clearing a persistent variable makes it no longer persistent

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CM 2024-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/2121306-clearing-a-persistent-variable-makes-it-no-longer-persistent

回答： CM 2024-5-22

This is something that keeps catching me out. I do not think it is explicitly documented anywhere, though it could be implied in the matlab instructions to clear persistent variables by clearing the function.

If you clear a persistent variable using clear variablename from within the function, it ceases to be persistent.

Something like this would be handy to have as there are situations where I want to clear some, but not all persistent variables in a function. Clear allows you to do it all in one line. The alternative is to set them all to [], which is a bit more verbose.

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Abhishek Kumar Singh 2024-5-22

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Hi @CM, I think it is mentioned in documentation of persistent that clearing the function that declares the variable clears the persistent variable. You can refer to the Tips section of this page: https://www.mathworks.com/help/matlab/ref/persistent.html#:~:text=To%20clear%20a%20persistent%20variable%2C%20use%20clear%20with%20the%20name%20of%20the%20function%20that%20declares%20the%20variable.%20For%20example%2C%20clear%20myFun

When the clear command is used within a function to clear a variable, it removes the variable from the workspace entirely. This means that not only does it cease to be persistent, but it also gets cleared from memory.:

function foo()
    persistent counter;
    counter = 43;
    
    % Display
    disp(['Counter value: ' num2str(counter)]);
    
    disp("Before clearing counter..")
    disp(whos('counter'))
    
    % Clear the persistent variable (ceases to be exists)
    clear counter; 
    disp("After clearing counter..");
    
    disp(whos('counter'));
end
foo()
Counter value: 43
Before clearing counter..
          name: 'counter'
          size: [1 1]
         bytes: 8
         class: 'double'
        global: 0
        sparse: 0
       complex: 0
       nesting: [1x1 struct]
    persistent: 1

After clearing counter..

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Answer 1

埃博拉酱 2024-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/2121306-clearing-a-persistent-variable-makes-it-no-longer-persistent#answer_1461341

You can use deal to set multiple variables to the same value at once (such as an empty value, in your case). Using clear in a function is very unusual and is not the right choice in most cases.