How do I filter an array with AngularJS and use a property of the filtered object as the ng-model attribute?

You can use the "filter" filter in your controller to get all the "C" grades. Getting the first element of the result array will give you the title of the subject that has grade "C".

$scope.gradeC = $filter('filter')($scope.results.subjects, {grade: 'C'})[0];

http://jsbin.com/ewitun/1/edit

The same with plain ES6:

$scope.gradeC = $scope.results.subjects.filter((subject) => subject.grade === 'C')[0]

<div ng-repeat="subject in results.subjects | filter:{grade:'C'}">
    <input ng-model="subject.title" />
</div>

Here is a modified JSBin with a working sample:

http://jsbin.com/sezamuja/1/edit

Here is what I did with filters in the input:

<input ng-model="(results.subjects | filter:{grade:'C'})[0].title">