Array elements are skipped

edit

I want to move the joints of my robot at 0, 90, 180, 270 degrees, but it only displays the state of 90 and 270 degrees on gazebo repeatedly. I output the results to the terminal, and actually four angles show periodically, and counts are formally 0,1,2,3,4,5,6.... However I do the different tests, the robot always skips one element between two array elements on gazebo. Does anyone know how to solve this problem? Here is my plugin code.

#include <functional>
#include <gazebo/gazebo.hh>
#include <gazebo/common/common.hh>
#include <ignition/math/Vector3.hh>
#include <gazebo/physics/physics.hh>
#include <stdio.h>

using namespace std;

namespace gazebo
{
  int count = 0;
  float pos[4] = {0, 1.57, 3.14, 4.71};

  class ModelPush : public ModelPlugin
  {
    public: void Load(physics::ModelPtr _parent, sdf::ElementPtr /*_sdf*/)
    {
      this->model = _parent;
      this->updateConnection = event::Events::ConnectWorldUpdateBegin(std::bind(&ModelPush::OnUpdate, this));
    }

    public: void OnUpdate()
    {
       this->model->GetJoint("joint1")->SetPosition(0, pos[count%4]);
       this->model->GetJoint("joint1")->>Update();
       this->model->GetJoint("joint2")->SetPosition(0, pos[count%4]);
       this->model->GetJoint("joint2")->>Update();

       cout << fixed << this->model->GetJoint("joint1")->GetAngle(0).Radian() << "\t";
       cout << fixed << this->model->GetJoint("joint2")->GetAngle(0).Radian() << "\t" << count << endl;

       count++;
       usleep(500000);
    }

    private: physics::ModelPtr model;
    private: event::ConnectionPtr updateConnection;
  };
  GZ_REGISTER_MODEL_PLUGIN(ModelPush)
}