When a global variable is justified

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Valtar 2023-7-4

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评论： Jan Kappen 2024-9-4

I use few global vars in my app. The Code Analyzer suggests to get rid of them, but this would involve--in my view--more complicated coding. I'd like to learn the best programming practice in this regard and maybe a fix is simpler than I think.

I use global vars for two main reasons.

First, I need a debug flag in all my functions. This is to spit out debug info if app is run with a debug flag (on the command line). To fix this, I would need to intorduce an additional input variable in all definitions and calls. To me, this is more ugly compared to a line of global definition inside each function. But in general this is easy to fix (introducing addional input argument).

Second, this is more difficult, I think. One function logit() writes log info in both GUI text box and in the file on disk. Thus, it needs FID of the file, and a text box handler, plus another flag which tells if the app runs in batch mode (w/o GUI) or not. For the latter, probably there is a way to find out batch or not via a GUI ask, as well as it shall be possible to find the text box handler via a GUI ask, i.e. w/o explicit handler in a varable. So, I guess it is possible to fix my logit(), where it would have more logic to find out: batch or not, find the needed handler for the text box, and have the file FID in a persistent local variable, maybe (if other funcs do not write to this file).

Thanks

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Valtar 2023-7-4

@Stephen23, I think I have confused all by using "app" as equivalent to "software" ;) My software (eventually a compiled Matlab porgram) is runing in batch mode (w/o GUI) as well as in GUI mode. And if i can work on logit() internal logic to be able to find which mode I'm runing and then query for the text box handle, the issue with the debug flag stays. I struggle to understand the need to refactor it as an additonal input variable :) it's very benign. if something wrong happens to it, nobody will get hurt)

Stephen23 2023-7-4

" if something wrong happens to it, nobody will get hurt"

The important question is: if something goes wrong with the global, can it in any way (silently) affect the data being processed? If yes, then using globals is a risk. Otherwise... you could consider the global benign.

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Answer 1

Matt J 2023-7-4

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编辑：Matt J 2023-7-4

To fix this, I would need to intorduce an additional input variable in all definitions and calls.

No, you could add the debug flag as an app property. Then, it would get passed around to all the callbacks and class methods as part of the app object that gets passed to them automatically.

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Valtar 2023-7-4

Well, I have all the callbacks as nested funcs :) My app is rather simple. I can have them all as nested. And if those need to do more calc, I pass the data as explicit arguments to other, lower-level funcs. But the GUI sort of stays in one main func. And I have one function logit() who needs the GUI handlers (some of the warnings it will show up as a pop-up if the app is in GUI mode; otherwise just spit to the console). And, again, this logit() I call a lot from many places, it is like a fprintf() replacement, a wrapper which decides what to do depending on the exec mode (CLI aka no GUI vs GUI). So, naturally, I don't want to give an additional argument to logit() in which I could pass a structure with all the needed (e.g. debug flag, GUI flag, maybe a text box handler for writing in GUI).

My logit() has one input param--the message string. All the rest is taken from global vars ;)

I probably can get away figuring out inside logit() if I run with GUI or not, and if yes, to find the text box handle using its fixed object name. This will remove the need for those global vars. logit() does not actually need the debug flag (since it is too simple).

For all the other funcs, to convert the debug flag from a global to a normal, passed-as-input var, I think i would need to change the defnition and calls... There is no other way, since i don't really use OOP.

Valtar 2023-7-4

Only the GUI callbacks are nested. All the processing functions are not. And I need logit() all other the software; eveywhere instead of fprintf().

I have just realized that if I eliminate GUI related globals in logit(), and instead will search for the needed handle with findobj(gcf,....), then it could be slower than the global var... I can propably store the needed handle in guidata, and in logit() just check if guidata exists (meaning that the SW is in GUI mode), and if yes, just grab the needed for the output handle.

Matt J 2023-7-4

编辑：Matt J 2023-7-4

在 MATLAB Online 中打开

You can make wrapper for logit which will use the externally scoped variables. You can use the wrapper within your GUI as a local, specialized version of the general function.

function guiMain
   
    debugFlag=true;
    ...
    function logitLocal(varargin) %wrapper
       
        if debugFlag
            logit(something)
        else
            logit(something_else)
        end
        
    end
end

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Answer 2

Image Analyst 2023-7-4

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@Valtar by not using app designer because you think it gives you better control, well, I take issue with that. I'd like to see some example of that. But by doing all the tedious things of setting up callbacks, etc., I think you're actually creating more work for yourself, or you just end up with a simple, more basic app than you could otherwise if you'd have used App Designer.

See the FAQ on globals: https://matlab.fandom.com/wiki/FAQ#Are_global_variables_bad?

Globals create variables in the global workspace, which can stay there until you shutdown MATLAB or call "clear globals". This can cause problems if you end up using global variables with old, no longer appropriate, values in your new runs. With app designer the globals are all internal and don't live beyond the running of your app.

However you get the effect of globals with app designer by creating and using a property of the app object. Any function in there can automatically see app.varName (or whatever you call it) without you having to explicitly say "global varName" inside every function that needs to use that variable.

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Answer 3

Jan Kappen 2024-9-3

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What about a global logger class? You set select the log-levels at creation time.

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Rik 2024-9-4

Ah, I thought you meant people could use a global to store an instance of a logger class, not that an instance of a logger class could replace the use of a global.

Jan Kappen 2024-9-4

Ah understood. Sorry that was misleading. I was thinking of a singelton logger class, similar to logging — Logging facility for Python — Python 3.12.5 documentation, or Log4Matlab - File Exchange - MATLAB Central (mathworks.com), or mathworks/advanced-logger: Configurable and extensible logging for MATLAB applications (github.com)

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