jquery实现checkbox的全选功能

$(document).ready(function() {
    var num = $("#check").length;
    $("#role_manager input[id=selector]").click(function() {
        for (var i = 0; i < num; i++) {
            if ($(this).attr("checked") == true) {
                $("#check").each(function() {
                    $(this).attr("checked", true);
                });
            }
            else {
                $("#check").each(function() {
                    $(this).attr("checked", false);
                });
            }
        }
    });
});

页面如下:

<table id="role_manager" border="1" cellpadding="0" cellspacing="0" width="80%" style="border: 2px solid; border-collapse: collapse; border-color: #3399FF; border-collapse: collapse;">
        <tbody>
            <tr bgcolor="#ECE1E8">
                <td width="10%">序号</td>
                <td width="30%">名称</td>
                <td width="30%">描述</td>
                <td width="10%">全选<input type="checkbox" id="selector" name="selector" /></td>
            </tr>
            <?php
                while($object=mysql_fetch_object($result)){
            ?>
            <tr>
                <td>
                    <?php echo $object->Id; ?>
                </td>
                <td>
                    <?php echo iconv("gb2312","utf-8",$object->Name); ?>
                </td>
                <td>
                    <?php echo iconv("gb2312","utf-8",$object->Description); ?>
                </td>
                <td>
                    <input type="checkbox" id="check" name="check[]" />
                </td>
            </tr>
            <?php
                }
            ?>
        </tbody>
    </table>