class: how to use the class within itself

Hi there, I have used Processing (based on java) and recently turned to Matlab, and its syntax is quiet confusing..I am trying to make a simple spring simulation using class, and in the function 'reaction', I would like to use the object from this class as an input, and in the function 'move', I'd like to use the property 'position' initialised in the beginning and change it constantly.

But when I call this class, millions of errors occurs..can anybody help to tell me where I did wrong..I got something saying that the property 'position'must be accessed from a class instance because it is not a constant property..And I've never seen that..

classdef Particle

      properties
          rest = 100;
          mass=1000;
          damping=0.999;
          position=[0 0];
          velocity; %= [1.*rand(100,1) 1.*rand(100,1)];
      end

      methods (Static)

          function this = Particle()
              this.position=[0 0];
              this.velocity=[1.*rand(100,1) 1.*rand(100,1)];
          end

          function velocity=react(Particle)
              p1 = Particle.position;
              p2 = Particle.position;
              d = sqrt((p1(1)-p2(1))^2+(p1(2)-p2(2))^2);
              displacement = d-Particle.rest;
              a = normr(p1-p2);
              a = a.*(displacement/Particle.mass);
              velocity = Particle.velocity+a;
        %     Particle.bounce(position, velocity);
              velocity = velocity.*Particle.damping;
          end

          function [position] = move (velocity)
              position=Particle.this.position+velocity;
          end

          function draw (myposition)
              viscircles(myposition, 0.1);
          end

      end

end

and this is my main sketch calling class

a = Particle;
b = Particle;

a.react(b);
b.react(a);
a.move();
b.move();
line([a.position(1) b.position(1)],[a.position(2) b.position(2)]);
a.draw();
b.draw();

Thanks so much! I think I'm still using the java syntax rather than matlab's....

Answer 1

[David Ding]

Hi Vapeur,

It is difficult to tackle the issue without seeing the actual error. However, seeing your code, there is an obvious error in your methods, which is that you must pass in the object as the first argument of each method. This is expected for MATLAB. For example, you have:

 function this = Particle()
          this.position=[0 0];
          this.velocity=[1.*rand(100,1) 1.*rand(100,1)];
 end

The syntax is incorrect. Instead you need to have:

function Particle(obj)
          obj.position=[0 0];
          obj.velocity=[1.*rand(100,1) 1.*rand(100,1)];
end

Note the first argument in the "Particle" method is the object itself. Also this is a void function so no output is specified.

Another example is with regards to your "move" method.

function [position] = move (velocity)
    position=Particle.this.position+velocity;
end

I am guessing that you are outputting the updated position by the velocity input. Remember that the first argument MATLAB expects is the object itself. So you need to have something like this:

function [position] = move (obj, velocity)
    position = obj.position+velocity;
end

Try to fix other parts of your code in the methods section. For example, in your "react" method, you need to start with:

function velocity = react(obj)
...
end

Best of luck on your endeavors!

Thanks,

David