MATLAB: Does the method doesn’t recognise the variable U that I defined as global

callbackfunctionvariable

I wrote a method in the designer app, so that I can call the functions that I need without writting them every time for each button. But I get a little error while executing the third function ( in the for =1:numel(U)) line:
methods (Access = public)
        
        function implement = loadingdata(app)  % first function for loading data
                load data.mat Table1
                load data.mat Table2   
 ..      end
        
        function input = timeandfrequency(app) % 2nd function for the input windows
             answer = questdlg('Is there a time stamp?',...
                'Please give the time stamp',...
                'yes','no','no');
                if (answer == "yes") 
                prompt ={' type the start time in [HH:MM:SS] ','Type the endtime in [HH::MM:SS]'};
                dlgtitle = 'Time in HH:MM:SS';
                dims =[1 40];
                definput = {'13:20:00','13:50:00'}; 
                time_answer = inputdlg(prompt,dlgtitle,dims,definput);
                st = time_answer{1,:};
                et = time_answer{2,:};
                end
                answer = questdlg(' Which Frequency would you like to plot ?', ...
                '', ...
                'Frequency 1','Frequency 2','Frequency 3','Frequency 3');
        end
        
        function plotting = diagrams(app) % third function for plotting the data
        for k = 1:numel(U)
        
            Var = Variableunits.Variable;
            Num = size(Var,1);
            X = T.Variable==U(k);
            Uhrzeit = round(seconds(T.Time(X)/1000)); %Umwandlung der Uhrzeit zum Format HH:MM:SS
            Uhrzeit.Format='hh:mm:ss';
            Uhrzeit = sort( Uhrzeit,'ascend');
            Wert = T.Value(X);
            if (answer == "yes") % Wenn ein Zeitstempel vorhanden ist
                ind = find(Uhrzeit >= st & Uhrzeit <= et);%Die Uhrzeit zwischen Anfangsuhrzeit und Endzeit einschränken
                Uhrzeit = Uhrzeit(ind);
                Wert = Wert(ind);
            end
            switch answer
                    case 'Frequency  1'
                    ....
                    case 'Frequency 2'            
                    ....
                    case 'Frequency 3' 
                    ....
            end %switch
        end %for
        end %function 
        end %method
After executing the first button as an example, it calls the 3 function and then I get the error: Unrecognised function or variable U in the method, despite defining it as a global variable below:
      % Button pushed function: Variable1Button
        function H2_TEMPERATURE_Pt01CelsiusButtonPushed(app, event)    
             loadingdata(app)  % first function
            timeandfrequency(app)% second function
            global  Us
            global U; 
            Us = ["Variable1"];
            U = categorical(Us); 
            %for loop
            diagrams(app) % third function
Why does the function cant recognise the variable U? How can the method recognises it from the button function?

Best Answer

Don't use Global variables, especially in AppDesigner!
Instead, declare variables that are shared between app functions as private or public properties.
Private properties are for variables shared interally within the app. Public properties are for variables shared externally to the mlapp file.
See Matlab's documentation on app properties
https://www.mathworks.com/help/matlab/creating_guis/share-data-across-callbacks-in-app-designer.html
Step 1: Declare the variable as a private|public property.
Add the property using the green "+" in the image below (in code view, within AppDesigner)
A private | public declaration will be automatically added to the code. Change the name of "Property" to the variable name.
It's also a good idea to give the variable a default value. I named the variable "myDescriptiveVariableName" and set it as NaN.
Step 2: Use the variable anywhere in the app
To use this variable, simply use app.myDescriptiveVariableName. Any function can update the value stored in that property.
For public variables accessed outside of the app, you just need the app handle and then you can access the variable in the same way as shown above.
Here are some reviews about how to get the app handle outside of the app
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