2.item(0).checked=true;
f_obj3.item(0).checked=true;
}
}
//-->
</script>