I'd like to save a particular class property to disk, while ignoring the rest of the class properties. I think MATLAB allows the saveobj method to be overridden for this purpose. However, this saves the class object with only that property attached. I want to save the property itself, without any of the class information.
I might think that a suitable method would look like:
classdef myClass
properties
myProp
end
methods
def b = saveobj(a)
b = a.myProp;
end
def Save(a,fname)
save(fname,'a.myProp');
end
end
end
But neither of these have the desired effect. Can anyone help me, please?
You can overload the save function itself without having to go through saveobj:
classdef myClass
properties
myProp
end
methods
function [] = save(a,fname,varargin)
myProp = a.myProp; %#ok<PROP,NASGU>
save(fname,'myProp',varargin{:});
end
end
end
Then on the command window:
>> foo = myClass();
>> foo.myProp = 4;
>> foo.save('var.txt');
>> bar = load('var.txt','-mat');
>> bar.myProp
ans =
4
The first method (the one involving saveobj) is actually correct. For the purposes of discussion let us consider this simple class:
classdef testclass
properties
x
end
methods
function this = testclass(x)
this.x = x ;
end
function a = saveobj(this)
a = this.x ;
end
end
end
When you ask MATLAB to save an instance of your class, it will use the saveobj method when you call save if one exists. The output of this method can be an object, a struct, an array, whatever. You want to test that this has occurred, and you do something natural like this:
>> obj = testclass('hi')
obj =
testclass with properties:
x: 'hi'
>> save tmp.mat obj
>> clear all
>> load tmp.mat
>> obj
obj =
testclass with properties:
x: []
>>
And this is where I suspect your confusion arises. You expect obj to be a char but instead it's an empty object of class testclass. (You can verify that it is just an instance of the object based on the saved definition of the class, and that it is not created by calling the empty constructor.)
This may seem rather confusing until you understand how loadobj works. In order for MATLAB to know which static method to call on load, it saves the class definition in conjunction with whatever output you are providing from your custom saveobj method. When you call load it then loads the class definition and calls the loadobj static method if one exists. We can test this by modifying the class definition above:
classdef testclass
properties
x
end
methods
function this = testclass(x)
this.x = x ;
end
function a = saveobj(this)
a = this.x ;
end
end
methods( Static )
function this = loadobj(a)
this = testclass(a) ;
end
end
end
If you set a breakpoint in the loadobj method you can verify that the type of a is indeed char as you expect.
Related
I'm having a weird problem using object oriented matlab, and I'm not sure if I'm completely missing something, or I've hit some unusual behaviour.
I have a superclass defined as follows:
classdef (Abstract) AbstractSimulationData < matlab.mixin.Heterogeneous
properties (Abstract)
name
data
end
properties
timeStamp = -1
end
methods (Abstract)
CalculateData(obj, t)
end
methods
function data = GetData(obj, t)
if obj.timeStamp == t.t
data = obj.data;
else
obj.timeStamp = t.t;
obj.CalculateData(t);
data = obj.data;
end
end
end
end
When the concrete class implements the method CalculateData I intend it to set the superclass property data.
Two concrete classes from this are:
classdef A < AbstractSimulationData
properties
name = 'A'
data = []
end
methods
function obj = A
% No special initialisation
end
function CalculateData(obj, t)
test = t.B.GetData(t)
obj.data = rand(2,10);
end
end
end
And:
classdef B < AbstractSimulationData
properties
name = 'B'
data = containers.Map
end
methods
function obj = B
% No special initialisation
end
function CalculateData(obj, t)
if isempty(obj.data)
obj.data('left') = 1;
obj.data('right') = 2;
end
% Other operations
end
end
end
Within the A.CalculateData method I call the GetData method for B. This in turn calls the B.CalculateData method.
Now, as it filters back up, the data property gets set correctly so that B.GetData returns the value that was set in B.CalculateData.
But, when I go the next step up to A.GetData it returns [], the initialised value. Going through debug mode, the calculations definitely work in A.CalculateData and it seems to set data correctly, but when it jumps back up to the superclass obj.data is the default value from A.
A minimum working example to get this behaviour:
t.B = B();
t.t = 1;
a = A;
a.GetData(t);
>> test =
Map with properties:
Count: 2
KeyType: char
ValueType: any
>> ans =
[]
This is all part of a pretty big project, so it's hard to give the full context, but if it's possible from this, can someone explain why setting superclass data works for B but not A?
If it makes a difference B is returning a container.Map and A is returning 2xn matrix.
As far as I can tell from the documentation, it should work.
I like the OO programming style that matlabs App Designer uses (or at least the way I'm using it). Now I'm wondering if I can use the same style in my "normal" matlab class.
What I have now:
classdef myClass
properties
myVar;
end
methods
function Main(obj)
obj.myVar = "a";
obj = DoSomething(obj);
disp(obj.myVar) % outputs "c"
end
function obj = DoSomething(obj)
if(obj.myVar == "a")
obj.myVar="c";
else
obj.myVar = "b";
end
end
end
end
Which can be called externally using:
myClassInst = myClass;
myClassInst.Main()
I would like to get rid of all the "obj = " in the classdef, as is possible in App Designer. So something that would look like this:
classdef myClass
properties
myVar;
end
methods
function Main(obj)
obj.myVar = "a";
DoSomething(obj); % Just call the function without "obj = "
disp(obj.myVar) % outputs "a" because I didn't overwrite obj
end
function DoSomething(obj)
if(obj.myVar == "a")
obj.myVar="c";
else
obj.myVar = "b";
end
end
end
end
The equivalent of this seems to work in App Designer. So it appears you can modify variables in a class (instance?) in App Designer, while also being able to access the modified variable without explicitly overwriting your old class instance.
I noticed App Designer has all methods an properties set to (Access = private), though I'm not sure that has anything to do with it. Of course if I set everything to private, then I can't access the Main() method from outside anymore.
So my question is, how can I program in "normal" matlab, the same way as is possible in App Designer?
EDIT:
The following works in App Designer (I left out the methods/properties for the GUI elements):
classdef tmp < matlab.apps.AppBase
properties (Access = private)
myVar; % Description
end
methods (Access = private)
function doSomething(app)
if app.myVar == "a"
app.myVar = "c";
else
app.myVar = "b";
end
end
end
% Callbacks that handle component events
methods (Access = private)
% Code that executes after component creation
function startupFcn(app)
app.myVar = "a";
doSomething(app);
disp(app.myVar); % outputs "c"
end
end
end
You definitely can! All you have to do is inherit from the handle class, as opposed to a value class which is the default for matlab. You can also define private and public methods as in other languages.
The only thing you have to do is:
classdef myclass < handle % this is how you inherit from base class
properties
public_property
end
properties (Access=private)
private_property
end
methods
function obj = myclass() % class constructor
...
end
function public_function()
...
end
end
methods (Access=private)
function private_function()
...
end
end
end
Now every time you pass an object of this class to a function, you are not passing it by value, you are passing by reference (as you might be used to from python) and modifying it's properties modifies them also in the original object.
You need to inherit (< at the top of the class) from a handle class
classdef myClass < handle
properties
var
end
methods
function obj = myClass( varargin )
% Constructor function, called automatically when object is created
end
function someFunction( obj )
obj.randomizeVar(); % Equivalent to randomizeVar( obj );
end
function randomizeVar( obj )
obj.var = rand();
end
end
end
See the documentation for the difference between a "handle" and "value" class:
A value class constructor returns an object that is associated with the variable to which it is assigned. If you reassign this variable, MATLAB® creates an independent copy of the original object. If you pass this variable to a function to modify it, the function must return the modified object as an output argument. For information on value-class behavior, see Avoid Unnecessary Copies of Data.
A handle class constructor returns a handle object that is a reference to the object created. You can assign the handle object to multiple variables or pass it to functions without causing MATLAB to make a copy of the original object. A function that modifies a handle object passed as an input argument does not need to return the object.
Moreover, if you edit matlab.apps.AppBase, the class which you app designer code inherits, you can see that the first line is
classdef AppBase < handle
So you are literally doing the same thing, without the AppBase middle-man.
We have this code in 'Reconstruction the subclass object from a saved struct' from MATLAB OOP documentation.
classdef MySuper
% Superclass definition
properties
X
Y
end
methods
function S = saveobj(obj)
% Save property values in struct
% Return struct for save function to write to MAT-file
S.PointX = obj.X;
S.PointY = obj.Y;
end
function obj = reload(obj,S)
% Method used to assign values from struct to properties
% Called by loadobj and subclass
obj.X = S.PointX;
obj.Y = S.PointY;
end
end
methods (Static)
function obj = loadobj(S)
% Constructs a MySuper object
% loadobj used when a superclass object is saved directly
% Calls reload to assign property values retrived from struct
% loadobj must be Static so it can be called without object
obj = MySuper;
obj = reload(obj,S);
end
end
end
I have a question about obj = MySuper. What is purpose of this line? How we can call MySuper object from this function without insert any object to loadobj function?
You first question is: What is the purpose of the obj = MySuper; line?
The answer is that the obj = MySuper; line initiates the variable obj as an element of the class MySuper. Non-static functions in a class will only run if the first input parameter is an instance of the class, so if obj is not initiated as a MySuper-object, then matlab will look for other functions called reload to run, and if none is found give you an error.
For your second question, I am not 100% sure what you mean. But I hope one of the following points will answer your question:
If you want to make a function that relates to a class, but not to a specific instance of the class, you can make a static function, these can take any input (also (if you want it that way) no input at all) - that is they don't need to have a first input parameter of the specific class.
To run a static function, use the class name followed by a dot and then the function name, so here you would type MySuper.loadobj(S) to run the function with the parameter S.
I would suggest that you try this out with the given example to better get to know the way oop works in matlab, you may for example try:
S.PointX = 1;
S.PointY = 2;
obj = MySuper.loadobj(S)
I hope this answers your questions.
If I have a class
classdef foo
properties
a = 0;
end
methods
function obj foo(obj)
obj.a = 5;
end
end
end
And a function
function result = GetFoo()
result = foo();
end
At the command interpreter:
>> x = GetFoo()
x =
foo
Properties:
a: 5
Methods
>> x.a = 10
x =
foo
Properties:
a: 10
Methods
this yields an instance of foo and the value 'a' can be assigned. However, doing the same in one step:
>> GetFoo().a = 10
GetFoo =
a: [1x1 struct]
This creates a new structure called GetFoo that overrides the class and gives it a member called 'a'. Rather than the code getting the result of GetFoo() (the class instance) and settings its property 'a' = 10, it does this instead. I can see that it is likely a hangover from the ability to create weakly typed structures on the fly and the code having to be backward compatible It also seems to be related to the fact that MATLAB has no concept of a pointer so every input/output argument is a deep copy and that the above even if it would work would be setting the value of a copy of the value GetFoo() is using to source its return object, then throwing it away.
Nonetheless the goal really is to be able to do all the work I need without creating and requiring to clear temporary variables. The intent is code maintainability as much as cosmetic style.
You're mixing together quite a few different issues/complaints in your question, but I suspect what you're looking for is a handle class. Inherit your class from handle (i.e. write classdef foo < handle, and your class will have what is basically pass-by-reference behavior. You're right that MATLAB doesn't use pointers, but with handle classes it does have references.
Separately, it's not possible in MATLAB to index in to the output of a function (i.e. to write GetFoo.a or GetFoo().a - you do need a temporary variable for that, whether or not it returns a value or handle class.
Note that in your example above, the result of the GetFoo().a = 10 is not an instance of foo with the property a equal to 10 - it is a structure with a field a equal to 10. Type class(GetFoo), and you'll see it's a struct, not a foo.
This is really ugly, but is just to raise the glove that was thrown about doing it in one line. :-)
First of all, I'll modify a bit the class so you'll see that the constructor is called:
classdef foo
properties
a = 0;
end;
methods
function obj = foo()
obj.a = 5;
display('foo!');
end;
end;
end
And the one-liner, ran in Command Window:
>> subsasgn(GetFoo(), struct('type', '.', 'subs', 'a'), 10);
foo!
I am creating a class in MATLAB and while I have little experience with objects, I am almost positive I should be able to set a class property using a class method. Is this possible in MATLAB?
classdef foo
properties
changeMe
end
methods
function go()
(THIS OBJECT).changeMe = 1;
end
end
end
f = foo;
f.go;
t.changeMe;
ans = 1
Yes, this is possible. Note that if you create a value object, the method has to return the object in order to change a property (since value objects are passed by value). If you create a handle object (classdef foo<handle), the object is passed by reference.
classdef foo
properties
changeMe = 0;
end
methods
function self = go(self)
self.changeMe = 1;
end
end
end
As mentioned above, the call of a setting method on a value object returns the changed object. If you want to change an object, you have to copy the output back to the object.
f = foo;
f.changeMe
ans =
0
f = f.go;
f.changeMe
ans =
1