var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  var vm =this
  vm.elements=["test","test2"];
  vm.addElement = addElement;
  
      function addElement(element) {
        console.log('hi');
        vm.elements.push(element);
    }
});
<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link href="style.css" rel="stylesheet" />
    <script src="https://code.angularjs.org/1.5.4/angular.js"></script>
    <!-- By setting the version to snapshot (available for all modules), you can test with the latest master version -->
    <!--<script src="https://code.angularjs.org/snapshot/angular.js"></script>-->
    <script src="app.js"></script>
  </head>

  <body ng-controller="MainCtrl as vm">
<form name="myForm">
  <div layout="row">  
    <label>Element
    <input type="text" name="input" ng-model="vm.element">
    </label>
    <button ng-click="vm.addElement(vm.element)">
    Increment
    </button> 
  <div>
  <div layout="row">{{vm.elements}}
    <ul>
     <li  ng-repeat="elt in vm.elements" class="">
         <div ng-if="$index %3 ==0" > <font size="3" color="red">{{elt}}{{$index}}</font></div>
         <div ng-if="$index %3 !==0"  >{{elt}}-{{$index}}</div>
    </li>
    </ul>
  </div>
</form>
  </body>
</html>

/* Put your css in here */