Matlab OOP (Basic): How to change the local variables
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From: Tom on 29 Apr 2010 17:31 Hi I'm trying to learn Matlab OOP and started out implementing a simple digital-filter. Now, I have a class that does the filtering and the memory of the filter is contained within the class and is updated whenever a new filter output is computed. The filter object is called in a for-loop for every sample iteration. I notice that even though I do update the memory of the filter in the class-method the memory (a vector) is reset to zero so this the code doesn't work. Can someone please reveal how I ensure that the filter memory update is retained for the next sample iteration? Thanks Tom
From: us on 29 Apr 2010 17:38 "Tom " <stoltzo(a)gmail.com> wrote in message <hrctqo$fj8$1(a)fred.mathworks.com>... > Hi > > I'm trying to learn Matlab OOP and started out implementing a simple digital-filter. > > Now, I have a class that does the filtering and the memory of the filter is contained within the class and is updated whenever a new filter output is computed. The filter object > is called in a for-loop for every sample iteration. I notice that even though I do update the memory of the filter in the class-method the memory (a vector) is reset to zero so this > the code doesn't work. > > Can someone please reveal how I ensure that the filter memory update is retained for the next sample iteration? > > Thanks > Tom a - very - small amout of code is needed... us
From: Tom on 29 Apr 2010 17:44 "us " <us(a)neurol.unizh.ch> wrote in message <hrcu7r$bsi$1(a)fred.mathworks.com>... > "Tom " <stoltzo(a)gmail.com> wrote in message <hrctqo$fj8$1(a)fred.mathworks.com>... > > Hi > > > > I'm trying to learn Matlab OOP and started out implementing a simple digital-filter. > > > > Now, I have a class that does the filtering and the memory of the filter is contained within the class and is updated whenever a new filter output is computed. The filter object > > is called in a for-loop for every sample iteration. I notice that even though I do update the memory of the filter in the class-method the memory (a vector) is reset to zero so this > > the code doesn't work. > > > > Can someone please reveal how I ensure that the filter memory update is retained for the next sample iteration? > > > > Thanks > > Tom > > a - very - small amout of code is needed... > > us Hi us Thanks for your quick reply - here is my code (implementing a digital state variable filter) Hope you can see what I'm doing wrong.... classdef StateVariableFilterClass properties f = []; fs = []; q = []; input = []; output = []; mem; y_lp = []; y_hp = []; y_bp = []; Norder = []; tmp = []; xx = 1; end methods % Class constructor function obj = StateVariableFilterClass(Norder,f,Q,fs) obj.f = 2*sin(pi*f/fs); obj.q = 1/Q; obj.mem = zeros(Norder,2); obj.Norder = Norder; end % State Variable Filter function y = filterState(obj,x,counter) % Iterate over biquad sections for jj=1:obj.Norder % Lowpass filter output obj.mem(jj,2) = obj.mem(jj,1)*obj.f+obj.mem(jj,2); obj.y_lp = obj.mem(jj,2); % Highpass filter output obj.y_hp = x-obj.mem(jj,2)-obj.mem(jj,1)*obj.q; % Bandpass filter output obj.y_bp = obj.y_hp*obj.f+obj.mem(jj,1); obj.mem(jj,1) = obj.y_bp; x = obj.y_hp; end y = x; end end end The class is instantiated and used in the following way: % Filters filt = StateVariableFilterClass(Norder,fc,q,fs); for ii=1:length(x) y(ii) = filt.filterState(x(ii),ii); end
From: Tom on 29 Apr 2010 18:05 Maybe I just need to use "persistent"? "us " <us(a)neurol.unizh.ch> wrote in message <hrcu7r$bsi$1(a)fred.mathworks.com>... > "Tom " <stoltzo(a)gmail.com> wrote in message <hrctqo$fj8$1(a)fred.mathworks.com>... > > Hi > > > > I'm trying to learn Matlab OOP and started out implementing a simple digital-filter. > > > > Now, I have a class that does the filtering and the memory of the filter is contained within the class and is updated whenever a new filter output is computed. The filter object > > is called in a for-loop for every sample iteration. I notice that even though I do update the memory of the filter in the class-method the memory (a vector) is reset to zero so this > > the code doesn't work. > > > > Can someone please reveal how I ensure that the filter memory update is retained for the next sample iteration? > > > > Thanks > > Tom > > a - very - small amout of code is needed... > > us
From: David Young on 29 Apr 2010 18:29
You need to be aware of the difference between a value class and a handle class. Objects belonging to normal classes (i.e. value classes) obey the same rules as other Matlab objects: that is, they are effectively copied when assigned or passed to a function. This excellent rule means that you never accidentally update the state of an object that you didn't mean to update, and fits in the general philosophy of Matlab as a language. However, if you are used to the behaviour of other object-oriented languages, it can come as a surprise. What this means is that when you update the properties of an object, you have to assign the updated object to a variable. So your filterState method needs to return obj as a result (say the first), and you need to call it using [filt, y(ii)] = filt.filterState(x(ii),ii); or, perhaps more neatly, store y in the object and have another method that just gets the value of y. There's another way to deal with this: make your class a handle class. This makes its behaviour more like that of Java classes - changes to the object will persist as you expect without an assignment. My own view is that you shouldn't do this unless there is a very convincing reason (e.g. for GUIs or complex data structures that need to be updated), as it breaks the normal Matlab model, but I am sure that many programmers use handle classes for convenience. |