在select if中设置空值如果为空则选择AngularJS

Question

有人可以告诉我如何制作空值，如果在选择列表中没有任何内容，则显示“不适用”

demo

https://codepen.io/Turqus/pen/wPrKrr?editors=1010

模板

    <div ng-app="test" ng-controller="select">
  <select class="form-control" ng-model="selectedBoard" ng-change="changeBoard(selectedBoard)">
     <option ng-repeat="item in boards" value="{{$index}}">{{$index}}</option>   
 </select>

 <select class="form-control" ng-model="selectedList">
   <option ng-repeat="item in downloadedLists" value="{{$index}}">{{$index}}</option> 
 </select>                  
</div>

Answer 1

<option ng-repeat=" item in downloadsLists" >{{$index || "n/a"}}</option>

Answer 2

sudo apt-get install clang

clang -v //Now see installed version

Answer 3

 <option value="">N/A </option>  
 <option ng-repeat="item in boards" value="{{$index}}">{{$index}}</option>