This code in lua will apply a function to a value
function Apply(f, value)
return f(value)
end
I can then use it like this apply arbitrary functions calls on my game object like so
Apply(Draw, GameObject)
Apply(Update, GameObject)
Is it possible to instead do what I would, probably incorrectly, call a higher order method
function GameObject:Apply(f)
return self:f()
end
What I eventually want to do is have a table of GameObjects, that I can call Methods on in batch. So using this "higher order method" concept that may not even exist I would create code that does the following.
...
--Create the batch object with three bullets in it
BatchGameObjects = BatchGameObject:new(Bullet1, Bullet2, Bullet3)
--Call equivelent to
--Bullet1:DrawMethod()
--Bullet2:DrawMethod()
--Bullet3:DrawMethod()
--Bullet1:UpdateMethod()
--Bullet2:UpdateMethod()
--Bullet3:UpdateMethod()
BatchGameObjects:Apply(DrawMethod)
BatchGameObjects:Apply(UpdateMethod)
You'll probably want to pass function NAME if you're dealing with methods on other objects, because methods with same name on different objects may resolve to very different functions.
function BatchGameObjects:Apply(function_name)
-- ... or iterate on objects in any other way that matches how you store them ...
for idx = 1, #self.object_list do
local object = self.object_list[idx]
object[function_name](object)
end
end
function GameObject:Apply(f)
return f(self)
end
Related
I'm trying to learn how to use classes in Matlab, having never used them in any language before, so apologies if this is a bit basic.
I have defined a class called car, with the properties of colour, type, serial number and speed, with a function to change the speed.
classdef car <handle
properties
colour
type
speed
end
properties (SetAccess = private)
SerialNumber
end
methods
function faster(obj, v)
obj.speed = obj.speed + v;
end
end
end
In another script I can type
car.colour = "red", and when I disp(car), the class has the property colour with label "red". When I call faster(100) however, instead of setting car.speed=100, it throws the error
Check for missing argument or incorrect argument data type in call to function 'faster'.
I built the class and method using the same sort of code structure as in this question:
https://www.mathworks.com/matlabcentral/answers/395472-how-to-call-a-method-from-a-class-called-a-within-another-method-from-a
where the user seemed to not have the issue I do. I'm not sure where I'm going wrong - my function seems like it should work. Can anybody point me in the right direction?
You need to call the function on the class
myCar = car();
myCar.faster( 10 ); % equivalent to 'faster( myCar, 10 )'
If you hadn't specified the < handle type, you would also need to assign it back to the class, i.e.
myCar = myCar.faster( 10 );
But you don't need this with a handle class.
I come from a Java background. I am having issues with classes in Matlab particularly getters and setters. getting a message saying conflict between handle and value class I'm a little lost with what to do so any help for lack of a better word will be helpful.
classdef Person
properties(Access = private)
name;
age;
end
methods
% class constructor
function obj = Person(age,name)
obj.age = age;
obj.name = name;
end
%getters
function name = get.name(obj)
end
function age = get.age(obj)
end
%setters
function value = set.name(obj,name)
end
function value = set.age(obj,age)
end
end
end
Implementation
Since your class is currently a subclass of the default Value class, your setters need to return the modified object:
function obj = set.name(obj,name)
end
function obj = set.age(obj,age)
end
From the documention: "If you pass [a value class] to a function, the function must return the modified object." And in particular: "In value classes, methods ... that modify the object must return a modified object to copy over the existing object variable".
Handle classes (classdef Person < handle) do not need to return the modified object (like returning void):
function [] = set.name(obj,name)
end
function [] = set.age(obj,age)
end
Value vs. Handle
Going a bit deeper, the difference between a Value class and a Handle class lies mostly in assignment:
Assigning a Value class instance to a variable creates a copy of that class.
Assigning a Handle class instance to a variable create a reference (alias) to that instance.
The Mathworks has a good rundown on this topic.
To paraphrase their illustration, the behavior of a Value class is
% p is an instance of Polynomial
p = Polynomial();
% p2 is also an instance of Polynomial with p's state at assignment
p2 = p;
and of a Handle class is
% db is an instance of Database
db = Database();
% db2 is a reference to the db instance
db2 = db;
Quick'n Dirty from the Java perspective:
- "handle" classes are what your mind is set to. proper object instances with pointers to them. use them.
- "value" classes are always returning a full clone of whatever object (which has been modified by what you just did, e.g. setting a name).
the reason they have both in Matlab is that in Matlab you would expect the "value" behaviour natively. Imagine you have a matrix A = [1 2; 3 4], then assign that via B = A. if you now set B(1) = -1 you'd hope that A(1) is still 1, right? this is because matlab keeps track of "copies" and truly creates them as you modify different variables initially set to the same matrix. in OOP you'd have A(1)=-1 now as everythings an object reference.
furthermore, "native" matlab routines dont have a "this/self/me" variable that contains the instance reference to access from within functions. instead, the convention is that the class instance will be prepended to the function's argument list.
so for a function call myclass.mymethod(arg1,arg1), the declaration must be
function mymethod(this, arg1, arg2)
% Note that the name you choose for "this" is arbitrary!
end
mind you, this is the java-perspective (and also my favourite one), the above function call is equivalent to mymethod(myclass,arg1,arg1). this is more native to matlab-style, but somehow makes it harder to see you're calling an objects method.
now, regarding setters/getters: for handle classes, everything feels java-ish now:
classdef MyClass < handle
properties
MyProp;
end
methods
function set.MyProp(this, value) %Note: setMyProp is also valid!
... % do checks etc, trigger calls,
this.MyProp = value;
end
function value = get.MyProp(this)
... % notify, update, triggers etc
value = this.MyProp;
end
end
Of course it goes without saying that you dont need to define a getter if you just want to return the value, i.e. myclassinstance.MyProp will work without any just as well.
Finally, getters/setters for value classes are something that [never encountered me/i never needed] in my 7 years of matlab oop, so my advise would be to go with handle classes and enjoy happy matlab coding :-)
otherwise, the above explanation & official matlab docs is doing the job for value class getter/setters.
In my program when two functions with the same name are defined for the same table, I want my program to give an error. What's happening is that it's simply just calling the last function and executing it.
Here's a sample code
Class{'Cat'}
function Cat:meow( )
print("Meow!")
end
function Cat:meow()
print("Mmm")
end
kitty = Cat:create()
kitty:meow()
The result of the execution is only: "Mmm"
Instead I want something like an error message to be given.
Unfortunately, __newindex does not intercept assignments to fields which already exist. So the only way to do this is to keep Cat empty and store all its contents in a proxy table.
I don't know the nature of your OOP library, so you'll have to incorporate this example on your own:
local Cat_mt = {}
-- Hide the proxy table in closures.
do
local proxy = {}
function Cat_mt:__index(key)
return proxy[key]
end
function Cat_mt:__newindex(key, value)
if proxy[key] ~= nil then
error("Don't change that!")
end
proxy[key] = value
end
end
Cat = setmetatable({}, Cat_mt)
I have defined an abstract base class measurementHandler < handle which defines the interface for all inherited classes. Two subclasses of this class are a < measurementHandler and b < measurementHandler.
I now have a function which should return a handle to an instance of either of these subclasses (depending on the function arguments) to it's caller. Consider something like this:
function returnValue = foobar(index)
if index == 0
returnValue = a();
else
returnValue = b();
end
end
This function is enclosed in a MATLAB Function block in Simulink (2013a). When I try to simulate the system, I get the following error:
Type name mismatch (a ~= b).
Can anybody suggest a workaround for this which still allows me to take advantage of OOP & inheritance when using Simulink?
This kind of pattern is possible in MATLAB Function block only if the "if" condition can be evaluated at compile time. The types cannot be switched at run-time. Can you make the index value a constant at the call site?
The main reason to use this pattern was to iterate over a measurementHandler Array, while these all can have custom implementations. I was able to do this by unrolling the loop with the coder.unroll directive. Example for the enclosing MTALAB Function block:
function result = handleAllTheMeasurements(someInputs)
%#codegen
for index = coder.unroll(1:2)
measurementHandler = foobar(index);
measurementHandler.handleMeasurement(someInputs);
end
result = something;
end
This way, the for loop gets unrolled at compile time and the return type of the function is well defined for each separate call.
Writing a subclass of dynamicprops allows to me to add properties dynamically to an object:
addprop(obj, 'new_prop')
This is great, but I would also love to create set / get functions for these properties on the fly. Or analysis functions that work on these dynamic properties.
My experience with Matlab has been so far, that once I create an instance of a class, adding new methods is not possible. That is very cumbersome, because my object may contain a lot of data, which I'll have to re-load every time that I want to add a new method (because I have to do clear classes).
So is there a way to add methods on the fly?
You cannot add methods like you add dynamic properties. However, there are two ways for implementing new methods during development that won't require you to re-load the data every time.
(1) I write standard methods as separate functions, and call them as myMethod(obj) during development. Once I'm sure they're stable, I add their signature into the class definition file - this requires a clear classes, of course, but it is a much delayed one, and from time to time you may have to shut down Matlab, anyway.
(2) With set/get methods, things are a little trickier. If you are using dynamicprops to add new properties, you can also specify their set/get methods, however (most likely, these methods/functions will want to receive the name of the property so that they know what to refer to):
addprop(obj,'new_prop');
prop = findprop(obj,'new_prop');
prop.SetMethod = #(obj,val)yourCustomSetMethod(obj,val,'new_prop')
EDIT
(2.1) Here's an example of how to set up a hidden property to store and retrieve results (based on jmlopez' answer). Obviously this can be improved a lot if you have a better idea what you're actually designing
classdef myDynamicClass < dynamicprops
properties (Hidden)
name %# class name
store %# structure that stores the values of the dynamic properties
end
methods
function self = myDynamicClass(clsname, varargin)
% self = myDynamicClass(clsname, propname, type)
% here type is a handle to a basic datatype.
self.name_ = clsname;
for i=1:2:length(varargin)
key = varargin{i};
addprop(self, key);
prop = findprop(self, key);
prop.SetMethod = #(obj,val)myDynamicClass.setMethod(obj,val,key);
prop.GetMethod = #(obj)myDynamicClass.getMethod(obj,key);
end
end
function out = classname(self)
out = self.name_;
end
end
methods (Static, Hidden) %# you may want to put these in a separate fcn instead
function setMethod(self,val,key)
%# have a generic test, for example, force nonempty double
validateattributes(val,{'double'},{'nonempty'}); %# will error if not double or if empty
%# store
self.store.(key) = val;
end
function val = getMethod(self,key)
%# check whether the property exists already, return NaN otherwise
%# could also use this to load from file if the data is not supposed to be loaded on construction
if isfield(self.store,key)
val = self.store.(key);
else
val = NaN;
end
end
end
end
I'm adding this answer because I think that this is not intuitive. At least not to myself at this moment. After finding this question I thought I had what I needed to be able to define the set/get methods for my dynamic class. All I wanted to achieve with this was something similar to what python does with its __setattr__ method. In any case, here is a continuation of the class made by #jonas a while ago with a few modifications to add the our custom set method.
classdef myDynamicClass < dynamicprops
properties (Hidden)
name_ %# class name
end
methods
function self = myDynamicClass(clsname, varargin)
% self = myDynamicClass(clsname, propname, type)
% here type is a handle to a basic datatype.
self.name_ = clsname;
for i=1:2:length(varargin)
key = varargin{i};
addprop(self, key);
prop = findprop(self, key);
prop.SetMethod = makefunc(key, varargin{i+1});
end
end
function out = classname(self)
out = self.name_;
end
end
end
function h = makefunc(key, argtype)
h = #newfunc;
function newfunc(obj, val)
obj.(key) = argtype(val);
end
end
With this class I'm defining the set method so that the parameter passed to the attribute is copied to the right type. To see what I mean consider the following usage:
>> p_int = myDynamicClass('Point', 'x', #int8, 'y', #int32);
>> p_int.x = 1000
p_int =
myDynamicClass with properties:
y: []
x: 127
>> class(p_int.x)
ans =
int8
With this we have forced the x attribute to be an integer of 8 bits which can only hold integers from -128 to 127. Also notice how the class of each attribute gives us the intended type.
My experience with Matlab has been so far, that once I create an instance of a class, adding new methods is not possible. That is very cumbersome, because my object may contain a lot of data, which I'll have to re-load everytime that I want to add a new method (because I have to do clear classes).
It's worth noting for present-day readers of this question that this is no longer true. As of MATLAB R2014b MATLAB updates class definitions at the moment you save them, and the behaviour of existing class instances automatically updates accordingly. In the case of adding new methods, this is uncomplicated: the new method simply becomes available to call on class instances even if they were created before the method was added to the class.
The solutions given for choosing set/get methods for dynamic properties still apply.
There are still cases where you might want to add methods to an instance dynamically and the method doesn't constitute a property set/get method. I think the only answer in this case is to assign a function handle as the value to a dynamic property. This doesn't create a bona fide method, but will allow you to call it in the same way you would a method call:
addprop(obj, 'new_method');
obj.new_method = #(varargin) my_method(obj,varargin{:});
Calls to obj.new_method(args) are thus passed to my_method; however this only works with a scalar obj; an array of instances will have separate values for the new_method property so obj.new_method no longer resolves to a single function handle that can be called if obj is an array.