title: Angularjs update other fields when one change
body: <p>I rewrote the question.</p>
  
  <p>My data is a list of objects, lets say rows. For every row I build an excel-like table of cells, this way</p>
  
  <pre><code>&lt;div ng-repeat="row in dataRows"&gt;
     &lt;input ng-model="row.a"&gt;&lt;input ng-model="row.b"&gt;{{row.c}}
  &lt;/div&gt;
  </code></pre>
  
  <p>What I want to achieve is an automatic update of <code>c</code> field based on changes of <code>a</code> and <code>b</code>.
  I tried to use <code>$scope.$watch</code> but this monitor my whole dataRows and I would like to fire a calculating value of <code>c</code> for those rows which changed</p>