How can I read global variables?

Dear everyone, I used for the first time a global variable. This is my script:

    emotion = ('aa_ANGER.png');
    global answer; % define global variable

    % Create push button with different emotions
    hButton1 = uicontrol('Units','normalized','Position', UL,'Style','pushbutton','String','HAPPINESS','FontSize',16,'callback', @fCallback_F);

   w = waitforbuttonpress;       
    if w == 0
        RT = toc(tOn);
        res = emotion(4:6);
        nPt = emotion(1:2);
        a = answer(1:3);     
            if strcmp(a,res)             
                response = 1;
            else
                response = 0;  
            end
       close all         
    end

but in the first loop an error occurred: Index exceeds matrix dimensions Referring to a = answer(1:3); If I run the script again (without clear anything) it works. I don't know WHY!!! It something about the global variable definition but I don't know what Do i have to do.

Any help will be appreciated!

Thank you

------------------------------------------------------------ where fCallback_f is define as function elsewhere in this way:

function fCallback_F(src,evt)
    global answer 
    answer = get(src,'String');  
end

Answer 1

[David Young]

The problem is not connected with your use of a global variable. I think there are two possibilities:

• If you click in the figure window, but not on the button, waitforbuttonpress will return, but nothing will have been assigned to answer . In this case, answer will be empty and you will see the error.
• I think it is possible that waitforbuttonpress can return before the callback function has run. The following code will then get an empty value for answer and you will see the error. The next time through, answer will have received a value.

Your description and my tests indicate that it's the second of these; more time with the documentation might support the hypothesis.

Either way the solution is this. Replace the call to waitforbuttonpress with

uiwait;

Remove if w == 0 and the corresponding end .

Insert into the callback function as the final statement before end

uiresume;

Then the progress of the main script is tied in properly with the execution of the callback function.

[Fed]

wowow! It's working perfectly now! Thank you so much!

[David Young]

Glad to know it's working!

[mat]

Thanks! This solved my problem too! But what if I want to use a button (stopbutton for example) in a continuous loop where uiwait is no option?

[David Young]

Then you'd need to use the button to set a variable and test its value each time round the loop. The variable could be global, but as Adam says in his answer, that's usually a bad solution, and it would be better to share it with the callback function by using nested functions.

Answer 2

[Adam]

To use a global variable in a function you have to call

global answer;

within that function if the global variable was created in some other function. That will create the variable within the scope of the current function allowing you to use it there.

Using global variables is almost always a bad idea and I can think of no circumstance in which it should ever be considered a good solution, but that is your decision. The above should allow you to use your global variable in any scope if you always add that line in a function in which you wish to use it.

[Image Analyst]

FAQ: http://matlab.wikia.com/wiki/FAQ#Are_global_variables_bad.3F

[Fed]

Dear Adam, thanks for the quick reply. I know that using global variables can be a problem but I don't know other ways to save the output of callBack function I used to check which button has been pressed.

Anyway, as you can see I define global answer before the uicontrol function AND within the callback function.

After that I just want to check if that variable is equal to the correct response, therefore I need to compare two strings. I don't understand where I should call it again. Thanks

[Image Analyst]

FAQ: http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

[Fed]

thanks for the links, but Just to get where the mistake is in my script. Where I should call the global answer? What I am missing?

[Adam]

Ah, ok, I didn't fully interpret all the code.

The error message suggests the problem is simply that whatever is in answer does not have 3 elements rather than that it can't access the global variable correctly.

This may be because the global variable is not behaving correctly (it is very easy to have them not behave correctly) or simply because there simply isn't a 3rd element in the variable at the point it is queried.

If I understand your code correctly you are storing the string of a pushbutton in your global variable.

Yet you have the handle for the pushbutton as hButton1 so you can simply query

get( hButton1, 'String' )

within your main code body as far as I can see, though I am not too familiar with the flow of execution where waitforbuttonpress is used.

The string on that pushbutton is not going to change though unless you are explcitly changing it via code, in which case you can access it (like above) in the same way you set it.

[Fed]

Actually I can't write in the callback function get( hButton1, 'String' ) because I have 6 other hButton I defined. And I am not sure it is a problem of length of string, since If I run the script (when the global variable is already inside the workspace) again it works! It should be a matter of access in the working space of the global variable. Probably, it is simply not behaving correctly as you suggested. Thanks in any case

[Adam]

Yes, I see the problem if the same callback is assigned to all pushbuttons.

This would be a lot easier if done via GUIDE, but otherwise David Young's solution will likely allow it to carry on working with global variables.

[Fed]

Next time I ll go with different solutions rather then global variables :)

Answer 3

[Raja Awais Liaqait]

Dear All,

I want help in the following code.

global min_realvar ;

global max_realvar ;

Firstly, I want to get the value of these variables and secondly i want to write them in such away that I can give the values of these variables as an input.