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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 dont have (direct) access to variables created in other functions withinthe 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 methodsfor 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 X Y Z

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. Thedocumented example in the MATLAB helps shows this pretty well:

function ticglobal TICTOC %define/incorporate global variable at start of functionTICTOC = clock;

function t = toc

global TICTOC %accesses variable TICTOC (or creates it if TICTOC isundefined)if nargout < 1

elapsed_time = etime(clock, TICTOC)else

t = etime(clock, TICTOC);end

Many hard-core coders prefer to avoidusing global except for constants. The reason

behind this is because its generally considered poor form to lock up a variable name (See

Steve Ls comment below for another reason!). While this wont 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 yourglobal 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 shouldnt everhave to use global command when designing aGUI because the handles structure does the job so well. GUIs and global dont mix kids!

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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 areall just stored variables that can be accessed anywhere within the GUI (aka global

variables!). Since we dont need to edit any property of the handles structure (e.g.

handles.static_text, String), we dont need to use the get/set commands. Creating theglobal 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 globalvariable 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!) inyour 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 couldnt get this right, so ended

up storing all my global variables via setappdata(gcbf, string, data). Ive neverhad a problem with this, although it is more cumbersome than the handles method

you describe here.

I think Ill use your method in future.

2. on 30 Jun 2009 at 9:35 am 2Steve L

Zane,

In the first method you said Many hard-core coders prefer to avoid using global

except for constants. The reason behind this is because its generally considered

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poor form to lock up a variable name. Thats 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 thecontext of a GUI.

Suppose I write a function that uses a global variable x.

function y = myglobalsquareglobal xif isempty(x)

x = 5;endy = 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 myGUIs uicontrols and have another uicontrols 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 nolonger works! Whats going on? I havent made any changes to the Square button

in hours!

The problem, I find out after spending a LOT of time looking at myglobalsquareand 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 thatcallback functions 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 youre trying to solve, not the data youve used to test

the function) for a visit.

On a side note, I suppose that would make a nested function an apartment in anapartment 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 doesnt have the right to do certain things [like paint the walls.]

3. on 30 Jun 2009 at 10:22 am3Zane Montgomery

I love the analogy. Thanks for the tip Steve!

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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 feelsuntidy 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 thisvideo:

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 thecause 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?

like

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ud=get(0,userdata);

ud.x=42;

set(0,userdata,ud);

on 02 Jul 2009 at 2:38 pm 6Anonymous

Im writing GUIs that contain some pretty large data arrays, and dont want these to beunnecessary duplicated, using up RAM. Im 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 GUIs 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 dont 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/*.extC:/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/*.extC:/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 dont want to select these files manually with uigetfile, because itsvery time consuming. Could anyone help me please. I tried something with fullfile(path,

*.ext) but then I still have

to specify each path.

P.s. The folders levelx/levelx_y/level/x_y_z are named otherwise actually. I just wanted to

illustrate the structure

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who can everyone help me!

im unstill about the gui ,i want to show the transfer function iin the Guide ,but how dontknow 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 setin the Guide

i hope you can make it help me

thank you

on 23 Jan 2011 at 11:45 am9Ameya

Having a Problem trying to plot an imported array,

the gui does not except a code from another .m file even though its shown in the

workspaceKindly 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, were going to learn how

to work with table data within a GUI framework. For example, say you wanted to take thecontents of the table manipulate the data. This tutorial will explain how to do that, and

much more.

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Contents

Accessing Table Data within GUI Callbacks

Accessing only the Selected Data

Next Time

Links and Downloads

Accessing Table Data within GUI Callbacks

Lets say you have the following GUI:

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For simplicity sake, lets 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 doingthis? Its actually quite straightforward. Lets 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 datatableData = get(handles.uitable1,'data');

%add 3 to the tabletableData = tableData + 3;

%update the tableset(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.

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Accessing only the Selected Data

Now, lets up the difficulty level a bit. Lets say you selected a couple of cells that youwant 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 wedoing this? Enabling this callback will allow us to keep track of what cells are being

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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) currentlyselecteds% handles structure with handles and user data (see GUIDATA)

In addition, the CellSelectionCallback field should now be populated as shown in the

image below:

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Adding the Necessary Code

First, lets 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 (seeVARARGIN)

%initialize this variablehandles.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)

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%get the number of rows and columns[rows,columns] = size(handles.selectedCells);

%get the data from the UITABLEtableData = get(handles.uitable1,'data');

%initialize the sumsum = 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 GUIset(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, were 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

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Download Source Files

The MathWorks Documentation for UITABLE

Cool Things You can do with UITABLEDougs 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

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Within the GUIDE framework, you can add a table to your GUI using the following icon

from the toolbar: .

Heres 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 PopulateTable button. Thus, were going to need to add some code to the pushbuttons 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 rowsmyData = rand(5,4);

%now populate the table with the above valuesset(handles.uitable1,'data',myData );

Now, lets run the GUI and push the button!

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A neat feature is that the table is smart enough to fill out the table according to the size ofthe 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 can

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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:

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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.

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Once youre done with that. you should see the following:

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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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Sometimes its easier to work from within the m-file framework, rather than the GUIDE

framework. We could have done exactly what we did above programmatically through the

m-file. In the opening function we could have done the following:

%store the row headers into a cell arrayrowHeaders = {'Blink','Dagger','Loves','MATLAB','!!!!!!'};

%set the row labelsset(handles.uitable1,'RowName',rowHeaders);

%do the same for the column headerscolumnHeaders = {'Quan','Daniel','Rob','Zane'};set(handles.uitable1,'ColumnName',columnHeaders);

In this example, we assumed that we knew the dimensions of our table. If you dont knowthe size of your table beforehand, then it can be difficult to apply data labels that are

meaningful. By working through the m-file, you obtain more flexibility since you wont

have to go back and modify the .fig file every time you want to make a change. And if you

are going to apply dynamic labeling, then working from the m-file is going to be mucheasier.

Next Time

Next time, were going to talk about how to work with manipulating the data within the

table.

Links and Downloads

Download Source FilesThe MathWorks Documentation for UITABLECool Things You can do with UITABLE

Dougs Video on UITABLE

MATLAB GUI - Tool Tips are your Friends!

What is a Tool Tip?

A tool tip is supplementary information about a GUI component that appears when the user

hovers the mouse cursor over the GUI component. As you can imagine, this can be quite

useful for the user. See below for examples.

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Tool Tip for the add button:

Tool tip for the first input parameter:

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Tool tip for the second input parameter:

How to Add Tool TipsWithin the GUIDE editor, you can add a tool tip by modifying the TooltipString propertyusing the Property Inspector. Simply double click on the component to bring up the

Property Inspector. The image below shows that the TooltipString property has been

modified for the Add! button.

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Adding Tool Tips Programmatically

If you would rather add the tool tips within your code, you can use the SET command. For

instance, if you wanted to add a tool tip for the add button, you could do the following inthe opening function of the GUI:

%put this code into the opening function of the GUIset(handles.input1_editText,'TooltipString','This is the first input.')set(handles.input2_editText,'TooltipString','This is the second input.')set(handles.add_pushbutton,'TooltipString','This is a tool tip! Pressto add.')

If you have a large list of tooltips, you may want to do it in separate m-file and call that m-

file in the opening function.

Download the Source Files and Other LinksDownload the source files here.

Yairs Undocumented Tips on How to Spice up Tooltips

Pimp My Gui Part 3: Help Me Help Yourself

Blinkdagger proudly presentsPimp My Gui, a series that will provide our readers with tipsand tricks on how they can make their GUIs beautiful on both the inside and outside.

In the previous post, we talked about using a custom menu bar to retain

functionality while clearing up space on a GUI. In this post, were going todiscuss how to implement a help feature for your GUI. The help feature

should be informative and simple so that any person can learn how to use the

GUI quickly. The help feature should offer other useful information such as the versionnumber, author, and other tidbits of information.

In the last post, we ended up with this GUI, but we never did anything for the help

portion of the GUI:

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Contents

How to use this GUI: Quick User Guide About this GUI

Putting it all Together

Addpath

Next Time

How to use this GUI: Quick User Guide

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A simple but effective way to implement a help screen is to create another standalone GUI.

For example, I created a standalone GUI that consists of only a static text component. I

populated the text field with some helpful information regarding the GUI and detailed asuccinct users guide.

When the GUI is called by the user, the user will see the following:

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Of course, you can add more tidbits and information depending on the actual GUI. Youmight also want to add a FAQ section here if you feel your GUI needs some extra

explanation. If you cant fit all the information into one screen, you can also utilize

pushbuttons that will change the static text when the button is pressed.

About this GUI

I also created a stand alone GUI for the About. This part should inform the user about

what version of MATLAB the GUI was designed on, date of creation, name of author,

some contact information, and the version number of the GUI.

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When the GUI is called by the user, the user will see the following:

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Putting it all together

All we have to do now is to call the appropriate help GUI in the appropriate callback, pretty

straightforward stuff. When the menu items for the help menu was created, MATLAB

automatically created callbacks for each menu item.

the callback for the How to use this GUI menu item is:

function helpGUI_Callback(hObject, eventdata, handles)%this is the name of the help GUI that contains the help informationhelpGUI_description

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the callback for the About this GUI menu item is

function aboutGui_Callback(hObject, eventdata, handles)%this command calls the GUI that has the "about" informationaboutGUI_description

Addpath

I have found that the addpath command within MATLAB can really help you keep your

files organized. If your GUI utilizes other m-files or images, it can be advantageous to store

them in appropriate subdirectories.

you can store them in a separate directory and simply add the path of that directory. It canget a little crowded in the main directory of your GUI, so its usually a good idea to put all

your sub functions and sub GUIs in a lower level folder.

For example, the following image shows how I originally had my m-files organized. The

main GUI files are Tutor_Commuter_QQ_05.figand Tutor_Commuter_QQ_05.m. The restof the files are used in creating the help.

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In the following image, you can see how I reorganized the files. All the files except the

main files are placed in thesubfunctions folder.

Now, all that has to done is to add the path of the subfunctions within the OpeningFunction of the main GUI.

%the addpath adds the subfunctions folder onto the MATLAB path. m-files%within this directory will be run as if it were in the same directoryas the%main GUI itself%the fullfile command combines directory names and parts%pwd is the "present working directory", which in this case will be

where%the main GUI is locatedaddpath(fullfile(pwd,'subfunctions');

Next Time

It looks like weve revamped Zanes GUI so that its obtained a shade of respectability. If

you guys have any more ideas on what can be done, wed love to hear about it. In the nearfuture, well open up the floor and ask for GUIs from our readers! Until next time . . .