How to clear a persistent variable in a MATLAB method

Question

I have a MATLAB class that contains a method that employs a persistent variable. When certain conditions are met I need to clear the persistent variable without clearing the object to which the method belongs. I've been able to do this, but only by employing clear functions which has excessively broad scope for my purposes.

The classdef .m file for this problem:

classdef testMe

    properties
        keepMe
    end

    methods
        function obj = hasPersistent(obj)
            persistent foo

            if isempty(foo)
                foo = 1;
                disp(['set foo: ' num2str(foo)]);
                return
            end
            foo = foo + 1;
            disp(['increment foo: ' num2str(foo)]);

        end

        function obj = resetFoo(obj)
            %%%%%%%%%%%%%%%%%%%%%%%%%
            % this is unacceptably broad
            clear functions
            %%%%%%%%%%%%%%%%%%%%%%%%%
            obj = obj.hasPersistent;
        end
    end

end

A script that employs this class:

test = testMe();
test.keepMe = 'Don''t clear me bro';

test = test.hasPersistent;
test = test.hasPersistent;
test = test.hasPersistent;

%% Need to clear the persistent variable foo without clearing test.keepMe

test = test.resetFoo;

%%
test = test.hasPersistent;
test

The output from this is:

>> testFooClear
set foo: 1
increment foo: 2
increment foo: 3
increment foo: 4
set foo: 1

test = 

  testMe

  Properties:
    keepMe: 'Don't clear me bro'

  Methods

which is the desired output. The problem is that the line clear functions in the classdef file clears all functions in memory. I need a way to clear with a much smaller scope. For example, if hasPersistent' was a function instead of a method, the appropriately scoped clear statement would beclear hasPersistent`.

I know that clear obj.hasPersistent and clear testMe.hasPersistent both fail to clear the persistent variable. clear obj is similarly a bad idea.

are you trying to emulate "static properties" in the C++/Java sense (properties of the class not class instances)? — Amro
I don't think so, although my C++/Java is very weak, so it's a possibility that I am, but that's not my intention. The reasons I don't think so are that the persistent variable foo is not constant across instances of the class. I need it to be local to an instance of the object and it's only needed inside the function hasPersistent. — Sevenless
to understand what I mean, create another instance of the class test2 = testMe(); and call test2.hasPersistent... Both instances share the same persistent variable foo. If you dont want such behavior, why not use a regular property (like keepMe property) and make it private? — Amro
Thank you. I didn't realize that was the behavior with multiple testMe objects in the workspace. Your suggestion to use private properties is totally correct and should have been obvious to me. That probably means it's time to stop for the night and go to bed. If you post an answer of "use a private property instead" I'll select and upvote. Thanks! — Sevenless

Amro Amro · Accepted Answer · 2012-05-28T00:20:18

Following the discussion in the comments, I think you want to use make foo a private property, accompanied with appropriate increment/reset public functions.

How to clear a persistent variable in a MATLAB method

2 Answers