Implementation code of jQuery operation input type=checkbox

<input type="checkbox">: 
2012欧洲杯"死亡之组"小组出线的国家队是：<br> 
<input type="checkbox" name="nation" value="Germany">德国 
<input type="checkbox" name="nation" value="Denmark">丹麦 
<input type="checkbox" name="nation" value="Holland">荷兰 
<input type="checkbox" name="nation" value="Portugal">葡萄牙

　1. The first and second place in the group qualify, so we have to limit ourselves to two choices.

var len = $("input[name='nation']:checked").length; 
if(len==0) { 
alert("请选择出线的国家队！"); 
return false; 
}else if(len<2) { 
alert("请选择两个国家队！"); 
return false; 
}else if(len>2) { 
alert("只能选择两个国家队！"); 
return false; 
}else { 
return true; 
}

　2. Traverse the selected national teams.

$("input[name='nation']:checked").each(function(){ 
alert("已选择的国家队: "+$(this).val()); 
});

　3. Cancel all selected national teams.

$("input[name='nation']:checked").attr("checked",false);

　4. Designate two national teams.

$("input[name='nation'][value='Germany']").attr("checked",true); 
$("input[name='nation'][value='Holland']").attr("checked",true);

　5. It is forbidden to choose the national team.

$("input[name='nation']").attr("disabled",true);

　6. Allowed to choose the national team.

$("input[name='nation']").attr("disabled",false);

　7. Select the national team whose index is even or odd (the index starts from 0).

//选中索引为偶数的国家队 
$("input[name='nation']:even").attr("checked",true); 
//选中索引为奇数的国家队 
$("input[name='nation']:odd").attr("checked",true);

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