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>