angular过滤器实现排序功能

<div id="box">      <!--第一个下拉框-->      <select ng-model="a">        <option value="age">按照年龄排序</option>        <option value="code">按照编码排序</option>        <option value="name">按照姓名排序</option>      </select>      <!--升序还是降序-->      <select ng-model="b">        <option value="">升序</option>        <option value="-">降序</option>      </select>      <!--一个搜索框-->      <input type="text" placeholder="请输入要搜索的内容" ng-model="c"/>    </div>    <!--渲染的数据-->    <div id="wrap">      <table>        <tr>          <th>编码</th>          <th>姓名</th>          <th>年龄</th>        </tr>        <tr ng-repeat="item in data|filter:c|orderBy:b+a">          <td>{{item.code}}</td>          <td>{{item.name}}</td>          <td>{{item.age}}</td>        </tr>      </table>    </div>

<script>      var app = angular.module("mk",[]);      app.controller("test",function($scope,$http){        $http.get('json/index.json').success(function(data){            $scope.data = eval(data);            $scope.a = "code";        })      })    </script>