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clearing a persistent variable makes it no longer persistent

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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.
  1 Comment
Abhishek Kumar Singh
Abhishek Kumar Singh on 22 May 2024
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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Accepted Answer

埃博拉酱
埃博拉酱 on 22 May 2024
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.

More Answers (1)

CM
CM on 22 May 2024
Thanks 埃博拉酱 and Abhishek. Deal looks like what I was after.

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