Best method to clear persistent variables

Hello all,

Recently I came to know that it is not a good idea to use

clear all;

So, I started using

clear variables

Now, I am wondering what I should do to clear only the persistent variables that I use inside my functions. It is absolutely critical that I clear the persistent states of my adaptive algorithms. Is there a way I can clear only persistent variables so that I don't have to re-run the entire script to generate the same data only to be used in a differently initialized algorithm?

Currently, I do this by calling the particular function with a

myfunction('init', init_method)

to clear the persistent variables or re-initialize them. Is it the best way? Your comments will help,

Thank you,

Answer 1

[Fangjun Jiang]

When you use Vars=whos, it returns flag indicating global variable and persistent variable. You can use that to pick up persistent variables to clear them. The example below uses 'global' because it's easy for me. You can try it replacing [Vars.global] with [Vars.persistent].

Vars=whos;
PersistentVars=Vars([Vars.global]);
PersistentVarNames={PersistentVars.name};
clear(PersistentVarNames{:});

[Gurudatha Pai]

I think that would work for me. I will try it soon.

Thank you

[mcsefl]

Should line 2 be corrected to read "PersistentVars=Vars([Vars.persistent]);"?

[Toby Hijzen]

I think that is what is meant. However this doesn't work for me. Also when clearing the global variables it is important that they are known within the current scope.

[Noam Greenboim]

doesn't work.

[Kenneth Leung]

Persistent variables are only visible within the function itself, so I like to write an 'if' statement at the beginning of the called function

input.flush = true;  

if input.flush
  vars = whos;
  vars = vars([vars.persistent]);
  varName = {vars.name};
  clear(varName{:});

    output = [];
    return

end

This way I have more reliably been able to 'flush' the persistent variables away whenever I want to by setting flush to true and calling my function again.

[Walter Roberson]

Yes but this transfers the problem to be how to set input.flush from outside of the function.

When persistent variables are used to represent ordinary state (e.g. usage statistics,) that are not big, then postponing the clear until the next call to the function can be ok. But persistent is also used for cache and when you are finished with the cache or it has gotten too big then you would like to clear it without calling the function. Even if the memory directly used is small, it could be handles to large objects and you might need to clear the reference to release the objects.

Answer 2

[Walter Roberson]

I don't know about the "best" way, but you can clear all the persistent variables in a particular function by asking to clear the function itself; e.g.,

clear myfunction

This will probably have the side effect of removing the stored JIT'd copy of it, causing it to be reparsed the next time it is invoked.

[Daniel]

This worked perfectly, thanks!

[Leo Simon]

I spent many many hours today trying to track down a bug that was clearly related to a failure to clear a persistent variable. One of the most impenetrable bugs I've seen in a long time. Clearing the parent program whose child caused the problem fixed the problem. What an incredibly easy fix to a really nasty problem. Thanks very much!

[Puneet Kumar]

Not working for me

[Walter Roberson]

Puneet Kumar, please describe the difficulty in more detail.

[Gavriel Aminov]

@Leo Simon,

Leo, you are right, that the problem must be solved at the parent of the calling function, I thank you.

I just want to stress the point, that I got the desirable result of clearing the persistent variable by the next way of:

clearing the function, the persistent variable is declared in, at the parent function of the calling function.

That is:

function FunParent()
clear FunWithPersistent
[a,b,c]=FunCalling();
end

------------------------------------

function FunCalling()
[x,y]=FunWithPersistent();
end

------------------------------------

function FunWithPersistent()
persistent OurHero
...
end

I'm sorry to say, that it is indeed seems to be a bug of MATLAB...

[Dimitrios Patikas]

Thank you

Answer 3

[John D'Errico]

You can also use

clear persistent

Which will clear all persistent variables.