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How do global variables differ from regular (local) variables?

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How do global variables differ from regular (local) variables?

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Each function in MATLAB contains a set of variables specific to that function. Even in the same .m file, you don’t have (direct) access to variables created in other functions within the file. Global variables give you the ability to create/change a variable in one function and have that updated variable accessible elsewhere. This post will discuss two methods for handling (no pun intended) global variables, one of which is perfectly integrated into Graphical User Interfaces (GUIs).

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  1. How do global variables differ from regular (local) variables? Each function in MATLAB contains a set of variables specific to that function. Even in the same .m file, you don’t have (direct) access to variables created in other functions within the file. Global variables give you the ability to create/change a variable in one function and have that updated variable accessible elsewhere. This post will discuss two methods for handling (no pun intended) global variables, one of which is perfectly integrated into Graphical User Interfaces (GUIs). METHOD 1: global VARIABLE The first (non-GUI) way to create a global variable is to use the function ‘global’. Create the global variables X, Y, and Z with the command: global XYZ The ‘global’ function needs to be called in each separate function (usually in the beginning) where the variables will be called. Stylistically, the variable names are usually longer names and all in CAPS to indicate global variables within the functions. The documented example in the MATLAB helps shows this pretty well: function tic global TICTOC %define/incorporate global variable at start of function TICTOC = clock; function t = toc global TICTOC %accesses variable TICTOC (or creates it if TICTOC is undefined) if nargout < 1 elapsed_time = etime(clock, TICTOC) else t = etime(clock, TICTOC); end Many hard-core coders prefer to avoid using ‘global’ except for constants. The reason behind this is because it’s generally considered poor form to lock up a variable name (See Steve L’s comment below for another reason!). While this won’t matter for smaller programs and functions, when the files get to be many hundreds (or thousands or millions) of lines long, it can be very difficult to keep track of all of the global variables and to remember to call all the necessary variables at the start of each function. The great thing about GUIs is that they already have a built-in global structure to deal with all of your global variables: the handles. The handles structure is an input (and therefore accessible) to every function in the GUI, making it perfectly capable doing everything the ‘global’ command can. In fact, you shouldn’t ever have to use ‘global’ command when
  2. designing a GUI because the handles structure does the job so well. GUIs and ‘global’ don’t mix kids! METHOD 2: handles.variable As you may have seen from many of the blinkdagger GUI tutorials, the handles structure is an extremely useful method to manipulate GUI boxes/buttons/tools. But the tool data are all just stored variables that can be accessed anywhere within the GUI (aka global variables!). Since we don’t need to edit any ‘property’ of the handles structure (e.g. handles.static_text, ‘String’), we don’t need to use the ‘get’/’set’ commands. Creating the global variable is as easy as saying: handles.x = 42; %And of course, don't forget to update your handles structure: guidata(hObject, handles); handles.x is now an independent variable and note that it has no relation to the local variable x. x = 43; is a completely valid command in the same function that would not overwrite your global variable ‘handles.x’. Remember, these variables can range from constants (e.g. 12) to strings (e.g. ‘Hello World’) to structures, cells, and arrays of constants/strings. Hopefully you can see the usefullness of global variables and will use them (properly!) in your coding adventures. 11 Responses to “MATLAB - Global Variables” 1. on 30 Jun 2009 at 9:02 am 1Andrew Scott When I started using GUIs a couple of years ago I couldn’t get this right, so ended up storing all my global variables via setappdata(gcbf, ’string’, data). I’ve never had a problem with this, although it is more cumbersome than the handles method you describe here. I think I’ll use your method in future. 2. on 30 Jun 2009 at 9:35 am 2Steve L Zane,
  3. In the first method you said “Many hard-core coders prefer to avoid using ‘global’ except for constants. The reason behind this is because it’s generally considered poor form to lock up a variable name.” That’s one reason to avoid using global variables, but a much stronger reason to avoid using global variables is that it can lead to bugs that are very difficult and time consuming to locate, particularly in the context of a GUI. Suppose I write a function that uses a global variable x. function y = myglobalsquare global x if isempty(x) x = 5; end y = x^2; In isolation, this function works perfectly fine. I set the value of the global variable x and then call myglobalsquare, and I get back the value x^2. Now I incorporate this function into a GUI. I set it as the callback for one of my GUI’s uicontrols and have another uicontrol’s callback set the value of the global variable x. I test it and it works, and I go on finishing up the GUI. But suddenly, when I introduce a new object into the GUI and set up its callback, my “Square” button no longer works! What’s going on? I haven’t made any changes to the Square button in hours! The problem, I find out after spending a LOT of time looking at myglobalsquare and the callback that sets the global variable x, is that the callback for the new object I introduced in the GUI _also_ uses the global variable x. Because of that callback function’s manipulation of the global variable, the myglobalsquare function no longer works the way I expect it to. If you think of a function workspace as a house, with the input arguments coming in the front door and the output arguments leaving by the back door, a global variable is an open window that anyone can crawl through and move your furniture around, potentially without you realizing it until you invite a very important guest (the data for the real problem you’re trying to solve, not the data you’ve used to test the function) for a visit. On a side note, I suppose that would make a nested function an apartment in an apartment complex, where the landlord has the right to come in and move stuff around in certain circumstances [mainly maintenance or emergency situations] and the resident doesn’t have the right to do certain things [like paint the walls.] 3. on 30 Jun 2009 at 10:22 am 3Zane Montgomery
  4. I love the analogy. Thanks for the tip Steve! 4. on 01 Jul 2009 at 6:43 am 4MATLABDoug ARGHH! Globals make me crazy… The handles structure is not global. It is passed around to all the callbacks in a GUI by default, but it is not global. I know this is partially a religious issue, but I see way too many people abusing true global variables because they do not understand scoping of variables and then they end up in a real mess later. Please do not use global variables unless you can explain to your teddy bear (http://sjbdeveloper.blogspot.com/2006/03/teddy-bear-code-reviews.html) why you really need them. My bet is you would be better off without them. See below for alternatives. On to my next religious issue: the handles structure is for handles. It just feels untidy to put data in a structure specifically labeled for handles. I see people putting just tons of data into the handles structure and it becomes a real mess. Is it going to cause as many problems as globals? Not likely. However, I highly recommend the use of GETAPPDATA and SETAPPDATA as shown in this video: http://blogs.mathworks.com/videos/2005/10/03/guide-video-part-two/ To me, this is cleaner. Loren is a fan of nested functions http://blogs.mathworks.com/loren/2008/01/16/nested-functions-and-variable- scope/ Please consider the alternative before using unneeded globals. Globals are often the cause of really tricky errors. Globals are just quicker, easier, more seductive. Use nested functions or appdata, not as clumsy or random as a global. An elegant weapon, for a more civilized age. -Doug  Anonymous Is it ok to use the userdata from the root?
  5. like ud=get(0,’userdata’); ud.x=42; set(0,’userdata’,ud);  on 02 Jul 2009 at 2:38 pm 6Anonymous I’m writing GUIs that contain some pretty large data arrays, and don’t want these to be unnecessary duplicated, using up RAM. I’m currently using global variables. Whilst I appreciate the elegance of Method 2, would switching to this effectively double my memory usage (with one copy of the data in the GUI’s handles structure, and the other in the local handles copy)?  on 01 Aug 2009 at 10:43 am 7Dey Hi all I apologize in advance because I don’t have a clue how start a new post… I need a script which does the following.: I have the following directory structures C:/Big/ecm/sDDP/new/level1/level1_1/level1_1_1/*.ext C:/Big/ecm/sDDP/new/level1/level1_1/level1_1_2/*.ext C:/Big/ecm/sDDP/new/level1/level1_1/level1_1_3/*.ext . . . C:/Big/ecm/sDDP/new/level2/level2_1/level2_1_1/*.ext C:/Big/ecm/sDDP/new/level2/level2_1/level2_1_2/*.ext C:/Big/ecm/sDDP/new/level2/level2_1/level2_1_3/*.ext . . . etc. I want to go through the folders levelx_y_z(where x, y, z stands for 1_1_1, 1_1_2 etc.) to get the files *.ext. I don’t want to select these files manually with ‘uigetfile’, because it’s very time consuming. Could anyone help me please. I tried something with ‘fullfile(path, ‘*.ext’)’ but then I still have to specify each path.
  6. P.s. The folders levelx/levelx_y/level/x_y_z are named otherwise actually. I just wanted to illustrate the structure  who can everyone help me! i’m unstill about the gui ,i want to show the transfer function iin the Guide ,but how don’t know begin my following topic: for G(S)=(7s^2+16s+10)/(s^4+5s^3+11s^2+16s+10) and have to show : + time set + bulk acceleration +error set in the Guide i hope you can make it help me thank you  on 23 Jan 2011 at 11:45 am 9Ameya Having a Problem trying to plot an imported array, the gui does not except a code from another .m file even though it’s shown in the workspace…Kindly help Dey You can solve your problem in one line with a regular expression: regexp(text, expression) You can get the text input from dir and the expression below should match your files: ‘C:/Big/ecm/sDDP/new/level\d+/level\d+_\d+/level\d+_\d+_\d+/.*\.ext’ It may be a good idea to experiment a bit with regexp to get familiar with it since it can be a bit tricky to get the expressions right. MATLAB GUI Tutorial - UITABLE Part 2, How To Access Table Data Last time, we learned how to display data onto a table. This time, we’re going to learn how to work with table data within a GUI framework. For example, say you wanted to take the contents of the table manipulate the data. This tutorial will explain how to do that, and much more.
  7. Contents Accessing Table Data within GUI Callbacks • Accessing only the Selected Data • Next Time • Links and Downloads • Accessing Table Data within GUI Callbacks Let’s say you have the following GUI:
  8. For simplicity sake, let’s assume that you would like to create a button that will add 3 to each of the entries of the table when the button is pressed. How would you go about doing this? It’s actually quite straightforward. Let’s take a look at the callback for the add button: function add_pushbutton_Callback(hObject, eventdata, handles) % hObject handle to add_pushbutton (see GCBO) % eventdata reserved - to be defined in a future version of MATLAB % handles structure with handles and user data (see GUIDATA) %get the table data tableData = get(handles.uitable1,'data'); %add 3 to the table tableData = tableData + 3; %update the table set(handles.uitable1,'data',tableData); So now, when I press the “Add 3″ button, it adds 3 to the table! This is just a simple example to show how to extract the data from the UITABLE, and to perform an operation on it.
  9. Accessing only the Selected Data Now, let’s up the difficulty level a bit. Let’s say you selected a couple of cells that you want to sum, as shown in the image below (You can hold onto the CTRL button while clicking on individual cells to select multiple cells). How would you go about doing this? Read on and all will be revealed. Enabling CellSelectionCallback The first thing we need to do is to enable the Cell Selection Callback. But first, why are we doing this? Enabling this callback will allow us to keep track of what cells are
  10. being selected on the table. You can do this by bringing up the Property Inspector for the UITABLE, and then clicking the following icon as shown in the image below. If you did it correctly, your m-file should have been updated to include the following: % --- Executes when selected cell(s) is changed in uitable1. function uitable1_CellSelectionCallback(hObject, eventdata, handles) % hObject handle to uitable1 (see GCBO) % eventdata structure with the following fields (see UITABLE) % Indices: row and column indices of the cell(s) currently selecteds % handles structure with handles and user data (see GUIDATA) In addition, the CellSelectionCallback field should now be populated as shown in the image below:
  11. Adding the Necessary Code First, let’s create and initialize a variable to hold the table cell selection information. We will call this handles.selectedCells, and initialize it in the opening function. % --- Executes just before uitable_tutorial_02 is made visible. function uitable_tutorial_02_OpeningFcn(hObject, eventdata, handles, varargin) % This function has no output args, see OutputFcn. % hObject handle to figure % eventdata reserved - to be defined in a future version of MATLAB % handles structure with handles and user data (see GUIDATA) % varargin command line arguments to uitable_tutorial_02 (see VARARGIN) %initialize this variable handles.selectedCells = []; Next, we go to the uitable1_CellSelectionCallback, which is the callback that we just enabled. % --- Executes when selected cell(s) is changed in uitable1. function uitable1_CellSelectionCallback(hObject, eventdata, handles) % hObject handle to uitable1 (see GCBO) % eventdata structure with the following fields (see UITABLE)
  12. % Indices: row and column indices of the cell(s) currently selecteds % handles structure with handles and user data (see GUIDATA) %every time the cell selection changes, we update this data %eventdata stores the indices of the selected cells handles.selectedCells = eventdata.Indices; %update the gui data guidata(hObject, handles); Adding the Selected Numbers Together First, we’re going to add another button and a static text component to display the sum. The modified GUI looks like this: Now, we need to write the callback for the button we just added: % --- Executes on button press in sumNumbers_pushbutton. function sumNumbers_pushbutton_Callback(hObject, eventdata, handles) % hObject handle to sumNumbers_pushbutton (see GCBO) % eventdata reserved - to be defined in a future version of MATLAB % handles structure with handles and user data (see GUIDATA)
  13. %get the number of rows and columns [rows,columns] = size(handles.selectedCells); %get the data from the UITABLE tableData = get(handles.uitable1,'data'); %initialize the sum sum = 0; %loop through each selected cell and keep a running sum %can anyone thing of a better way to do this? for x=1:rows sum = sum +tableData(tableIndices(x,1),tableIndices(x,2)); end %display the sum on the GUI set(handles.sum_text,'String',num2str(sum)) And there you have it, now you can select any number of cells, and then sum up the value of the contents! Next Time Next time, we’re going to talk about some of the cool features of the UITABLE that we have not yet discussed, including different data types within the UITABLE. Links and Downloads
  14. Download Source Files The MathWorks Documentation for UITABLE Cool Things You can do with UITABLE Doug’s Video on UITABLE MATLAB GUI Tutorial - UITABLE Part 1, How to Display Data With the release of MATLAB 2008b, you are now able to add tables to a GUI. In the past, there was no easy way to display your data in tabular form. With the UITABLE component, displaying your data in tabular form is easy, and most importantly, looks great! Contents Adding a Table to Your GUI using GUIDE • Displaying Data on the Table • Adding Column and Row Labels • Modifying your Table through the m-file • Next Time • Links and Downloads • Adding a Table to Your GUI using GUIDE
  15. Within the GUIDE framework, you can add a table to your GUI using the following icon from the toolbar: . Here’s what the GUI will look like within GUIDE: Displaying Data on the Table We are going to populate the UITABLE component with data by pushing the “Populate Table” button. Thus, we’re going to need to add some code to the pushbutton’s callback. In the populate_pushbutton callback, we use the following code: % --- Executes on button press in populate_pushbutton. function populate_pushbutton_Callback(hObject, eventdata, handles) % hObject handle to populate_pushbutton (see GCBO) % eventdata reserved - to be defined in a future version of MATLAB % handles structure with handles and user data (see GUIDATA) %first, create a data matrix that has 5 columns, 4 rows myData = rand(5,4); %now populate the table with the above values set(handles.uitable1,'data',myData ); Now, let’s run the GUI and push the button!
  16. A neat feature is that the table is smart enough to fill out the table according to the size of the data matrix that you feed it. So if I had done the following instead: myData = rand(100,100); The table would incorporate the use of scroll bars, as shown below. Adding Column and Row Labels A good way to spruce up your table is to add row and column labels. This helps differentiate your data and makes it easy to identify. Within the GUIDE framework, we
  17. can modify the labels by first bringing up the Property Inspector for the UITABLE. This can be done by double clicking the UITABLE component. Now, if you click on any of the fields in the above picture, it will bring up the Table Property Editor. This is where you can add Row and Column labels. For example:
  18. Make sure you click on the “Rows”, and that you select the “Show names entered below as the row headers” option. Finally, you just need to modify the names. Similarly, you can do the same for the columns.
  19. Once you’re done with that. you should see the following:
  20. And once you run your GUI, you can see the final result. A well labeled table that displays your data beautifully! Modifying your Table through the m-file
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