Global variables inside a Matlab GUI

Dear all,

I have a Matlab GUI with several callback functions. I need to pass a variable of type struct from one callback function to another.

I therefore inizialized a global variable "config" in the very first function of the .m file.

Then, in the callback functions I implemented the following code:

    % Callback #1
    function clbk1_Callback(hObject, eventdata, handles)
    global config
    config.clbk1=get(hObject,'String');

    % Callback #2
    function clbk2_Callback(hObject, eventdata, handles)
    global config
    config.clbk2=get(hObject,'String');

Etc.

The problem is that "config" seems to be initialized in every callback function. That is to say:

1) I call the 1st callback function, "config" is:

      config.clbk1='mickey'

2) After the 1st, I call the 2nd callback function, "config" should be

      config.clbk1='mickey'
      config.clbk2='mouse'

Instead, it's only

      config.clbk2='mouse'

What I'm missing?

Many thanks to anyone could help, Claudio

P.s.: Matlab version 2010b, Windows 8.1, both 64 bit

Answer 1

[Image Analyst]

You probably have a

clear global;

or

clear global config;

or

config = [];

somewhere in the code. I don't see any other way that it could be cleared, except "clear all". You don't have a clear all anywhere do you? Search your code for the word "clear" and tell me what you find.

[Claudio]

No "clear" has been found in the code :(

[Image Analyst]

What Julia said about "global config" initializing the variable is not true. If config already has a value, then as soon as you execute that global line, you will now see the variable in your function and the config value will have all value(s) that it had prior to that point. It will not be wiped clean.

Make a small script demonstrating the problem and I'll look at it. For example, put this code in test3.m and run it.

function test3
  clc;
  global g
  g = pi; % Assign a value.
  % Call f1
  fprintf('Prior to calling f1, g = %f\n', g);
  f1
  fprintf('After calling f1, g = %f\n', g);
  
function f1
  global g;
  fprintf('In f1, g = %f\n', g);
  g = 42.5;
  fprintf('In f1, g = %f\n', g);

Or maybe copy your program and start hacking out unrelated code that has nothing to do with config and see if it ever starts working. I still think you're either not assigning it in the first place, assigning it to null, or calling clear global. You can attach your code if you want but make sure it can run, i.e. attach any data files it might need, etc.

Answer 2

[Julia]

I am not sure if you really need a global variable to do what you want. You can simply define a new variable in the opening function:

handles.test=hObject;
guidata(hObject, handles); % updates handles structure

and configure it in the callbacks:

handles.test = ...
guidata(hObject, handles); % updates handles structure

If you have to use a global, I still think you have to update the handles structure after assigning a value.

[Claudio]

Thanks for the very fast reply. Actually, handles are correctly updated. As a matter of fact, I run the following test:

      % Callback #1
      function clbk1_Callback(hObject, eventdata, handles)

      % Callback #2
      function clbk2_Callback(hObject, eventdata, handles)
      config.clbk1=get(handles.clbk1,'String')
      config.clbk2=get(hObject,'String');

Calling the 2nd callback function after the 1st the result is correctly:

      config.clbk1='mickey'
      config.clbk2='mouse'

Anyway, I hope to find a better way to solve the problem because I have a large number of callback functions and recalling every time all the handles is not efficient.

[Julia]

Your test probably works because you don't do anything in the first callback. In the second you use the handles structure to get the value from the 1st callback. (that should always work)

I do not often use global variables so I don't know how they are initialized correctly. Could it be that you initialize config each time you write

global config

?

[Claudio]

Quote: "Your test probably works because you don't do anything in the first callback. In the second you use the handles structure to get the value from the 1st callback. (that should always work)"

It happens also if I make something inside the 1st callback function :(

Quote: "I do not often use global variables so I don't know how they are initialized correctly. Could it be that you initialize config each time you write..."

It seems so, but I should not be, according to the Matlab help...

Answer 3

[Julia]

Found this in the Matlab help:

To use a global within a callback, declare the global, use it, then clear the global link from the workspace. This avoids declaring the global after it has been referenced. For example,

cbstr = sprintf('%s, %s, %s, %s, %s', ...
   'global MY_GLOBAL',                ...
   'MY_GLOBAL = 100',                 ...
   'disp(MY_GLOBAL)',                 ...
   'MY_GLOBAL = MY_GLOBAL+1',         ...
   'clear MY_GLOBAL');

uicontrol('style', 'pushbutton', 'CallBack', cbstr, ...
   'string', 'count')

[Claudio]

I find this help, but it's not very clear to me. I'll try to study it better. Many thanks