Today I learned how to set the default value of the ng-options drop-down data in angular.Leave a note

Directly on the code

   <div>
        <label>teacher</label>
             <div>
              <select
                  ng-model="editcourse.teachername"
                  ng-options="teacher.username for teacher in teacherlist" required>
                <option value="">select teacher</option>
               </select>
             </div>
      </div>

angularjs

   //data is the id of the course
   $scope.edit=function (data) {
      //Get course object by course id
      courseservice.getbycourseid (data) .then (function (result) {
        $scope.editcourse=result.data;
        //Default value setting
        //First get the teacher object by the teacher id in the course
        courseservice.getteacherbyteacherid (result.data.teacherid) .then (function (result) {
         //$scope.teacherlist is a list of all teachers
          for (i=0;i<$scope.teacherlist.length;i ++) {
            //If the id of the teacher of the current course is equal to the id of the teacher currently traversed, give the object of the teacher currently traversed to ng-model="editcourse.teachername"
            if (result.data.userid == $scope.teacherlist [i] .userid) {
              $scope.editcourse.teachername=$scope.teacherlist [i];
            }
          }
        });
        angular.element ("#edit"). modal ({
          show:true
        })
      })
    }

Demo