Ng-if creating problem

"<div class="list">
  <label class="item item-input item-select">
    <div class="input-label">
      Unit to be Converted
   <select ng-model="factor2" ng-change="getResult(catagory,unit1,factor1,factor2)">
   <option value="{{array.factor}}"  ng-repeat="array in arraynew.arr">{{}}</option>

Above code is working fine. But when we edit as below
<div class="list" ng-if="catagory<18"> and is not working. factor2 is undefined showing. What is the problem. Here catagory is modal for select list.
Plz help