'Global' Variables in a guide GUI

I have a GUI guide function, with multiple buttons. I want each button to change a 'global' variable to a different value. How do I go about declaring the variable correctly, and assigning the values correctly?

Answer 1

[Jan]

Using global variables is a bad idea in general. They impede the debugging and the increase the level of complexity. You find many corresponding discussions here in the forum, but this problem concens other programming languages also. Ask an internet search engine for details.

But of course it is possible:

• Define the global variables in each function, which needs to access them:

function y = myComputations(x)
global gParameter
y = gParameter * x;
end

• This is done inside the callbacks also:

function Button1_Callback(hObject, EventData, handles)
global gParameter
gParameter = 1;
end

function Button2_Callback(hObject, EventData, handles)
global gParameter
gParameter = 2;
end

• If you need the variable in so base workspace, define them as global here also.

If you need the variable in the base workspace only, e.g. as input for a Simulink method, do not define it as global but using:

function Button1_Callback(hObject, EventData, handles)
assignin('base', 'gParameter' 1);
end

This is not as evil as global variables, but you cannot trace the source of the current value of these variables reliably also.

If 'global' means, that the variable needs to be shared between callbacks of a figure only, use either set/getappdata, guidata or the 'UserData' of the corrsponding GUI element. This allows e.g. to run two instances of the same GUI without conflicts. Please ask, if you need further explanations for this.

[Marcus Blackburn]

I don't need the variable to be global, I just need it to go between callbacks of the different pushbuttons. Can you give a little more detail on how to do that?

[Stephen23]

@Marcus Blackburn: MATLAB has documentation which is free to read online:

https://www.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html

https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

Personally I like to use nested functions to pass data between callbacks: neat, intuitive, easy. You will also find one thousand threads on this forum discussing how to pass data between callbacks. Your favorite internet search engine can search this forum and the MATLAB documentation.

[sonR]

It was stated in another forum (not sure where) and in the matlab help files that using nested callback functions to share variable data was not recommended. Can anyone elaborate on why? It seems that using the set/getappdata, guidata format makes the variables long and, to me, adds a little more confusion.

[Stephen23]

@sonny revell: that advice applies to GUIs written using GUIDE, but it does not apply to GUIs written from scratch. The advice can be found here in the MATLAB documentation:

https://www.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html

The documentation does not explain why GUIDE is unable to use nested functions.

Personally I always code my own GUIs from scratch, and I prefer to use a combination of local and nested functions within my GUIs: they are simpler, more intuitive, and much easier to use than any other ways of transferring data between callbacks (and the main calling function).

You can find an example here:

https://www.mathworks.com/matlabcentral/fileexchange/48930-interactive-regular-expression-tool

So far I have not found any reason to recommend GUIDE to anyone.

[Gopinath Karuppannan]

How can we alter the color of each string of gparameter? especially it should be different colors

[Rik]

Do you mean in the editor? Or do you mean the values of that variable?

It also sounds like you missed the advice to avoid global variables.