Help with class methods please

I'm pretty new to Matlab. I'm writing code that creates simulated images of rod-like particles diffusing in a liquid. I'm trying to use OOP to do this so I can keep track of positions and angles of the particles. My constructor method for the rods works fine, but I have a method to make them move and rotate that doesn't work. When I just write out the code in a function to test it it works so I'm guessing it's just something in the syntax that makes it not work when I call the method.

classdef rod

      properties
          rodlength;
          i_pos=0;
          j_pos=0;
          angle;
          frame;
methods  
function step(d,z,stepsize)%z is the simulated image the rod is put into

I want step to change the position and angle of some rod object d (I didn't include all the code inside the method because its long and boring). When I call the method, by saying rod_1.step(frame, 50) nothing happens, i.e. the image before and after I invoke the method is the same. However if I write out all the code outside the object it works so I think it has something to do with the way I call it. Any suggestions? Thanks.

Answer 1

[per isakson]

Change

    classdef rod

to

    classdef rod < handle

.

--- Cont. ---

Value objects are immutable. Their state (property values) cannot be be changed. When a new value is assigned to a property of a value object a new object is created and that new object must be assigned to a variable name or it is lost.

    new_object = old_object.method1( some_arguments );

It's hubris to try to write a simple explanation. I'm convinced the documentation pros at The Mathworks have tried hard to do that. "MATLAB numeric variables are of value objects."

    x = 17; x + 3;

The value of x is still 17.

.

Handle objects are mutable. Their state may be changed. An object may have multiple handles. Most of the common Design Patterns apply to handle objects. The Matlab documentations hardly mentions Design Patterns.

[John Kerin]

Thanks this seems to work! What's the simple explanation for the difference between value and handle classes?

[Sean de Wolski]

Value: pass by value; handle: pass a handle around that points to the value and inherit all of the cool features of the handle class.

Answer 2

[Eric]

I think per isakson's answer is a bit simplistic. You should probably read up on the difference between Handle classes and Value classes and then decide which type of class you need. Handle classes are not always the right solution. See

http://www.mathworks.com/help/techdoc/matlab_oop/brfylwk-1.html

If you're only ever going to create one instance of a rod object, then a Handle class is probably fine. Without the "< handle" suffix in the classdef line, you have a Value class. In that case you should use

obj = obj.step(d, z, stepsize);

Good luck,

Eric

[per isakson]

I would rather call it a "hint".

[Eric]

My apologies, simplistic is indeed overly harsh. I just wanted to make sure the OP is aware of what he's doing and that it may induce some other unexpected behaviors. I've had colleagues suffer from this same issue with Matlab classes. They've gone to great lengths in writing overly complicated code because they think Matlab only allows shallow copies of objects. I believe they may have been using the Handle class type without fully understanding what that means.