jq遍历数组

html结构如下：

<div id="demo">
    <div class="checkbox">
        <label>a:</label>
        <input type="radio" value="1" name="a" />1
        <input type="radio" value="2" name="a" />2
    </div>
    <div class="checkbox">
        <label>b:</label>
        <input type="radio" value="1" name="b" />1
        <input type="radio" value="2" name="b" />2
    </div>
    <div class="checkbox">
        <label>c:</label>
        <input type="radio" value="1" name="c" />1
        <input type="radio" value="2" name="c" />2
    </div>
    <div class="checkbox">
        <label>d:</label>
        <input type="radio" value="1" name="d" />1
        <input type="radio" value="2" name="d" />2
    </div>
</div>
<input type="button" value="btn" />

要求是点击btn,判断哪个类别没有选择，输出它的label

比如这个就输出"a没有选择"

用jq怎么实现

---

<div id="demo">
    <div class="checkbox">
        <label>a:</label>
        <input type="radio" value="1" name="a" />1
        <input type="radio" value="2" name="a" />2
    </div>
    <div class="checkbox">
        <label>b:</label>
        <input type="radio" value="1" name="b" />1
        <input type="radio" value="2" name="b" />2
    </div>
    <div class="checkbox">
        <label>c:</label>
        <input type="radio" value="1" name="c" />1
        <input type="radio" value="2" name="c" />2
    </div>
    <div class="checkbox">
        <label>d:</label>
        <input type="radio" value="1" name="d" />1
        <input type="radio" value="2" name="d" />2
    </div>
</div>
<input type="button" id="btn" value="btn" />
    <script type="text/javascript" src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
<script>
$('#btn').click(function(){
    var result = [];

    $('#demo div.checkbox').each(function(index){
        if($('input:checked', this).length === 0){
            result.push($('label:first', this).text() + '\t第' + (index + 1) + '个没有选择');
            //如果只需要找到一个就不再往后找了的话,把下面这行的注释删除掉
            //return false;
        }
    });

    alert(result.join('\n'));
});
</script>