Bug in Persistent variables? Simulink M-function

Question

John 2018-1-30

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/379876-bug-in-persistent-variables-simulink-m-function

编辑： John 2022-9-8

I am unable to initialize a Persistent variable. Here's a snippet of code in a Simulink M-function which does not actually set 'a' no matter what I do:

persistent a
if isempty(a)
    a = 1;
end

Either printing out 'a' or evaluating 'a' gives

a=[]

...NOT a=1 as it should.

It doesn't matter what the variable name is, or what I try to set it to, or how many variables there are. This bug happens with two different sets of Persistent Variables.

However, oddly, later on in the SAME M-function, I do another persistent declaration using the exact same method and there initialization DOES work.

I tried this in both R2016a and 2017b

What is going on?

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John Pryce 2019-5-25

编辑：John Pryce 2019-5-25

在 MATLAB Online 中打开

This bug exists. My "main program" is a function with input arguments and the crash occurs some levels down inside an a function that is called by fzero(), so isolating the error was tricky.

I confirm what John says "It doesn't matter what the variable name is, or what I try to set it to". Further:

If it crashes and I immediately re-run with the same inputs it always goes on crashing.
If I then run with different inputs it may work correctly.
If after doing something else I re-run with the original inputs that caused the crash it may work correctly.
Changing the layout of code, or changing nearby code, may change it between working and crashing. E.g. I originally had if nargin>4, it = it0; end below, and changing it to what's written cured the problem – so I thought! But it reappeared with other inputs.
Stepping through in the debugger, with argument it0 absent so nargin>4 is false, I found that though it skipped the it = it0; statement, it nonetheless changed the value of it to an unpredictable value (e.g. 165) while going from if to end.
Once past this perilous initialisation, it seems it behaves as it should do.

The piece of code in question is

function [x,xp,it1] = searchAndInterp(tt,xx,xxp,t,it0)
% some comments
persistent it %Seems most economical way to implement this
itbefore=it %!!!DEBUGGING
if nargin>4
  it = it0;
end
if isempty(it), it=1; end
itafter=it %!!!DEBUGGING
%...

That evidence should help locate where in the compiler the error is located. I have a file of relevant command window output but can't see how to attach it.

I'm using

-----------------------------------------------------------------------------------------------------
MATLAB Version: 9.5.0.944444 (R2018b)
Operating System: Mac OS X  Version: 10.14.4 Build: 18E226 
Java Version: Java 1.8.0_152-b16 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
-----------------------------------------------------------------------------------------------------

amin ya 2020-1-29

I can confirm that this is a bug. I am on Matlab 2019b Update 3

John 2020-1-29

Amin, it's sad to see this major bug still exists after 2 years.

There has been no reply from Mathworks -- Mathworks, are there any plans to resolve this bug?

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Answer 1

John 2018-1-31

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/379876-bug-in-persistent-variables-simulink-m-function#answer_302618

在 MATLAB Online 中打开

Okay, now I'm certain it's a BUG. The code WILL -- or will NOT -- properly initialize a variable depending on where it is located in the function, and what is around it. Even if the init is unrelated to the surrounding code.

Eg

<code 1>
persistent a
if isempty(a)
    a = 1;
end
<code 2>
<code 3>

might NOT work, but

 <code 1>
 <code 2>
 persistent a
 if isempty(a)
     a = 1;
 end
 <code 3>

WILL work.

The surrounding Code 1, 2, 3 are completely unrelated to the variable 'a'. Likewise, an init that DID work at a certain section will no longer work if it is moved.

Unfortunately, I cannot make the code work in places that I need it to, as it seems to be random why it works or is buggy.

Mathworks -- any response?