# FAQ

*6*pages on

this wiki

## Introduction, Policy, CreditsEdit

### What is MATLAB?Edit

**MATLAB** math programming software package written by MathWorks. Quoting from their web page : "MATLAB is a high-level language and interactive environment for numerical computation, visualization, and programming."

**MATLAB** is a complete environment for high-level programming, as well as interactive data analysis. **MATLAB** excels at numerical computations, especially when dealing with vectors or matrices of data. Symbolic math is available through an add-on toolbox that uses a MuPAD kernel.

There are many add-on toolboxes that extend **MATLAB** to specific areas of functionality, such as statistics, finance, signal processing, image processing, bioinformatics, etc. You can find a list of toolboxes (*http://www.mathworks.com/products/product_listing/index.html*) produced by MathWorks. There is also a list (*http://www.mathworks.com/products/connections/*) of some add-on products produced and sold by other companies, and some users place their own functions or collections of functions on the **MATLAB** Central File Exchange.

Questions about the name "**MATLAB**" often arise. **MATLAB** stands for "MATrix LABoratory". See this overview of **MATLAB**.

**What is this wiki about?**Edit

This wiki contains answers to questions commonly answered on the newsgroup comp.soft-sys.matlab (sometimes abbreviated cssm), which is a forum for discussing issues related to the use of **MATLAB**. It occasionally includes questions related to similar software packages like Octave. Any topic related to **MATLAB** is appropriate. Additionally, there will be occasional discussions regarding related math topics in a more abstract form.

The original charter for the group, created in early 1993, can be found on Google groups (http://groups.google.com/group/comp.soft-sys.matlab/msg/46d872ea3ad3aecd).

Before posting, please skim through this document to see if your question has already been answered. If it is has not, there may be information here that may help you better understand the issue and phrase your question.

This Wiki FAQ was started in November 2010 from the Google cache of the old original Mathworks FAQ.

Original Mathworks FAQ URL (now it simply points here to this page): http://matlabwiki.mathworks.com/MATLAB_FAQ

Newsgroup discussion about the FAQ changeover: http://www.mathworks.com/matlabcentral/newsreader/view_thread/295865

Google cache from which this FAQ was started: http://webcache.googleusercontent.com/search?q=cache:RXorVGb-NrQJ:matlabwiki.mathworks.com/MATLAB_FAQ+matlab+faq&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a

**Where can I find an archive of comp.soft-sys.matlab?**Edit

You can read past messages and post from the **MATLAB** Central Newsreader (*http://www.mathworks.com/matlabcentral/newsreader.html*) application.

If you are looking for content prior to 1997, you can check other Usenet mirrors of cssm at mathforum.org (*http://mathforum.org/kb/forum.jspa?forumID=80*) or google groups (http://groups.google.com/group/comp.soft-sys.matlab/).

**What other software packages exist to do similar work?**Edit

GNU Octave is a freely available software package with a language "mostly compatible with **MATLAB**": http://www.gnu.org/software/octave/ .

Scilab "is a scientific software package for numerical computations in a user-friendly environment". It is fully open source and has a parallel version. http://www.scilab.org/.

IDL (Interactive Data Language) is a commercial software package with applications similar to **MATLAB**. It is very well suited to image processing and 3D visualization. IDL was formerly produced by Research Systems Inc., now part of IIT Visual Information Solutions http://www.ittvis.com/ProductServices/IDL.aspx.

Igor Pro is a commercial graphing, data analysis, image processing and programming software package that combines extensive programmability and numerical analysis tools with powerful visualization tools. http://www.wavemetrics.com

O-Matrix is a commercial **MATLAB**-like program. In fact it has a **MATLAB** compatibility mode, which the authors claim can execute native **MATLAB** code 5-10 times faster than **MATLAB**. Readers who have used this package are encouraged to send me a more detailed explanation. http://www.omatrix.com

LyME runs a reasonable subset of **MATLAB** code on the Palm platform. Available for free at http://www.calerga.com. Thanks to Martin Cohen for this info.

Fredrik Hekland suggests: Back in the days when I used OS/2, Euler was a good replacement for **MATLAB** (at least for the simple operations I needed at that time). I see that Euler is still living, now as GNU GPL'ed OSS. http://mathsrv.ku-eichstaett.de/MGF/homes/grothman/euler/

Stefan Mueller is involved in developing a **MATLAB**-like program called JMathLib, written in Java. See http://mathlib.sourceforge.net/.

SciPy and NumPy are open source Python packages for scientific computations. http://www.scipy.org

FreeMat looks very similar to **MATLAB**, but it is open source. It runs under Linux, Mac OS X and Windows and the newest version 4.2 was released 2013-06-31. http://freemat.sourceforge.net/

### What is the FAQ editorial policy?Edit

### ContributorsEdit

## BasicsEdit

**What is a cell array?**Edit

A cell is a flexible type of variable that can hold any type of variable. A cell *array* is simply an *array* of those cells. It's somewhat confusing so let's make an analogy. **A cell is like a bucket.** **You can throw anything you want into the bucket: a string, an integer, a double, an array, a structure, even another cell array.** Now let's say you have an array of buckets - an array of cells or a "Cell Array". Each bucket can contain something different, or they might all contain the same type of variable. Bucket 1 could contain a string, while bucket 2 could contain an image (array of uint8's), while bucket 3 could be a int32. Or all buckets could contain strings of various lengths. It's totally flexible.

ca{1} = myString; ca{2} = myInteger; ca{3} = myDoubleArray; ca{4} = rgbImage; ca{5} = myStructure;

The braces should be read as "contents of", so if you say ca{4} = rgbImage, you are saying that "the content of" cell #4 is the variable rgbImage.

Another way to use the cell is to refer to *the cell itself*, rather than the *contents* of it, and for that you use parentheses. The item it refers to must be a cell. For example ca(1) is a cell, ca(2) is a cell, and ca(3) is a cell, even though those cells contain variables of arbitrary, and possibly different, types. To make something a cell, you enclose it in braces, like this:

ca(1) = {myString}; ca(2) = {myInteger}; ca(3) = {myDoubleArray}; ca(4) = {rgbImage}; ca(5) = {myStructure};

This set of code is entirely equivalent to the first set of code. For the first line, it's basically like saying "Let's get a bucket (a cell) and put the string into it - that's what {myString} by itself is. Then let's take that bucket and make it bucket #1, replacing any bucket that was already there." In other words, take the cell {myString} and make it be element #1 (bucket #1) of the cell array called "ca." It uses parentheses which means it refers to the whole single bucket (the bucket plus the contents) while the first set of code used braces which refers to *only the contents* of the bucket. So ca(1) equals the *cell* "{myString}", while ca{1} equals the *string* "myString" because the braces said to get the *contents* of the cell. In other words, ca{1} says don't give me the bucket with the string inside, just give me the string alone, without the bucket. It's just a slight difference - a slightly different way of considering it. Saying

ca(1) = {myString}; % or saying ca{1} = myString;

**are equivalent** for the most part. You can use either way and I don't really think one way or the other is really preferred. **You can use whatever way is easier for you to think about it.** Maybe one way will be more intuitive for you than the other way, but again, they are equivalent.

Cell arrays are *similar* to *structures*, which you probably are more familiar with, in that both are *containers* that can hold variables of a variety of different types (arrays, strings, scalars, even other structures or cells). The difference is that with structures you refer to the different "members" or "fields" by their name (e.g. UserSettings.myString), while with cells you refer to them by their index number (e.g. ca{1}).

Here is some demo code that may help explain cell arrays, and the type of classes you get when you use braces or parentheses:

% Initialize a cell array with three different types of contents. % First cell contains an int32, second cell contains a string, % and the third cell contains a double array. ca = {int32(123), 'abcdef', pi*ones(3)} % Let's see what's in cell #1 and the difference between % using ( ) and using { }. ca1cell = ca(1) fprintf('\nThe class of ca1cell is %s\n', class(ca1cell)) ca1contents = ca{1} fprintf('The class of ca1contents is %s\n\n', class(ca1contents)) % Let's see what's in cell #2 and the difference between % using ( ) and using { }. ca2cell = ca(2) fprintf('The class of ca2cell is %s\n', class(ca2cell)) ca2contents = ca{2} fprintf('The class of ca2contents is %s\n\n', class(ca2contents)) % Let's see what's in cell #3 and the difference between % using ( ) and using { }. ca3cell = ca(3) fprintf('\nThe class of ca3cell is %s\n', class(ca3cell)) ca3contents = ca{3} fprintf('The class of ca3contents is %s\n\n', class(ca3contents)) % Now let's see what gets displayed when we use the % celldisp() function specially made for displaying cells: fprintf('\nHere is what celldisp returns:\n') celldisp(ca);

One use of cell arrays is to hold lists of strings of different lengths. Since arrays are rectangular, you can't have an character array of strings unless each string was the same length (or padded with blanks to be as long as the longest string). To get around that, you can use a cell array instead of a character array. Each cell in the cell array would hold a string of a different length - they don't have to all be the same length like with a character array. For example:

ca = {'Short'; 'A little longer'; 'A really really long string'}

If you get strange error messages while working with cells or cell arrays, one easy thing to try is to change your braces into parentheses, or your parentheses into braces, and see if that eliminates the errors.

It's also possible to mix indexing of the row and column of the *cell array* with the indexing of the *contents* of the single cell at that row and column of the cell array. For example, let's create a cell array of 2 rows and 3 columns, and in every cell of that let's put a 4 element integer array. Then we'll access the second element of the integer array at the cell in row 1, column 2 of the cell array.

% Create an empty cell array of 2 rows and 3 columns. % Each element in the array is a single cell. rows = 2; columns = 3; c = cell(rows, columns); % Now, for each cell in the cell array, % put an array of 4 random integers (in the range 1-99). for column = 1: columns for row = 1 : rows randomNumberArray = randi(99, [1, 4]); c{row, column} = randomNumberArray; % An alternate way of specifying is given on the next line. %c(row, column) = {randomNumberArray}; % Note changes in braces and parentheses. fprintf('The integer array in row #%d, column #%d of the cell array = [%d, %d, %d, %d]\n',... row, column, randomNumberArray(1), randomNumberArray(2), randomNumberArray(3), randomNumberArray(4)); end end % Print out the second element of the 4-element integer array % from the cell at the (1,2) position of your 2D array of cells. ourValue = c{1,2}(2) fprintf('The second element of the integer array in cell row %d, column %d is %d\n',... row, column, ourValue);

To visualize, imagine you had an array of buckets arranged in 2 rows and 3 columns (this is our cell array), and in each bucket are 4 billiard balls arranged in a line. The above example goes to the bucket in the first row and second column, and reads off the number of the second billard ball in that bucket.

For more infomation, this link gives good examples about accessing cell data: http://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

**Does MATLAB only calculate to 4 significant digits?**Edit

It doesn't. It uses full double-precision floating point numbers to calculate everything. By default it only prints a few decimal places to the screen. You can change this using the command:

>>format long

Type:

>>help format

for more information.

**Why does the transpose operator take the complex conjugate?**Edit

When performing linear algebra operations on complex matrices, it is almost always the complex conjugate transpose (also called the Hermitian transpose) that is needed (see Gilbert Strang's linear algebra book for discussion- page 293 in edition 3). The bare apostrophe is an operator that takes the complex conjugate transpose. The non-conjugating transpose operator is a period followed by an apostrophe. Type help punct for more info.

>> A' % complex conjugate transpose >> A.' % transpose

**How can I detect NaN values in a matrix or vector?**Edit

By definition, `NaN`

is not equal to any number, not even `NaN`

itself. Therefore there are two ways to detect `NaN`

values:

% Generate sample data x = rand(1, 10); x(x > 0.5) = NaN; % Find NaN values in two different ways y1 = isnan(x) ; y2 = (x ~= x) ;

For speed purposes the use of `isnan()`

tends to be 20%-30% faster. Here's a test snippet if you want to see the comparison:

A = rand(1000); %Random 1000x1000 matrix A(rand(size(A))>.75) = nan; %Populate with NaNs t1 = 0; %time of isnan t2 = 0; %time of ~= for ii = 1:100 tic idx1 = isnan(A); t1 = t1+toc; tic idx2 = A~=A; t2 = t2 + toc; end ratio = t2/t1; %ratio of ~= to isnan isequal(idx1,idx2) %Insure same results %{ ratio = 1.2179 ans = 1 %}

**How can I make MATLAB use the full window width for displaying matrices?**Edit

In R12 (**MATLAB** 6.0), this can be controlled via a preference. Select the File | Preferences... menu item, and select Command Window in the Preferences dialog that appears. In the Display section, there's a checkbox labeled Limit matrix display width to eighty columns. Unchecking that box allows matrix displays to make full use of the Command Window's width. [Unchecked is the default.]

Starting with **MATLAB** R12.1, users can access the current command window size using the root property CommandWindowSize. That is, `sz=get(0, 'CommandWindowSize');`

. In R12.0, there is no way to do this unless you call undocumented C functions from a MEX file.

**How do I comment out a large block of code?**Edit

Starting in **MATLAB** 7 (R14), the syntax is:

%{ Stuff to be commented out %}

You can also highlight a section of code and type **control-r to comment out the code** -- this will place a percent symbol (%) at the beginning of the line. Typing **control-t will uncomment the lines** by removing any percent symbol that is the first non-blank character on the line.

If you have an older version, the built-in editor in **MATLAB** 6.0 has a block-comment feature, which will simply put a comment character on each line. Or you can use matlab-mode for Emacs, which supports this as well.

If you are using an even older version, use this:

if 0 commented out code end

This is not the best solution, since parse errors inside that block will cause an error.

**How do I save default settings across sessions?**Edit

The key is to create a startup.m file. Look at the online helpfor more detailed instructions specific to your operating system.

**How can I find local maxima in a vector array?**Edit

You can use the following one-line function to determine the indices of the local maxima.

function index = localmax(x) index = find( diff( sign( diff([0; x(:); 0]) ) ) < 0 );

If you have the Image Processing Toolbox, you can use the imregionalmax() function. If you have the Signal Processing Toolbox you can use the findpeaks() function.

**Why is 6*i not a complex number in my program?**Edit

You may have used a variable called "i" earlier in your program or session, thus overwriting the imaginary constant i with your own number. In this case, MATLAB will use your new value for i instead of treating i as sqrt(-1). Five ways to ensure that you receive a complex result are:

- Use the syntax 6i; MATLAB always interprets this as 6*sqrt(-1)

y = 6i;

- Redefine i back to sqrt(-1)

i=sqrt(-1) y = 6*i;

- Clear your redefinition of i

clear i y = 6*i;

- Use j instead of i (assuming you haven't used a variable called "j" earlier in you program or session)

Note: these are very good reasons for not using i & j as indexes (in FOR loops, for example)

y = 6*j;

- Use the COMPLEX function

y = complex(0, 6);

**How can I modify the MATLAB path?**Edit

Easiest solution: use the PATHTOOL GUI. Or if you want command line access:

Suggested by Joshua Stiff: You can use addpath to add directories from the command line, and path2rc to write the current path back to `pathdef.m'. If you do not have write permissions for `pathdef.m', path2rc can write to a different file, which you can execute from your 'startup.m'.

**What are the versions of MATLAB and associated runtime files?**Edit

These are documented in some technical solutions Solution 1-4GSNCF Solution 1-1IW46N

R14 - Matlab 7.0 - MCR 7.0 - compiler 4.0 - mclmcrrt70.dll R14SP1 - Matlab 7.0.1 - MCR 7.1 - compiler 4.1 - mclmcrrt71.dll R14SP2 - Matlab 7.0.4 - MCR 7.2 - compiler 4.2 - mclmcrrt72.dll R14SP3 - Matlab 7.1 - MCR 7.3 - compiler 4.3 - mclmcrrt73.dll R2006a - Matlab 7.2 - MCR 7.4 - compiler 4.4 - mclmcrrt74.dll R2006b - Matlab 7.3 - MCR 7.5 - compiler 4.5 - mclmcrrt75.dll R2007a - Matlab 7.4 - MCR 7.6 - compiler 4.6 - mclmcrrt76.dll R2007b - Matlab 7.5 - MCR 7.7 - compiler 4.7 - mclmcrrt77.dll R2008a - Matlab 7.6 - MCR 7.8 - compiler 4.8 - mclmcrrt78.dll R2008b - Matlab 7.7 - MCR 7.9 - compiler 4.9 - mclmcrrt79.dll R2009a - Matlab 7.8 - MCR 7.10 - compiler 4.10 - mclmcrrt710.dll R2009b - Matlab 7.9 - MCR 7.11 - compiler 4.11 - mclmcrrt711.dll R2009bSP1 - Matlab 7.9.1 - MCR 7.12 - compiler 4.12 - mclmcrrt712.dll R2010a - Matlab 7.10 - MCR 7.13 - compiler 4.13 - mclmcrrt713.dll R2010b - Matlab 7.11 - MCR 7.14 - compiler 4.14 - mclmcrrt714.dll R2011a - Matlab 7.12 - MCR 7.15 - compiler 4.15 - mclmcrrt715.dll R2011b - Matlab 7.13 - MCR 7.16 - compiler 4.16 - mclmcrrt716.dll R2012a - Matlab 7.14 - MCR 7.17 - compiler 4.17 - mclmcrrt717.dll

**How do I fix the error "Function definitions are not permitted in this context."?**Edit

You can have multiple functions defined in one m-file, but **you can't have a script followed by one or more functions in the same m-file**. For example, this m-file:

% This is the m-file called test.m, composed of a script followed by a function. % It will generate an error. clc; % Clear the command window. m = magic(7); MyCustomFunction(m) % Define a custom function in the m-file function MyCustomFunction(m) disp(m);

will generate an error:

Error: File: test.m Line: 8 Column: 1 Function definitions are not permitted in this context

because lines 1-5 are a script, and 7-9 are a function. **To fix**, add a "function" line as the first line in your code (after any comments) and it will work. See below:

% This is the m-file called test.m composed of two functions. function test() clc; % Clear the command window. m = magic(7); MyCustomFunction(m) % Define a custom function in the m-file function MyCustomFunction(m) disp(m);

If you don't have that function line, it's a script, and you can't then define functions later (further down) in your code. By adding that line, you'll be making a function out of the first few script lines in your m-file, and when all code is contained in functions, there is no error.

## ProgrammingEdit

**How can I create variables A1, A2,...,A10 in a loop?**Edit

Please don't do this! You will find that **MATLAB** arrays (either numeric or cell) will let you do the same thing in a much faster, much more readable way. For example, if A1 through A10 contain scalars, use:

A = zeros(1, 10); % Not necessary, but it'sfasterto preallocate space for k = 1 : 10 A(k) = % some value, variable, or function ... end

Now refer to A(i) whenever you mean Ai. In case each Ai contains a vector or matrix, each with a different size, you want to use cell arrays, which are intended exactly for this:

for k = 1 : 10 A{k} = 1 : k; end

Note that each A{i} contains a different size matrix. And be sure to use the curly braces for the subscript, __not__ parentheses! Remember (from the "What is a cell array" FAQ entry above) that A(k) is the cell * itself*, while A{k} refers to the

*of the cell. So in our above example A(k) is a cell while A{k} is an integer. See the FAQ entry on cells if this is still unclear to you.*

**contents**Another approach is to **use structures with dynamic field names** instead of cell arrays. The fields of the structure can be the variable names you want. And you can index into them with dynamic field references. For example:

names = {'fred' 'sam' 'al'}; for index = 1 : length(names) s.(names{index}) = index; % Assign index to s.fred, s.sam, and s.al end

In this case, you end up with the variable s, a structure, containing fields specified by the * names* in the strings that are stored in the cells of the cell array. You can assign

*anything*to the field such as a scalar, an array, a string, another structure, a cell array, or whatever you want. In this example we just assigned the integer in the index variable.

Now, if you still really want to go against our advice and create variables with dynamically generated names, you need to **use the eval() function**. With eval(), you use **MATLAB** commands to generate the string that will perform the operation you intend. For example, eval('A=10') has the same effect as A=10, and eval(['A' 'B' '=10']) has the same effect as AB=10, only the eval() function executes much more slowly. So in a loop, you could use:

for i=1:10 eval(sprintf('A%d = [1:i]', i)); end

Notice how much more obfuscated this is. It could be made *possibly* clearer to split it up into multiple lines:

for i=1:10 lineOfCode = sprintf('A%d = [1:i]', i); % Now, execute lineOfCode just as if you'd typed % >> Ai=[i:i]' into the command window eval(lineOfCode); end

In addition, this can cause difficult-to-troubleshoot problems in your code, particularly if you try to dynamically create a variable with the same name as a function:

function y = mysin(x) eval('sin = 5;'); y = sin(x);

Calling this function with "y = mysin(1)" will not return y = 5 (the first element of the sin variable created by EVAL) -- it will return the sine of 1, because when the function was parsed there was no variable named sin and so the usage of sin on the last line was parsed as a call to the built-in SIN function. The fact that a variable named sin existed at runtime is irrelevant; the parsetime "decision" takes precedence.

__Repeat____: don't create variables at runtime using eval()__

**unless you have a very good reason**, such as someone gives you a MAT file with 2000 variables named A1428, for example. Even in that case, you can avoid eval() by using dynamic field names of a structure:

% Assume the MAT-file example1.mat contains 2000 variables, A1 through A2000 S = load('example1.mat'); % S is now a structure array with 2000 fields, S.A1 through S.A2000. % To access A1428, use: x1 = S.A1428; % If the "index" of the variable you want to access is stored in a variable: k = 1428; x2 = S.(sprintf('A%d', k)); x3 = S.(['A', num2str(k)]);

**How can I process a sequence of files?**Edit

We present two ways of doing this:

**If the files that you want to process are sequentially numbered**, like "file1.txt", "file2.txt", "file3.txt", etc. then you can use SPRINTF or NUM2STR to create the filename and LOAD, IMREAD, FOPEN, etc. to retrieve the data from the file. (Also note the three different ways of building the file name - you can use your favorite way.)

% Read files mat1.mat through mat20.mat, file1.txt through file20.txt, % and image1.jpg through image20.jpg. Files are in the current directory. % Use fullfile() if you need to prepend some other folder to the base file name. % Adapt to use which ever cases you need. for k = 1:20 % Create a mat filename, and load it into a structure called matData. matFileName = sprintf('mat%d.mat', k); if exist(matFileName, 'file') matData = load(matFileName); else fprintf('File %s does not exist.\n', matFileName); end % Create an image filename, and read it in to a variable called imageData. jpgFileName = strcat('image', num2str(k), '.jpg'); if exist(jpgFileName, 'file') imageData = imread(jpgFileName); else fprintf('File %s does not exist.\n', jpgFileName); end % Create a text file name, and read the file. textFileName = ['file' num2str(k) '.txt']; if exist(textFileName, 'file') fid = fopen(textFileName, 'rt'); textData = fread(fid); fclose(fid); else fprintf('File %s does not exist.\n', textFileName); end end

In the above code, matData, imageData, and textData will get overwritten each time. You should save them to an array or cell array if you need to use them outside the loop, otherwise use them immediately inside the loop.

The second method is if you want to **process all the files whose name matches a pattern** in a directory. You can use the DIR function to return a list of all file names matching the pattern, for example all .txt files or all .csv files, or all files named myFilennnn.dat (where nnnn is some number). Note that while this example uses *.jpg as the pattern and IMREAD to read in the data, as with the previous example you could use whatever pattern and file reading function suits your application's needs:

% Specify the folder where the files live. myFolder = 'C:\Users\yourUserName\Documents\My Pictures'; % Check to make sure that folder actually exists. Warn user if it doesn't. if ~isdir(myFolder) errorMessage = sprintf('Error: The following folder does not exist:\n%s', myFolder); uiwait(warndlg(errorMessage)); return; end % Get a list of all files in the folder with the desired file name pattern. filePattern = fullfile(myFolder, '*.jpg'); % Change to whatever pattern you need. theFiles = dir(filePattern); for k = 1 : length(theFiles) baseFileName = theFiles(k).name; fullFileName = fullfile(myFolder, baseFileName); fprintf(1, 'Now reading %s\n', fullFileName); % Now do whatever you want with this file name, % such as reading it in as an image array with imread() imageArray = imread(fullFileName); imshow(imageArray); % Display image. drawnow; % Force display to update immediately. end

Or you can use the simpler, though not as robust, code inspired by this StackOverflow question:

csvfiles = dir('*.csv') for file = csvfiles' fprintf(1,'Doing something with %s\n',file.name) end

The simplistic code above assumes that all files will be in the current folder. Or you can try a "File Exchange Pick of the Week": FileFun.

### **How do I fix the error "Subscript indices must either be real positive integers or logicals."?** Edit

In MATLAB **all** array indices must be logical or positive numeric integers. This means that the following is permitted:

>> A = [123,456,789];

>> A(2) % 2 is an integer ans = 456

>> logicalIndexes = A > 400 % Produces a logical (boolean) array of true (1) and false (0) logicalIndexes = 0 1 1 >> A(logicalIndexes) ans = 456 789

But the following produces an error:

>> A(3.14159) % 3.14159 is not an integer Subscript indices must either be real positive integers or logicals.

>> A(0) Subscript indices must either be real positive integers or logicals.

>> A(-1) Index exceeds matrix dimensions.

Note that fractional numbers, negative integers, zero, and complex/imaginary numbers are not permitted indices. Note that zero is only permitted as an index if it's not really an integer or double zero, but *really* "false" - a *logical* data type. When MATLAB *displays* logical values it uses 0 and 1 rather than "false" and "true".

The reason is that the **indexes refer to rows and columns in the array**. So while you can have row #1 or column #3, you can't have row #3.14159, nor can you have the 0th row or column, or the (-1)th row or column.

To fix the error you must make sure that your indexes are real, positive integer numbers, or logicals. They can be scalars (single numbers) or vectors or arrays of many numbers. You might take the expression for your index and make it into a single variable, like myIndexes, and then examine that in the variable editor or use code like this to figure out it's real data type and value:

myIndexes = (some complicated expression) format long myIndexes % No semicolon so will print directly to the command line. whos myIndexes % Displays data type, e.g. int32, double, logical, structure, etc.

The official Mathworks answer to this question can be found here: [1]

**How do I fix the error "Function definitions are not permitted in this context"?**Edit

You cannot mix a script and function(s) in the same m-file. You can have a script, and that script can call functions in *other* m-files, * or* you can have

*all functions with no script*at all. Most likely you have forgotten to include the "function" keyword and the name of your m-file as the first executable line of your m-file. If you do that, it will probably work. See the following examples:

Example 1 : script and function = not allowed:

% This test.m contains a script and a function, SomeFunction(). % You can not do this. It does not work because you % cannot mix a script with a function in the same m-file. % You will get the error "Function definitions are not permitted in this context." % If SomeFunction() were in a separate m-file, SomeFunction.m, % instead of in the same m-file, test.m, then it would work. % Here is the script: clc; % Clear the command window. workspace; % Make sure the workspace panel is showing. format compact; m = randi(9, 2, 3) output = SomeFunction(m) % Here is the function: function output = SomeFunction(m) output = 2 * m; % Multiply m by 2.

Example 2 : two functions = allowed:

% This test.m contains two functions: test() and SomeFunction() % You can do this. Because it has only functions, % and does not have a script, it works. % Because the first executable line of code is "function test" % that turns it from a script into a function and allows it to work. % Here is the first function, that has the name of the m-file: function test % <----- Key function definition keyword! clc; % Clear the command window. workspace; % Make sure the workspace panel is showing. format compact; m = randi(9, 2, 3) % Define sample data. output = SomeFunction(m) % Call SomeFunction() % Here is the second function. It should not take on the name % of any built-in MATLAB function to avoid conflict. function output = SomeFunction(m) output = 2 * m; % Multiply m by 2.

Note: if the name of the first function declared in the m-file does not match the file name, it will execute when called by the file name, not the name after the function keyword. For example, in the second example above, if we had improperly called the function TestFunction

function TestFunction

then the code would execute by issuing the command "test" because that's the name of the m-file. Issuing the command "TestFunction" would give the error "Undefined function or variable 'testFunction'."

**Can I read a text file with a certain format?**Edit

Yes. If the file has nothing but numbers separated by whitespace, and has a constant number of columns through the entire file, you can just type load myfile.txt.

The function DLMREAD is more flexible and allows you to read files with fields delimited by any character.

If you have mixed data, such as columns of text and numbers, you can use READTABLE. The first line is the column headers line and says what the fields of the table will be called.

t = readtable('file.txt');

The function TEXTREAD is more flexible still and allows you to skip lines at the beginning, ignore certain comment lines, read text as well as numbers, and more. Type help textread for more info.

If you are dealing with a more complicated file try XLSREAD, for example when opening a csv file with only partially numerical content.

If none of these suit your needs, you can always use the low-level file I/O functions FOPEN, FREAD, FSCANF, FGETL, FSEEK and FCLOSE to read the data however you would like.

**Are global variables bad?**Edit

1) Using globals is perfectly okay (that's why they are there in first place), just like for's and while's and other intelligent 2nd generation computer language constructs (that MATLAB is built on), as long as you are careful about where, why, and how you use them.

2) However, using globals is not recommended by many programmers because it shows that she/he didn't think ahead but was rather sloppy... and just kept adding on those never-ending "important set up" parameters that she/he needed to use by all the axes - or whatever - of a one single project.

3) Using globals can be a problem in terms of bookkeeping if you end up having dozens of them hovering around in your workspace. If you are not careful and use the same names for global variables that represent different things, then you may start having problems if the global <myParameter> from function1 is mixed up with the global <myParameter> from function2. You can do <whos global> to show you all the global variables.

4) A nice way to *reduce the number of global variables* is to collect all your globals into a single structure (thereby uncluttering your workspace). For example

% Declare UserSettings (or whatever name you want) as global in this function. global UserSettings; % Get the user folder of the user who logged in to the computer. userFolder = getenv('USERPROFILE') % 'C:\Users\YourName % Set up global variable, UserSettings, for use in any other function that % declares UserSettings as global, but in ONLY those functions. UserSettings.dataFolder = fullfile(userFolder, '\Documents') UserSettings.resultsFolder = fullfile(UserSettings.dataFolder, '\Results') UserSettings.someVariable = 42*pi;

Just attach any variables that you want to make global to the UserSettings structure. Now, any other function that declares UserSettings as global will have access to all of the member fields of UserSettings. Other functions that *do not have* the "global UserSettings" line in them *will not be able to see* the UserSettings global variable. It is *not global unless* the global line is included in the function.

5) Alternatively you can not have UserSettings as global and instead *pass it as an input argument to only those functions that need it*. This is one preferred way of sharing variables in MATLAB ... and it shows your intelligence, to boot.

If you're using global variables because you want to share variables between functions, look at the section on How can I share data between callback functions in my GUI. That section of this FAQ lists some alternatives to using global variables.

**How does logical indexing work?**Edit

From the Getting Started book:

The logical vectors created from logical and relational operations can be used to reference subarrays. Suppose X is an ordinary matrix and L is a matrix of the same size that is the result of some logical operation. Then X(L) specifies the elements of X where the elements of L are nonzero.

Here's a short example of logical indexing to specify certain array elements:

m = magic(5) % Get the logical matrix which is zero where % m <= 20 and 1 where m >= 21 bigNumbersLocations = m > 20 % Extract those big numbers into an array % Method #1: bigNumbers = zeros(size(m)); bigNumbers(bigNumbersLocations) = m(bigNumbersLocations) % Method #2: bigNumbers2 = m; bigNumbers2(~bigNumbersLocations) = 0 % Display the big numbers. It will be a 1D vector. m(bigNumbersLocations) m = 17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9 bigNumbersLocations = 0 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 bigNumbers = 0 24 0 0 0 23 0 0 0 0 0 0 0 0 22 0 0 0 21 0 0 0 25 0 0 bigNumbers2 = 0 24 0 0 0 23 0 0 0 0 0 0 0 0 22 0 0 0 21 0 0 0 25 0 0 ans = 23 24 25 21 22

**Huge memory waste using array of structs?**Edit

The following example was posted to the newsgroup:

I've discovered to my horror that structs take up an obscene amount of overhead (I'm running version 5.3.1.29215a (R11.1) on a Dec ALPHA). I have a set of 10,242 observations, each consisting of 3+13=16 fields, which have 3*27 + 1*13 = 94 values. So the total size in bytes should be 10,242 * 94 * 8 bytes/double = 7,701,984.

I have this stored in a 1 x 10242 data structure, and when I issue the whos command, it tells me that the data now takes up 27,367,136 bytes!

Professor Cris Luengo answers:

My guess would be that a structure contains **MATLAB** arrays. Each array has some overhead, like data type, array sizes, etc. In your second implementation (index using data.latitude(observation)), there are 10,242 times less arrays allocated. Note that in your data, for each observation, you have 13 arrays with one value. I don't know how large the matrix header exactly is, but it is a waste putting only a single value in it!

I think Cris has hit it exactly. Every **MATLAB** matrix has an overhead of ~100 bytes, even matrices with a single element. In this example, there are 16 fields * 10242 elements = 163872 matrices. Each one of these matrices adds an additional 100 bytes, for 16.4Mbytes in pure overhead. This still comes up a little short of the amount reported, but it is fairly close.

It is much more efficient, both for storage and computation, to use a struct of arrays rather than an array of structs.

**Why is my MEX file crashing?**Edit

Memory errors are one likely reason. Greg Wolodkin suggests the debug memory manager:

The platform-independent way to use the debug memory manager is to set the environment variable MATLAB_MEM_MGR to contain the string "debug".

On Windows:

C:\> set MATLAB_MEM_MGR=debug C:\> matlab

On Unix with csh or tcsh:

% setenv MATLAB_MEM_MGR debug % matlab

On Unix with sh or bash:

$ MATLAB_MEM_MGR=debug matlab

The debug memory manager cannot catch your code the instant it writes out of bounds (tools like Purify can do this but the performance hit they induce is quite painful). What it will catch is that in general, when you write outside of one memory block you end up writing into another, corrupting it or (in the case of the debug memory manager) hopefully corrupting only a guard band. When you later free the memory, we can tell you that you walked off the end of the block and corrupted the guard band.

**Do boolean operators short-circuit?**Edit

In many programming languages, boolean operators like AND and OR will stop evaluating as soon as the result is known. For instance,

1 | error('Short-circuit')

would never get to the error part, since the 1 is always true.

**MATLAB** versions >= 6.5 include the new short-circuiting logical operators || and &&. Use these for all condition tests in loops and similar, and use the old | and & for element-by-element logical operations. You can find details here (*http://www.mathworks.com/help/techdoc/ref/logicaloperatorsshortcircuit.html*).

In older versions of **MATLAB**, the boolean operators | and & are only short-circuit evaluated inside the conditions of IF and WHILE statements. In all other contexts, all parts of the conditional are evaluated.

**How do I fix "Out of Memory" problems?**Edit

A frequent variant of this question is: "I have 3 GB of RAM, and 2 GB of swap space. Why can't I create this 600 MB matrix?"

First of all, issue the command "memory" in the command window to see how much memory is available on your computer. It should return something like this:

**>> memoryMaximum possible array: 520 MB (5.457e+008 bytes) *Memory available for all arrays: 1343 MB (1.409e+009 bytes) **Memory used by MATLAB: 447 MB (4.690e+008 bytes)Physical Memory (RAM): 3036 MB (3.184e+009 bytes)**

- Limited by contiguous virtual address space available.

** Limited by virtual address space available.

Remember that double precision floats take up 8 bytes. So a million element vector takes up 8Mbytes. Be sure you're estimating properly.

Many operations need to create duplicate matrices. For example, B = inv(A.') must create a tempory variable the same size as A to hold the transpose, and B is the same size as A.

If you're sure your matrices are reasonably sized, then read these Mathworks Technical Notes: Tech Note 1106: Memory Management Guide and Tech Note 1107: Avoiding Out of Memory Errors

Another variant of this question is: "How do I pre-allocate memory when using MATLAB?"

If the matrix size is not defined prior to populating it with data through a FOR loop, memory fragmentation problems may happen since MATLAB is not aware of the final matrix size upon the conclusion of the FOR loop. In order to work around this issue, one solution is to pre-allocate memory by creating an initial matrix of zeros with the final size of the matrix being populated in the FOR loop. You should read the following Mathworks article: Technical Solution 1-18150 for a more complete discussion of this problem.

**How do I dynamically generate a filename for SAVE?**Edit

You're probably trying

fname = 'foobag'; save fname variable;

To do this correctly, you need to use the "functional" form of save:

fname = 'foobar'; save(fname, 'variable');

In fact, it is true in general that the following two lines are equivalent:

command str1 str2 str3 command('str1', 'str2', 'str3')

This allows one replace any or all of the parameters with dynamically generated strings. This is also useful in commands like PRINT, LOAD, CLEAR, etc.

**What's the difference between M-files, Pcode, and MEX files?**Edit

- M-files are plain ASCII text that is interpreted at run time. Actually it is parsed once and "just-in-time" compiled, but this is transparent to the user. Use M-files for most of your
**MATLAB**development, and for platform independence and maintainability. - Pcode is a preparsed and encoded version of the M-file. Since it is preparsed, it saves on the load time of the function. This is most likely not an issue except for very large M-files, since most are parsed only once anyway. Pcode also lets you hide the source code from others. Careful, there is no way to convert Pcode back to the M-file source. Pcode is platform independent.
- MEX files are native C or C++ files that are dynamically linked directly into the
**MATLAB**application at runtime. They must be compiled for each hardware architecture on which they are to be run. MEX files have the potential to crash the**MATLAB**application, but rather large speed gains are possible, depending on the algorithm.

**Can MATLAB pass by reference?**Edit

If you are attempting to use pass-by-reference to modify the input argument passed into a function, the answer to the question depends on whether the input is a handle object or a value object. The two types of objects are described in the Object-Oriented Programming (*http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_oop/brfylq3.html*) and Programming Fundamentals (*http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/brtm8si.html*) sections of the documentation. By default, objects (including matrices, arrays, etc. of the built-in data types) are value objects.

Handle objects do exhibit reference behavior when passed as function arguments; value objects do not. When you pass a handle object to a function, **MATLAB** still copies the value of the argument to the parameter variable in the function (with one bit of subtlety; see below.) However, all copies of a handle object refer to the same underlying object.

If a function modifies a handle object passed as an input argument, the modification affects the object referenced by both the original and copied handles. In this case, the function does not need to return the result to be reassigned.

If instead you are attempting to use pass-by-reference to avoid unnecessary copying of data into the workspace of the function you're calling, you should be aware that **MATLAB** uses a system commonly called "copy-on-write" to avoid making a copy of the input argument inside the function workspace until or unless you modify the input argument. If you do not modify the input argument, **MATLAB** will avoid making a copy. For instance, in this code:

function y = functionOfLargeMatrix(x) y = x(1);

**MATLAB** will not make a copy of the input in the workspace of functionOfLargeMatrix, as x is not being changed in that function. If on the other hand, you called this function:

function y = functionOfLargeMatrix2(x) x(2) = 2; y = x(1);

then x is being modified inside the workspace of functionOfLargeMatrix2, and so a copy must be made.

For more information on this behavior, read this posting (*http://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/*) on Loren Shure's blog.

**How can I pass additional parameters into a "function function" like ODE45, QUAD, FSOLVE, FMINCON, GA, etc?**Edit

The term "function functions" refers to functions in **MATLAB** and the toolboxes that accept a function (usually a function handle) and evaluate that function repeatedly during the course of their work. Some examples of "function functions" are:

- the ordinary differential equation solvers like ODE45, ODE23, etc.
- the integration functions like QUAD and QUADL
- the optimization functions in the funfun directory in
**MATLAB**, like FMINSEARCH and LSQNONLIN - the optimization functions in the Optimization Toolbox, like FMINCON, FSOLVE, and LSQCURVEFIT
- the optimization functions in the Genetic Algorithm and Direct Search Toolbox, like GA and PATTERNSEARCH

There are several documents on The MathWorks support website that shows examples of how to pass additional parameters to the functions used by the "function functions".

- ODE solvers like ODE45 and integration functions like QUAD or QUADL (
*http://www.mathworks.com/support/solutions/data/1-1AIJF.html?solution=1-1AIJF*) - Optimization functions in toolbox/matlab/funfun and toolbox/optim (FMINCON, FSOLVE, FZERO, LSQCURVEFIT, etc.) (
*http://www.mathworks.com/support/solutions/data/1-19HM6.html?solution=1-19HM6*) - Genetic Algorithm and Direct Search Toolbox functions (GA, PATTERNSEARCH) (
*http://www.mathworks.com/support/solutions/data/1-1BZ6V.html?solution=1-1BZ6V*)

This information has also been incorporated into the documentation in recent versions of **MATLAB** and the toolboxes:

**MATLAB**(*http://www.mathworks.com/access/helpdesk/help/techdoc/math/f2-941087.html*)- Optimization Toolbox (
*http://www.mathworks.com/access/helpdesk/help/toolbox/optim/ug/f67947.html*) - GA function, Genetic Algorithm and Direct Search Toolbox (
*http://www.mathworks.com/access/helpdesk/help/toolbox/gads/bqlex3b-1.html*) - PATTERNSEARCH function, Genetic Algorithm and Direct Search Toolbox (
*http://www.mathworks.com/access/helpdesk/help//gads/f12810.html*)

**Why does MATLAB clear the breakpoints I've set in my M-file?**Edit

When a function is cleared from memory using the CLEAR function, breakpoints in that file are also cleared. That means that if you execute code including the statement "clear functions", "clear <the name of your function>", or "clear all", your breakpoints will be cleared.

If you want to have your program stop execution and enter debug mode regardless of whether or not you have cleared it, insert a call to the KEYBOARD function into the code at the location where you want to enter debug mode. This will not be cleared by the CLEAR function and will cause **MATLAB** to enter debug mode when the KEYBOARD function is called.

Alternately, you can use the "Stop if Errors/Warnings" item under the Debug menu in the **MATLAB** Editor to cause **MATLAB** to enter debug mode whenever the code you're running throws an error that is not caught by a TRY/CATCH block, whenever it throws an error that is caught by a TRY/CATCH block, whenever it throws a warning, or whenever the value of a variable becomes Inf or NaN. You can find more information on this item in the documentation (*http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_env/bq4hw4g-1.html*).

**Why do I receive an error about an undefined variable or the recursion limit when calling ODE45?**Edit

One common cause of an "Undefined function or variable" error or an error about the RecursionLimit being exceeded when using an ODE solver like ODE45 is that the function being called by the ODE solver itself contains the call to the ODE solver. For instance, if your code is:

function dy = myodefunction(t, y) dy = [y(2); 2*t]; y0 = [0; 1]; tspan = [0 10]; [t, y] = ode45(@myodefunction, y0, tspan);

If you call myodefunction with no inputs, you will receive an error on the second line, where **MATLAB** tries to use t and y to define dy. If you call myodefunction with two inputs, it will proceed without error to the ODE45 call. ODE45 will call myodefunction with two inputs, and that call will proceed without error to the ODE45 call. This process will repeat until ODE45 has recursively call myodefunction a number of times equal to the root RecursionLimit property, at which point **MATLAB** will throw an error.

To avoid these errors, do not include the call to the ODE solver inside your ODE function (the function you pass to the ODE solver as the first input argument.) Instead, define the ODE function in a subfunction or as a separate function file and use that in your ODE45 call.

The subfunction approach requires one function file:

% begin myodefunction.m function [t, y] = myodefunction y0 = [0; 1]; tspan = [0 10]; [t, y] = ode45(@myodesubfun, y0, tspan); function dy = myodesubfun(t, y) dy = [y(2); 2*t]; % end myodefunction.m

The separate file approach requires one function or script file that calls ODE45:

% begin myodefunction.m function [t, y] = myodefunction y0 = [0; 1]; tspan = [0 10]; [t, y] = ode45(@myodeseparatefun, y0, tspan); % end myodefunction.m

and a function file that ODE45 will call:

% begin myodeseparatefun.m function dy = myodeseparatefun(t, y) dy = [y(2); 2*t]; % end myodeseparatefun.m

**Why is it advised to avoid using the "eval" function?**Edit

The EVAL function is one of the most powerful, flexible, and potentially dangerous functions in MATLAB. Since EVAL is so powerful, it is easy to misuse the function. In a way, the EVAL function is a lot like global variables; both are tools that are *so *easy to use that it might be easier to use them rather than to search for a more elegant, safer, and appropriate solution. There is a major drawback to the EVAL function, although it can be avoided if you use EVAL carefully. EVAL can be used to alter arbitrary variables. In addition, two related functions, evalin and assignin, can be used to alter variables in different function workspaces. These functions can create bugs which are difficult to reproduce and nearly impossible to eliminate. Further explanation, and The Mathworks official warning against using eval, can be found in Mathworks Tech Note 1103.

**Where did my file go? The risks of using the cd function.**Edit

Sometimes if you're writing code that does file input/output without specifying the full path (folder + base file name) you may not find your file. It may not be there to read in or may not be written out to the folder that you expected. For example

storedStruct = load('mySavedParameters.mat'); imwrite('myOutputImage.png')

If another function call used the cd() function to change the current folder, then you would be looking to that folder when you tried to read in a file or write out a file. If you thought that you were looking at a different folder, then you'll get a "file not found" error upon trying to read in a file, or else not find the file in that folder that you thought you wrote out to. It would be best if any code that used cd() saved and restored the original folder:

originalFolder = pwd; % Do stuff. Then restore the original folder cd(originalFolder);

but you cannot always rely upon that. It's much, much better to not use cd() and instead create the full-blown explicit filename with functions such as sprintf(), fileparts(), and fullfile(). Because if you have the full path name of the file, you'll know for certain where it will get saved to or read from. See the following code for guidance:

% Code to ask user for a folder. startingFolder = pwd; returnValue = uigetdir(startingFolder,'Select folder'); % returnValue will be 0 (a double) if they click cancel. % returnValue will be the path (a string) if they clicked OK. if returnValue ~= 0 % Assign the value if they didn't click cancel. folder = returnValue; else folder = startingFolder; end % Example 1: Write out a .mat file with some variables. % Create the full file name. baseFileName = 'myFile.mat'; fullFileName = fullFile(folder, baseFileName); % Save out the variable into the mat file. stringToSave = 'It is risky to use cd().'; % Some sample variable. integerToSave = 42; save(fullFileName, 'stringToSave', 'integerToSave'); % Example 2: Read in a mat file baseFileName = 'myFile.mat'; fullFileName = fullFile(folder, baseFileName); if exist(fullFileName, 'file') % It exists. storedVariablesStructure = load(fullFileName); else % It doesn't exist. warningMessage = sprintf('Error reading mat file\n%s.\n\nFile not found', ... fullFileName); uiwait(warndlg(warningMessage)); end

## GraphicsEdit

### Edit

Unlike custom functions that you write, the callback functions take the predetermined input variables (hObject, eventdata, handles) so it's not clear how you can pass in other variables that you need to. There are several techniques you can use to share data between callback functions in your GUI. Which approach is easiest depends on whether you are creating your GUI using GUIDE to design the layout (GUIDE-based GUIs) or whether you are calling FIGURE, AXES, UICONTROL, etc. directly (programmatic GUIs.)

The main techniques are:

- Using the handles structure. You can dynamically add on new members to the handles structure that contain your variables that you want to pass in. Unlike global variables, which expose the variables only where you put the global statement, attaching to the handles structure and passing handles will share
the variables you have attached to the handles structure. This could expose variables to your function that you did not need or want exposed. Since variables passed in to MATLAB functions are "pass by value" and not "pass by reference" if you change any of the variables, you are only changing the local copy. If you change any of the variables attached as members of the handle structure, and you want to retain these changed values in the calling function, you will have to return the handles structure as an output of your function, and accept it in your calling function, or else use the guidata() function. Otherwise changes you make to the handles structure variables are only local and will not survive once the function has returned.**all**

% Add a brand new variable on to the handles structure. handles.myNewVariable = 42;

- Creating your callback functions as
functions inside the main GUI function. The function must be truly**nested***nested*with the main function's "end" statement and not merely listed as another separate, independent function just defined in the m-file. (There is a difference.) - Storing data in a property [often the UserData property] of a component in the GUI and retrieving it when needed. For example

set(handles.button1, 'UserData', myString).

- Storing data in the application workspace using the SETAPPDATA and GETAPPDATA functions.

% Do this to save variables to your figure's workspace. % handles.GUIHandle is the "Tag" property of your main GUI figure. % Double-click the figure to bring up the "Property Inspector" in GUIDE % to see what your figure is named. You may have called it something different % than GUIHandle. You can call it whatever you want but the name in the "tag" % field of the Property Inspector must match the name in setappdata(). setappdata(handles.GUIHandle, 'yourVariable', yourVariable) % Do this to retrieve variables from your figure's workspace. yourVariable = getappdata(handles.GUIHandle , 'yourVariable') % Do this to remove what you saved from your figure's workspace. rmappdata(handles.GUIHandle, 'yourVariable')

- Declaring the variable(s) global. For example, if you have this inside a function:

global myVariable; % Declare global. Any function with this line it it can see this variable.

then any other function that also has the "global myVariable" declaration in it will be able to see the variable called "myVariable". Functions that do not have the "global myVariable" line in them will * not* be able to see the variable. So it's actually more like a "friend" variable (if you're familiar with C++ programming) than a truly global variable because not every function or workspace sees it. The "myVariable" variable will not be seen in the "base" workspace - it will be seen only inside functions with the "global myVariable" declaration in them.

- Writing the data out to a file, such as a mat file with the save function, and then having the called function read in that mat file.

% Do this to save variables to your figure's workspace. % yourVariable could be a single variable that is a structure % with a bunch of variables as members of the structure % if you wish. That can be convenient because then there % is only one variable to save. save(fullMATFileName, 'yourVariable') % Do this to retrieve the variable. storedStructure = load(fullMATFileName); yourVariable = storedStructure.yourVariable;

- Use the assignin() function.

% Send the variable myGUIVariable from the GUIs workspace % to a variable called myBaseVariable in the "base" workspace. assignin('base', 'myBaseVariable', myGUIVariable); % They could both be the same name if you wish. assignin('base', 'myVariable', myVariable);

**Sharing between multiple GUIs**. If the "main" GUI calls other GUIs, then the best way to do it is by passing variables in via the input argument list, and accepting output variables via the output argument list. The output argument of GUI2 can then be sent into GUI3. So someplace in GUI1 (like the callback function of the "Go!" button of GUI1), you'd have this

[out2a out2b out2c] = gui2(in2a, in2b, in2c, in2d); [out3a out3b] = gui3(out2a, out2b);

or something along those lines. The arguments can be extracted out of the varargin cell array of your opening code for the GUI, for example in the GUI's "OpeningFcn" function if you used GUIDE. Once they are in your opening function, then they can be shared amongst the other functions in the GUI with the methods mentioned earlier in this section. This method will not let GUI1 control GUI2 and GUI3's parameters "live" - they can be changed only when calling the GUIs. To have GUI1 control GUI2 when GUI2 is already running, you can use the assignin() function.

This answer from Geoff Hayes in the Answers forum may also help you in the multiple GUI situation: [2]

**Official Documentation**. The documentation contains instructions for using these techniques to share data between callbacks in these Mathworks web pages:

- GUIDE-based GUI (
*http://www.mathworks.com/access/helpdesk/help/techdoc/creating_guis/f5-998197.html*) - Doug Hull's video tutorial Passing data between GUIDE callbacks without globals in MATLAB
- Doug Hull's Video Tutorial How to pass data from one GUI to another

- programmatic GUIs (
*http://www.mathworks.com/access/helpdesk/help/techdoc/creating_guis/f13-998197.html*)

**How do I adjust the fontsize of a ticklabel?**Edit

The ticklabel gets its properties from the axis to which it is attached. So set(gca, 'fontsize', 14) should do the trick. Type get(gca) to see what else you can set.

**Can I create a pi/sigma/superscript in my ticklabels?**Edit

Not directly... **MATLAB** does not interpret TeX strings in ticklabels. You can play games with placing text by hand. See the MathWorks solution for some ideas of how to work around this.

There are also some free third-party software packages you can use to accomplish this. One community-generated solution in the MATLAB File Exchange is Format Tick Labels (*http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do?objectId=15986*) as an example. It will replace axes tick labels with formatted text objects that can include both Tex and LaTex interpreted strings.

Doug Schwarz has written a Styled Text Toolbox that does this. It is freely available at: http://www.frontiernet.net/~dmschwarz/stextfun/index.html

**Can I open multiple files using uigetfile?**Edit

As of **MATLAB** 7.0 (R14), you can use the 'MultiSelect' parameter with UIGETFILE to allow the selection of multiple files. If you are using a version of **MATLAB** prior to version 7.0, you can use the `uigetfiles.dll' submission on the **MATLAB** Central File Exchange to do this on a Windows platform. This file is located here (*http://www.mathworks.com/matlabcentral/fileexchange/Files.jsp?type=category&id=&fileId=331*).

**I want to use a scrollbar to scroll my edit boxes/buttons**Edit

Sorry, there's no easy solution. **MATLAB** does not support hierarchical figures, so you can't have a container control holding your controls. If you really need this you'll have to create your own, using the callbacks from the scrollbar to modify the position properties of your controls.

Ghassan Hamarneh writes:

What I would do is to add 2 pushbuttons to the figure: one at the top right and another at the bottom right and use these buttons to control the vertical scroll of the content of the figure. (surely you can add another 2 horizontal pushbuttons, lower left and lower right).

Whenever any of these buttons is pressed, you loop over all the controls except the two pushbuttons, and increment/decrement the vertical/horizontal postition value of each control. Something like this:

% fig_handle is the handle of the figure containing the UI controls % pushb1_handle, pushb2_handle are the handles of the pushbuttons % described above % find handles of all the controls all_handles=findobj(fig_handle); % exclude the handles of the 2 pushbuttons and the figure itself move_handles=setdiff(all_handles, ... [fig_handle, pushb1_handle, pushb2_handle]); % loop over the remaining handles and change their positions for k=1:length(move_handles) set(move_handles(k),'position', ... (get(move_handles(k),'position'))-[0 0 0 10]); end

**How can I rotate ticklabels?**Edit

As of release R2014b, set the axes XTickLabelRotation, YTickLabelRotation, or ZTickLabelRotation properties. See the "Tick Values and Labels" section of the documentation for axes properties for more information.

For earlier releases, you cannot rotate tick labels directly but there are some community-generated solutions to simulate this in the MATLAB Central File Exchange. See Format Tick Labels (*http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do?objectId=15986*) as an example.

**How can I display data in a grid like Excel?**Edit

If you simply want to edit the matrix as if it were an Excel spreadsheet, you can use the builtin Array Editor. Type openvar(my_var) at the command prompt, or double click on the variable in the Workspace Browser. Or you can use the uitable control. See the help for uitable.

**How can I write a title or text with multiple lines?**Edit

This can be done using a cell array of strings:

title({'First line','Second Line'}) text(0.5, 0.5, {'First line','Second Line'})

Or can be done with sprintf():

filename = 'cameraman.tif'; variableValue = 1234; % Note the \n in the format string to cause a new line: axesCaption = sprintf('The file is %s\nThe value is %d',... filename, variableValue); title(axesCaption, 'FontSize', 18);

**How can I use a common colormap among several images?**Edit

One way to do this is to insert a CAXIS command in your plotting function/script. For this to work well, you first need to have a first look at all of your data to determine what are the minimum and maximum values over the entire set of images. For example, if the overall minimum value amongst all images is 40 and the overall maximum is 256, you may issue the command "caxis([40 260])" for each image.

Tech note 1215, at http://www.mathworks.com/support/tech-notes/1200/1215.shtml, addresses a related question, namely "How can I use multiple colormaps in a single figure". As a bonus, it includes a thorough discussion of colormaps in general.

**How can I set default handle graphics properties?**Edit

There are probably several hundred of these default handle graphics options. Rather than trying to remember any particular one, the best thing is to learn the general principle behind all of these default handle graphics properties. The basic call to insert into your startup.m file is :

set(0,'DefaultObjectnamePropertyName',Value)

For line objects, here are a few examples:

set(0,'DefaultLineMarkerSize',12); set(0,'DefaultLineMarker','d'); set(0,'DefaultLineLineWidth', 2);

Similarly, you can use these statements for axes objects:

set(0,'DefaultAxesLineWidth', 2); set(0,'DefaultAxesXGrid','on'); set(0,'DefaultAxesTickDir','out'); set(0,'DefaultAxesTickLength',[0.015 0.015]); set(0,'DefaultAxesFontName','Arial')

For more details, do a full text search for 'Defining Default Values' in the R12 online help, and click on the very first hit. Also see the following entries in the R12 online help:

- Graphics Object Hierarchy
- Types of Graphics Objects

**How can I modify the default bar color used for histograms?**Edit

A histogram is made up of patch objects. The trick is to modify the FaceColor property of these patches. A short example follows:

x=rand(400,1); hist(x); % Default facecolor is blue h=get(gca,'Children'); set(h,'FaceColor', 'm'); % magenta facecolor % Draw only the edges of the bars making up the histogram set(h,'FaceColor', 'none');

**How can I draw more than two lines with plotyy?**Edit

You can use the axes' handles to plot additional lines, as follows:

x1 = (1:10)'; x2 = x1; y1 = x1; y2 = x1.^2; %Plot one line against each of the left and right y-axis [ax, h1, h2] = plotyy(x1,y1,x2,y2); %Plot additional line against the left y-axis x3= x1; y3 = y1 + 3; h3 = line(x3,y3,'Parent', ax(1), 'Color',get(h1,'Color')); %Plot additional line against the right y-axis x4= x1; y4 = y2+3; h4 = line(x4,y4,'Parent', ax(2), 'Color',get(h2,'Color')); set(ax,'YLimMode','auto','YTickMode','auto')

**Is there a command available for plotting an arrow on a graph?**Edit

A user-contributed m-file (arrow.m) is available at http://www.mathworks.com/matlabcentral/fileexchange/Files.jsp?type=category&id=&fileId=278

Also look at arrow3.m at http://www.mathworks.com/matlabcentral/fileexchange/Files.jsp?type=category&id=36&fileId=1430

**Why does movie(M,1) display the movie M twice?**Edit

This behavior, where MOVIE displayed the movie one time more than was requested, was eliminated in **MATLAB** 7.4 (R2007a), as mentioned in the release notes (*http://www.mathworks.com/access/helpdesk/help/techdoc/rn/bq08z9s-1.html#bq08z9s-3*).

In older releases, the movie function displayed each frame as it loaded the data into memory, and then played the movie the requested number of times all the data was loaded. This eliminated long delays with a blank screen when you loaded a memory-intensive movie. The movie's load cycle was not considered one of the movie repetitions.

**How can I set the focus in my GUI?**Edit

You can't. One hopes that The MathWorks will include this often-requested feature in a future, but there is no guarantee.

Related to this, changing the stacking order of your GUI elements might allow you to set the tab order, but this seems to not always work. GUIDE in **MATLAB** version >= 6.5 includes a Tab Order Editor, which does a better job at this.

**How can I add text labels to data points?**Edit

The text command can be used in a vectorized form to automatically add text labels wherever needed. Say you have a matrix D, where the first column contains X coordinates and the second column contains Y coordinates. Then us illustrates:

plot(D(:,1),D(:,2),'+-'); n=num2str(D,'%5.3f/'); n=n(:,1:end-1); % Just to remove the trailing slash text(D(:,1),D(:,2),n);

**How can I create a movie from my MATLAB figures?**Edit

Try searching the File Exchange:

http://www.mathworks.com/matlabcentral/linkexchange/?term=movie

where you'll find entries such as these:

"How to combine MATLAB figures into a movie"

By Sandeep Somani, Center for Advanced Research in Biotechnology, U MD Biotechnology Inst

"How to Make a Movie from MATLAB"

I receive with a certain regularity the question: ``How do I make movies with Matlab?* In fact, ...Contributed by: MATLAB Central Team*

*http://www.mathworks.com/help/techdoc/creating_plots/f10-1460.html*

You might want to use code similar to this:

% Make an avi movie from a collection of PNG images in a folder. % Specify the folder. myFolder = 'C:\Users\yourUserName\Documents'; if ~isdir(myFolder) errorMessage = sprintf('Error: The following folder does not exist:\n%s', myFolder); uiwait(warndlg(errorMessage)); return; end % Get a directory listing. filePattern = fullfile(myFolder, '*.PNG'); pngFiles = dir(filePattern); % Open the video writer object. writerObj = VideoWriter('YourAVI.avi'); % Go through image by image writing it out to the AVI file. for frameNumber = 1 : length(pngFiles) % Construct the full filename. baseFileName = pngFiles(frameNumber).name; fullFileName = fullfile(myFolder, baseFileName); % Display image name in the command window. fprintf(1, 'Now reading %s\n', fullFileName); % Display image in an axes control. imageArray = imread(fullFileName); imshow(imageArray); % Display image. drawnow; % Force display to update immediately. % Write this frame out to the AVI file. writeVideo(writerObj, thisimage); end % Close down the video writer object to finish the file. close(writerObj);

**How do I save my figure, axes, or image? I'm having trouble with the built in MATLAB functions.**Edit

Many people encounter trouble when they try to use the various built in MATLAB functions (such as print, saveas, hgsave, etc.) for saving figures, axes, or images. For example, the colors may be different than expected, there may be undesired spacing around it, it may be all black or all white, the overlay graphics don't get saved along with the underlying image, the resolution is not correct, the wrong part of the window is being saved, etc. Try using export_fig in the File Exchange. This may solve your problems. Does this work?

**How can I get Greek letters and other symbols on my GUI?**Edit

Please visit this Answer: Answers Forum Tutorial on Greek Letters

## Math/AlgorithmsEdit

**Why is 0.3 - 0.2 - 0.1 **(or similar)** not equal to zero?**Edit

(The above example returns -2.7756e-17.)

As is mentioned frequently in the newsgroup, some floating point numbers can not be represented exactly in binary form. So that's why you see the very small but not zero result. See EPS.

The difference is that 0:0.1:0.4 increments by a number very close to but not exactly 0.1 for the reasons mentioned below. So after a few steps it will be off whereas [0 0.1 0.2 0.3 0.4] is forcing the the numbers to their proper value, as accurately as they can be represented anyway.

a=[0 0.1 0.2 0.3 0.4]; b=[0:.1:.4]; as=sprintf('%20.18f\n',a) as = 0.000000000000000000 % == 0.100000000000000010 % == 0.200000000000000010 % == 0.299999999999999990 % ~= bs ! 0.400000000000000020 % == bs=sprintf('%20.18f\n',b) bs = 0.000000000000000000 % == 0.100000000000000010 % == 0.200000000000000010 % == 0.300000000000000040 % ~= as ! 0.400000000000000020 % == -and- format hex; hd=[a.',b.'] hd = 0000000000000000 0000000000000000 % == 3fb999999999999a 3fb999999999999a % == 3fc999999999999a 3fc999999999999a % == 3fd3333333333333 3fd3333333333334 % ~= ! 3fd999999999999a 3fd999999999999a % ==

If you're trying to compare two floating-point numbers, be very careful about using == to do so. An alternate comparison method is to check if the two numbers you're comparing are "close enough" (as expressed by a tolerance) to one another:

% instead of a == b % use: areEssentiallyEqual = abs(a-b) < tol % for some small value of tol relative to a and b % perhaps defined using eps(a) and/or eps(b)

You can see this same sort of behavior outside **MATLAB**. Using pencil and paper (or a chalkboard, or a whiteboard, etc.) compute x = 1/3 to as many decimal places as you want. The number of decimal places must be finite, however. Now compute y = 3*x. In exact arithmetic, y would be exactly 1; however, since x is not exactly one third but is a rounded approximation to one third, y will not be exactly 1.

For a readable introduction to floating point arithmetic, look at Cleve's Corner article from 1996: Floating Points (PDF) (*http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf*)

For an official "Answer" by the Mathworks in the Answers forum, read Why-does-mat2str-0-005-90-return-0-0050000000000000001-in-matlab

For more rigorous and detailed information on floating point arithmetic, read the following paper: What Every Computer Scientist Should Know About Floating Point Arithmetic (*http://docs.sun.com/source/806-3568/ncg_goldberg.html*)

Another resource is Technical Note 1108 (*http://www.mathworks.com/support/tech-notes/1100/1108.html*) on the Support section of The MathWorks website.

And yet another is Loren's blog A Glimpse into Floating-Point Accuracy

**How do I create a circle?**Edit

To create a 2D logical image of a solid circle (a disc), you can use code like this:

% Create a logical image of a circle with specified % diameter, center, and image size. % First create the image. imageSizeX = 640; imageSizeY = 480; [columnsInImage rowsInImage] = meshgrid(1:imageSizeX, 1:imageSizeY); % Next create the circle in the image. centerX = 320; centerY = 240; radius = 100; circlePixels = (rowsInImage - centerY).^2 ... + (columnsInImage - centerX).^2 <= radius.^2; % circlePixels is a 2D "logical" array. % Now, display it. image(circlePixels) ; colormap([0 0 0; 1 1 1]); title('Binary image of a circle');

It can be extended to handle ellipses by putting in factors inside the sqrt() in the obvious places. If you want, this circle mask can be used to assign image values either inside or outside the circle to a new gray level:

% Assign values inside the circle. newGrayLevelInside = 50; grayImage(circlePixels) = newGrayLevelInside; % Or, assign values outside the circle. newGrayLevelOutside = 150; grayImage(~circlePixels) = newGrayLevelOutside ;

If you want to **plot **a circle, you can use the **rectangle**() function:

radius = 5; centerX = 20; centerY = 30; rectangle('Position',[centerX - radius, centerY - radius, radius*2, radius*2],... 'Curvature',[1,1],... 'FaceColor','r'); axis square;

Newer versions of the Image Processing Toolbox have the viscircles() function for plotting multiple circles simultaneously.

radius = 5; centerX = 20; centerY = 30; viscircles([centerX, centerY], radius); axis square;

Or, if you want a list of **x and y coordinates of the perimeter** of the circle, or of an arc, you can do this:

xCenter = 12; yCenter = 10; theta = 0 : 0.01 : 2*pi; radius = 5; x = radius * cos(theta) + xCenter; y = radius * sin(theta) + yCenter; plot(x, y); axis square; xlim([0 20]); ylim([0 20]); grid on;

Note that it will be a circle according to the tick marks on the axes, but whether it appears perfectly circular on your computer monitor depends on your video adapter settings. The above code can also be used to **create an arc** - a portion of a circle. Just change the line that assigns theta to start and stop at the desired angles of the arc.

If you want this to be in the overlay of an existing plot or image, issue the "hold on" command prior to issuing the rectangle command, or else the circle will destroy the existing plot or image.

The use of **meshgrid** (or **ndgrid**) can be easily extended to 3D to create a logical mask for a sphere.

% Create a logical image volume of a sphere with specified % diameter, center, and image size. % First create the image. imageSizeX = 100; imageSizeY = 100; imageSizeZ = 100; [columnsInImage rowsInImage pagesInImage] = meshgrid(1:imageSizeX, 1:imageSizeY,1:imageSizeZ); % Next create the sphere in the image. centerX = 40; centerY = 60; centerZ = 50; radius = 36; sphereVoxels = (rowsInImage - centerY).^2 ... + (columnsInImage - centerX).^2 + (pagesInImage - centerZ).^2 <= radius.^2; % sphereVoxels is a 3D "logical" array. % Now, display it using an isosurface and a patch fv = isosurface(sphereVoxels,0); patch(fv,'FaceColor',[0 0 .7],'EdgeColor',[0 0 1]); view(45,45); axis equal; title('Binary volume of a sphere');

### **How do I create an arc?** Edit

Creating an arc is very similar to creating a circle. Simply specify a starting and ending angle for theta.

% Define parameters of the arc. xCenter = 2; yCenter = 1; radius = 5; % Define the angle theta as going from 30 to 150 degrees in 100 steps. theta = linspace(30, 150, 100); % Define x and y using "Degrees" version of sin and cos. x = radius * cosd(theta) + xCenter; y = radius * sind(theta) + yCenter; % Now plot the points. plot(x, y, 'b-', 'LineWidth', 2); axis equal; grid on;

**How do I create an ellipse?**Edit

The code is very similar to the code to create a circle from above.

% Create a logical image of an ellipse with specified % semi-major and semi-minor axes, center, and image size. % First create the image. imageSizeX = 640; imageSizeY = 480; [columnsInImage rowsInImage] = meshgrid(1:imageSizeX, 1:imageSizeY); % Next create the ellipse in the image. centerX = 320; centerY = 240; radiusX = 250; radiusY = 150; ellipsePixels = (rowsInImage - centerY).^2 ./ radiusY^2 ... + (columnsInImage - centerX).^2 ./ radiusX^2 <= 1; % circlePixels is a 2D "logical" array. % Now, display it. image(ellipsePixels) ; colormap([0 0 0; 1 1 1]); title('Binary image of a ellipse', 'FontSize', 20);

Or you can create an ellipse in the overlay of a plot or image:

xCenter = 12.5; yCenter = 10; xRadius = 2.5; yRadius = 8; theta = 0 : 0.01 : 2*pi; x = xRadius * cos(theta) + xCenter; y = yRadius * sin(theta) + yCenter; plot(x, y, 'LineWidth', 3); axis square; xlim([0 20]); ylim([0 20]); grid on;

**How do I create a ring?**Edit

The code is very similar to the code to create a circle from above - just specify an inner and outer radius.

% Create a logical image of a ring with specified% inner diameter, outer diameter center, and image size. % First create the image. imageSizeX = 640; imageSizeY = 480; [columnsInImage rowsInImage] = meshgrid(1:imageSizeX, 1:imageSizeY); % Next create the circle in the image. centerX = 320; centerY = 240; innerRadius = 100; outerRadius = 140; array2D = (rowsInImage - centerY).^2 ... + (columnsInImage - centerX).^2; circlePixels = array2D >= innerRadius.^2 & array2D <= outerRadius.^2; % circlePixels is a 2D "logical" array. % Now, display it. image(circlePixels) ; colormap([0 0 0; 1 1 1]); title('Binary Image of a Ring', 'FontSize', 25);

**How can I fit a circle to a set of XY data?**Edit

An elegant chunk of code to perform least-squares circle fitting was written by Bucher Izhak and has been floating around the newgroup for some time. The first reference to it that I can find is in msgid:<3A13371D.A732886D%40skm.com.au>:

function [xc,yc,R,a] = circfit(x,y) %CIRCFIT Fits a circle in x,y plane % % [XC, YC, R, A] = CIRCFIT(X,Y) % Result is center point (yc,xc) and radius R. A is an optional % output describing the circle's equation: % % x^2+y^2+a(1)*x+a(2)*y+a(3)=0 % by Bucher izhak 25/oct/1991 n=length(x); xx=x.*x; yy=y.*y; xy=x.*y; A=[sum(x) sum(y) n;sum(xy) sum(yy) sum(y);sum(xx) sum(xy) sum(x)]; B=[-sum(xx+yy) ; -sum(xx.*y+yy.*y) ; -sum(xx.*x+xy.*y)]; a=A\B; xc = -.5*a(1); yc = -.5*a(2); R = sqrt((a(1)^2+a(2)^2)/4-a(3));

Tom Davis provided a more sophisticated approach that works for more cases in msgid:<3C76E5AA.350DA497@eng.usf.edu> and msgid:<3C785735.F7F192BC@eng.usf.edu>. Code included.

**How can I fit an ellipse or other shape to a set of XY data?**Edit

See this paper Least-squares orthogonal distances fitting of circle, sphere, ellipse, hyperbola, and parabola

**How do I create a set of random locations within a circle?**Edit

To create a set of (x,y) coordinates within the perimeter of a solid circle (a disc), you can use code like this from Roger Stafford in his Answers posting:

% Create a random set of coordinates in a circle. % First define parameters that define the number of points and the circle. n = 5000; R = 20; x0 = 50; % Center of the circle in the x direction. y0 = 90; % Center of the circle in the y direction. % Now create the set of points. % For a full circle, use 0 and 2*pi. angle1 = 0; angle2 = 2*pi; % For a sector, use partial angles. % angle1 = pi/4; % angle2 = 3*pi/4; t = (angle2 - angle1) * rand(n,1) + angle1; r = R*sqrt(rand(n,1)); x = x0 + r.*cos(t); y = y0 + r.*sin(t); r = R*sqrt(rand(n,1)); x = x0 + r.*cos(t); y = y0 + r.*sin(t); % Now display our random set of points in a figure. plot(x,y, '.', 'MarkerSize', 5) axis square; grid on; % Enlarge figure to full screen. set(gcf, 'units','normalized','outerposition',[0 0 1 1]); fontSize = 30; xlabel('X', 'FontSize', fontSize); ylabel('Y', 'FontSize', fontSize); title('Random Locations Within a Circle', 'FontSize', fontSize);

**How do I find the area of a polygon?**Edit

You can use the built-in function polyarea. If you wish to avoid using 'polyarea', one method is this (offered by Roger Stafford). Let x and y be vectors of the corresponding coordinates of the polygon's vertices taken in counterclockwise order around the polygon.

area = 1/2*sum(x.*y([2:end,1])-y.*x([2:end,1]));

**How do I find the areas of contours?**Edit

You can find the areas of closed contours you generated with the contour function with the following code:

[x, y, z] = peaks; % Generate sample data. [C, h] = contour(x, y, z, 20); % Find contours. ch = get(h, 'children'); areas = zeros(numel(ch), 1); for k = 1 : numel(ch) % Get the x and y coordinates of this contour. x = get(ch(k), 'xdata'); y = get(ch(k), 'ydata'); [x, y] = poly2cw(x, y); % Compute area of this contour. areas(k) = polyarea(x, y); end

**How does the backslash operator work? What does it do?**Edit

For full matrices, pseudocode describing the algorithm can be found in the MathWorks Technical Solution How does the backslash operator work when A is full? (*http://www.mathworks.com/support/solutions/data/1-172BD.html?solution=1-172BD*) or Operating on Sparse Matrices (*http://www.mathworks.com/access/helpdesk/help/techdoc/index.html?/access/helpdesk/help/techdoc/math/f6-8856.html*).

Also for sparse matrices, you can turn on monitoring routines that show you some of the steps. Use spparms('spumoni', 1), or spparms('spumoni', 2)

**How does one compute a factorial?**Edit

After **MATLAB**5.3, there is a factorial function, but it is not vectorized. Why this is so we will never know. Instead, you can use the gamma function, which is vectorized. Careful, factorial(n) = gamma(n+1). If you are trying to compute a ratio of factorials, see the next question.

**How can one accurately compute ratios of factorials?**Edit

If you are trying to compute "n choose k", just use the function nchoosek. Otherwise, Paul Skoczylas suggests in msgid:<bzAM5.2437$pQ4.21981@jekyl.ab.tac.net>

If I wanted to evaluate n!/(n-j)! for large values of n and/or j (but still assuming n>j), I would use the gammaln function.

gamma(m+1)=m! gammaln(m+1)=log(m!)

Rewrite and manipulate the equation:

A=n!/(n-j)! log(A)=log(n!/(n-j)!) log(A)=log(n!)-log((n-j)!) A=exp(log(n!)-log((n-j)!))

so,

A=exp(gammaln(n+1)-gammaln(n-j+1))

**Why does MATLAB return a complex number for (-8)^(1/3)**Edit

In the same way there are two solutions (plus and minus) for the square root of a positive number, there are multiple solutions for roots of negative (and complex) numbers. If you express the number in magnitude*exp(i*theta) form, the cube root (for instance) takes the form (magnitude^(1/3))*exp(i*(theta+2k*pi)/3), for k=0:2 or 1:3.

-8 is 8*exp(i*pi), so theta=pi and the cube roots are 2*exp(i*pi/3), 2*exp(-i*pi/3), and 2*exp(i*pi). The last one simplifies to -2.

**MATLAB** always returns the first solution counter-clockwise from the positive real axis, i.e. 2*exp(i*pi/3) or: 1 + 1.732i. Armed with this knowledge, you can compute all or some particular root. For instance, if you want the negative real cube root, simply take the cube root of the absolute value of the number, and negate it.

For a different wording and more information, see the related [MathWorks Tech Solution http://www.mathworks.com/support/solutions/data/1-15M1N.html?solution=1-15M1N].

Finally, Joe Sababa suggests in msgid:<eeada27.1@WebX.raydaftYaTP> a method to find all the roots at once:

Find the roots of a polynomial:

P=[1 0 0 27]; roots(P)

An alternate method to obtain -2 as a cube root of -8 is to use the NTHROOT function:

x = nthroot(-8, 3)

x =

-2

**Can I compute the DFT of an irregularly-sampled signal?**Edit

The easiest way to find the spectrum of irregularly sampled data is to resample it to uniform samples. You can do this with MATLAB's `interp1`

function. The accuracy of your spectrum will depend on the accuracy of the interpolation. You will want to experiment with several of the interpolation methods that are available in `interp1`

. I believe that for interpolation with a limited window (i.e. interpolating a sample value from N nearest neighbors), the Lagrange interpolation is optimal, but Lagrange is not one of the choices in `interp1`

.

If interpolation doesn't work there are other schemes available. The Lomb-Scargle periodogram is often mentioned in relation to this question and may be more appropriate if your data has very uneven spacing (e.g. very large or very small spacings). I know that this algorithm is listed in Numerical Recipes, but I don't have a good on-line reference (with **MATLAB** code) to point you to.

In general, the problem is that the spacing between points determines the "importance" of a particular point. For example, if several points are very close together, then small amounts of noise on those measurements will tend to have a greater effect on inferring the slope of the function (and with it, the high frequency energy) than the same amounts of noise on measurements that are further apart.

**How can I efficiently do row-wise vector-matrix multiplication?**Edit

Elaborating on the question, the for-loop implementation looks like:

for row = 1:N output(row, :) = vector(row) * matrix(row, :); end

In recent **MATLAB** versions (>= 6.5), this is actually not such a bad idea, as the accelerator will do a good job on a loop like this.

The "old-style" vectorized version of this uses repmat.

output = matrix .* repmat(vector, 1, M);

This is a fine solution, except that having to replicate vector uses memory unnecessarily, and is less cache-efficient. The accelerated single loop may run faster in some cases, due to the better cache-usage.

In newer versions of **MATLAB** you can use bsxfun. BSX stands for Binary Singleton eXpansion. It accomplishes the same thing without the memory footprint of repmat and is more compact than a for-loop.

output = bsxfun(@times,matrix,vector);

Finally, there is a solution using sparse. Ken Davis writes:

If A is MxN and B is Nx1 or 1xN, then A*sparse(1:N, 1:N, B) multiplies the columns of A by the elements of B. Similarly, sparse(1:M, 1:M, B)*A multiplies the rows of A by the corresponding elements of B. For division, use 1./B in place of B as the argument to sparse.

This solution requires the conversion of the full vector to a sparse format, but the actual computation will be very fast. The fastest solution depends on the sizes involved, so try them all!

**How do I find "kinks" in a curve?**Edit

If you have a set of (x,y) points that define a curve, and you want to find discontinuities in the slope, or "kinks" in the curve, you can check the radius of curvature between 3 points along the curve. Please see this reference: Posting by Roger Stafford in the Answers forum

**Installation, Crashes, Platform-Specific Issues**Edit

**How do I release or deactivate a MATLAB license?**Edit

Follow the instructions in technical support solution document 1-15HLG (*http://www.mathworks.com/support/solutions/data/1-15HLG.html?solution=1-15HLG*).

**During installation, the installer crashes, hangs, gives an error message, does not authenticate your license, or otherwise does finish installing correctly.**Edit

There are so many ways that you could have problems during installation and volunteers in the Answers forum can't possibly know the solutions to all of them or even some of them. The Mathworks gives free support for all installation problems, even by telephone and even for the Student Edition. Call them at (508)647-7000. Although the Student Edition normally does not get technical support, it does get support for installation issues.

**After installation, MATLAB crashes or gives an error message when I try to run MATLAB.**Edit

The causes for crashes are many and complex, and complicated to figure out, especially if you don't have access to the source code. This is why it's best for The Mathworks to work on your crash issue. Call The Mathworks at (508)647-7000 and explain to them the circumstances for your crash. The Mathworks gives free support for all installation problems, even by telephone and even for Student, Home, or Trial versions. Note, this is just if MATLAB itself crashes, not if it's your m-file throwing an error (red text in the command window) or a DLL or mex file causing the crash.

**Why doesn't Ctrl-C interrupt execution of my m-file?**Edit

This is likely to be a problem only under Windows, where **MATLAB** must poll for Ctrl-C events. If it is deep within matrix computation, it will simply not respond. If this occurs inside a loop construct, you can force **MATLAB** to poll more often by inserting drawnow or pause(0) into the loop. This will also update your figures and make GUIs more responsive.

**How can I make MATLAB open without the GUI?**Edit

Start **MATLAB** using the command `matlab -nodesktop`

. A related switch is `-nojvm`

, which starts **MATLAB** without the Java Virtual Machine, making it take much less memory. However many of the editor and browser features will not work.

**Why can't I type into the command window?**Edit

If you are running XFree86 4.0, try turning off your num lock key. Or upgrade to the very latest XFree86 4.0.x where I believe that bug is fixed.

As a short-term workaround you can use `matlab -nojvm`

to get back to the R11 xterm interface, which doesn't have this problem.

  [edit]

**Image Processing Toolbox**Edit

**How do I split an image into non-overlapping blocks?**Edit

See this link for demo code: Split_image_into_blocks

**How do I mask an image?**Edit

To set an image to some contant value either inside or outside some defined regions, you first need to make a binary image. See this page for demo code: Mask an image.

**How do I segment and measure objects in an image?**Edit

Please refer to this demo: Image Analyst's "Image Segmentation Tutorial"

**How do I find objects of a certain color?**Edit

To find objects or regions in a certain color range, please refer to: Image Analyst's Color Segmentation Demos

**How do I extract frames from a video file?**Edit

To see how to extract individual frames from a video file and save them to disk, see this demo: Extract Frames From Movie. The demo also shows how to do image processing and analysis on the individual frames and displays the results as a function of frame number.

**How do I build a movie from individual frames?**Edit

To see how to take individual frames from image files on disk and create and play a movie from them, see the bottom part of this demo: Extract Frames From Movie

**How do I measure a distance or area in real world units instead of in pixels?**Edit

You need to measure an object of known length so that you can get a spatial calibration factor. Put a ruler, a tape measure, a standard sheet of paper, or anything else of known length into the scene. Then snap a picture of it and use imdistline() to find the length in pixels. For example, let's say you laid down a sheet of A4 paper and measured it's distance as 2100 pixels. Knowing that it is 21 cm long, you get a spatial calibration factor of 21/2100, which you multiply your distances in pixels by to get the distance in cm. Multiply by that calibration factor squared to convert the area in pixels to the area in square centimeters.

calibrationFactor = 21/2100; distanceInCm = distanceInPixels * calibrationFactor; areaInSquareCm = areaInPixels * calibrationFactor ^ 2;

**MATLAB Compiler Toolbox**Edit

**How can I make a standalone executable from my MATLAB code?**Edit

To generate a standalone executable that you can execute outside **MATLAB** or on a computer without **MATLAB**, you will need to use the **MATLAB** Compiler.

http://www.mathworks.com/products/compiler/

The section titled "Deployment Process" in the documentation for this product gives instructions on what steps you need to perform to run the executable on a machine that does not have **MATLAB** installed.

If something doesn't compile as expected, please refer to MATLAB's page:

Limitations About What May Be Compiled

**My standalone executable won't run on the target computer. What can I try?**Edit

If you've deployed (installed) your standalone executable application on the end-user's computer and it won't run, there are various things you can try to fix the situation:

**1)** Make sure that you have installed the MCR (MATLAB Compiler Runtime) on the target computer that is** the same version that you have on the system that compiled the program**. A different version will not work. For example, a program compiled with version 8.1 will not work on a computer that does not have that exact 8.1 version installed on it. Earlier or later versions will not work. If this is the problem, then you should see an error message notifying you of this in the console window that appears as the application tries to launch. You can have multiple, even older, versions of MCR on the target computer without any conflict, but you * must *at least have the same version it was compiled with.

**2)** Run the Dependency Report to see what functions and files are called. On MATLAB since R2013b, click the down arrow on the title bar of the "Current Folder" panel and select "Reports/Dependency Report". On older versions, use the "Tools/Show dependency report" pull down menu item. See what files are called. Pay particular attention to those that are labeled "other."

**3)** Try fdep in the File Exchange. It's like a souped up version of the dependency report tool that's built into MATLAB. Look at all the modules that are listed. It sometimes finds modules that the Dependency Report tool doesn't.

**4)** Run DependencyWalker on the executable and look for any missing components. This must be run on the target computer, not the computer that compiled it, unless it won't run on that computer either.

**5)** Make sure any ancillary files that are needed at run time are shipped with the installation package. This includes DLL's, OCX's, initialization/configuration/mat files, sample data files, documentation, Excel templates, etc.

**6)** Call the Mathworks and let them figure it out. Sometimes the mistake is on their side. For example, one time they figured out that there was something missing in the image processing toolbox or the compiler so that compiled versions didn't run. They made a patch for it.

**7)** Also be aware that the MCR installer may claim that it will install the Microsoft Visual C redistributable. However that never seems to work for me and Dependence Walker will show that it can't find msvcr80.dll and msvcr90.dll. In this case, go to the Microsoft web site and download and install the redistributable yourself. You can also try here: Latest Supported Visual C++ Downloads. After doing that it seems to work most of the time. Sometimes the installer just quits after installing the Visual C++ redistributable and never installs the actual MATLAB run time library software. Some have found that rebooting after that first installation where it quits will allow you to rerun the MCRInstaller.exe and it will then finish.

**8)** Check your startup.m code for commands that should not be run in a deployed executable. When your compiled app runs, it also executes the startup.m file that you had when you compiled the code. So whatever code you had in *your* startup.m file will also get run on the target computer. It is not obvious to people that their startup.m code gets run before the actual code that they wanted to run gets run, but it does. So make sure that everything that's in there makes sense when run on the target computer. For example if your startup.m code changes directory to a particular directory, or reads a specific file, and that file does not exist on the target computer, it will throw an error or warning. **You can have code in your startup.m file that runs only on your computer while in development mode, and other code that is run only in deployed code on the target computer if you check the isdeployed variable**. It is a built-in variable that is true if the code is compiled and false if it's source code running in the normal MATLAB development environment. For example:

if isdeployed % Code that gets run only by compiled applications uiwait(msgbox('This is a compiled application')); else % Code that gets run only in development environment uiwait(helpdlg('You are in development mode.')); % For example, set current folder to your main working code folder. cd('C:/SomeFolderOfYours'); end

**9)** Have the startup code for your application print out ctfroot to the console window to see where the executable actually unpacks the executable to.

fprintf(1, '%s', ctfroot);

If you refer to subfolders in your application, they will actually be under this folder, which is usually in a temporary, hidden folder (in Windows, it's under C:\Documents and Settings\username\Local Settings\Temp) and not where you installed your executable.

**10)** If your application tries to call a DLL with a different number of bits, you will have trouble - it may not launch. For example, you cannot compile a 64 bit m-file that tries to call a 32 bit DLL. This is a common problem with users trying to call old DLLs from third party manufacturers (e.g. manufacturers of cameras or other peripherals). If you're calling a 32 bit DLL, you **MUST **use the 32 bit version of MATLAB to compile it, *and *install the 32 bit version of the MCR from the matching MATLAB Release on the target computer. The target computer can be either a 32 bit computer or a 64 bit computer. You can install * both *the 32 bit and 64 bit versions of MATLAB on

*your*computer and they will live happily together with no conflicts.

**11)** If you're running the executable by double-clicking the icon in Windows Explorer, and it encounters an unhandled error (an error that you don't catch in a try/catch block), then the application may just exit before you ever see any error messages. To make sure you see the error messages before they vanish, run the program from a console window (an MS-DOS command prompt). From the start menu, type cmd in the run box to get a console window. Then use cd to navigate to the folder where you have your executable. Then type in the name of the executable. When it runs, any errors will go to the console window but the console window will not close and you'll have the ability to see the errors. You might also be able to do this from the MATLAB command window prompt if you type an exclamation point (bang) followed by the name of the executable.

**12)** Some **Simulink **blocks implement portions of their functionality using shared libraries (DLL files on Windows). Additionally, in some cases EXEs generated from models using these blocks are dependent on these shared libraries as well. In this case it seems like the Simulink UDP Send block has some of these dependencies.

The **PACKNGO **function can be used to locate the needed shared libraries. Go into your modelname_target_rtw folder and load the 'buildInfo' object into the workspace from the "buildInfo.mat" file. Then you can execute the command

>> packNGo(buildInfo);

This will create compressed folder in the same directory containing the generated EXE. This compressed folder should contain any DLLs that the generated EXE is dependent on. When running the EXE outside of the MATLAB/Simulink environment these DLLs need to be on the system path or within the same directory as the EXE. This information came from the Answers forum

**Why can't my standalone compiled executable find my files?**Edit

The folder where you installed your compiled executable is not the actual folder that your application runs out of. For example, it's not really in C:\Program Files\MyApp (or wherever), it's really in some secret hidden folder - for example, in C:\Users\MyUserName\AppData\Local\Temp\MyUserName\mcrCache7.16\MyApp\. So if you put any files in the folder where you think the exe is (C:\Program Files\MyApp), it won't find them because the exe is not really there. The exe in *that* folder is actually a self-extracting archive that unzips the *real* executable to the secret folder. So your file would have to be located there in that secret folder for your executable to see it (that is, if you neglected to include the full path and just assume it would find your file in the *current working directory*, which is bad practice). The ways around this are to ship the files with the -a option of mcc, *or* include it in deploytool if you're using deploytool to create your executable. Or better yet, specify the full path of any files you use and make sure they're there. Use fullfile() to create full file names, and exist(filename, 'file') to check for them ebfore you use them. Alert the user with uiwait(warndlg(yourWarningMessage)) if the file does not exist. Create yourWarningMessage with sprintf() explaining the missing file to your user.

If you don't want your exe to unpack the archive to a secret hidden folder, then (in Windows) you can set the MCR_CACHE_ROOT environment variable to "." (dot). Then it will unpack a bunch of stuff (subfolders, etc.) to the current folder (where at least you can see it and know it's there).

**How can I suppress the black console window (DOS command window) when I run my standalone executable?**Edit

See the Compiler Documentation. You have to use the -e option.

"Suppress appearance of the MS-DOS command window when generating a standalone application. Use *-e* in place of the *-m* option. This option is available for Windows only. Use with -R option to generate error logging as such:

mcc -e -R -logfile -Rfilename-vfunction_name

or:

mcc -e -R '-logfilelogfilename'-vfunction_name

You can also suppress the MS-DOS command window when using deploytool by creating a Windows Standalone Application."

**What is the path in my deployed application?**Edit

Please read Loren's discussion here: Path management in deployed applications.

**Toolboxes, Blogs, Software Archives, Books, and Other Resources**Edit

**What toolboxes or other m-files does my m-file use?**Edit

First of all, you can find out what toolboxes are installed on your computer by issuing the "ver" command in the MATLAB command window. To find out what toolboxes are required by any specific m-file, you can:

- Select "
**Tools->Show Dependency Report**" from the main MATLAB pulldown menu. To get the Tools menu to show up, you must have focus in the Editor window (not the Command window). In other words, you must have clicked last in the Editor window to make it the active window.

- You can use "fdep" from the File Exchange. This sometimes finds dependent files that the dependency report doesn't find.

- You can use "depfun" from the File Exchange.

**Where can I find the release notes for all the versions?**Edit

The Release Notestell you what new features were introduced, or what bugs were fixed, in versions since the year 2004. Other than the direct link just given, here's a way to navigate to the release notes for current and older versions of MATLAB.

- Start at The MathWorks home page.
- Click on Support.
- Click on Product Documentation.
- Click on MATLAB.
- Click on MATLAB Release Notes.

Note that release notes are available there for releases back to R14 in 2004. The release notes summarize new features, compatibility considerations, and fixed and known bugs.

Similarly, you can find release notes for the Image Processing Toolbox going back to version 2.2.2 in 2000.

### Edit

Official Mathworks blogs:

Other users' blogs:

http://www.cb.uu.se/~cris/blog/

http://imageprocessingblog.com/

**Where can I find an archive of user-contributed MATLAB software?**Edit

The MathWorks has a website for user-contributed **MATLAB** files:

http://www.mathworks.com/matlabcentral/fileexchange/ .

**Where can I find a package to perform a specific task?**Edit

First, check the File Exchange for free, user-contributed code submissions:

http://www.mathworks.com/matlabcentral/fileexchange/

Next, look to the Mathworks web site's product listing:

http://www.mathworks.com/products/product_listing/index.html

Finally, you can look to other specific user-written toolboxes, such as:

Styled Text Toolbox: written by Doug Schwarz, this toolbox allows extremely flexible formatting of text strings, including symbols, math formulae, etc. Its TeX interpreter is much more complete than the builtin MATLAB interpreter. http://www.frontiernet.net/~dmschwarz/stextfun/index.html

MATLAB and LaTeX: Arno Linnemann has written an M-file to simplify the inclusion of MATLAB graphics into LaTeX documents, along with a nice document of tips and tricks. http://www.uni-kassel.de/~linne/**matlab**/WelcomeEng.html

Genetic Algorithm Optimization Toolbox: GAOT implements simulated evolution in the MATLAB environment. Written by the Meta-Heuristic Research and Applications Group at the North Carolina State University Department of Industrial Engineering: http://www.ise.ncsu.edu/mirage/GAToolBox/gaot/ (WARNING: as of Nov. 10, 2010 that link is defunct).

**What textbook would you recommend for learning MATLAB?**Edit

One choice is the image processing book written by Steve Eddins. Steve Eddins is a software development manager in the MATLAB and image processing areas at MathWorks. Steve coauthored Digital Image Processing Using MATLAB. He writes about image processing concepts, algorithm implementations, and MATLAB, both in the book and on his Steve on Image Processing blog.

**Where can I find MATLAB Style Guides or Coding Standards?**Edit

Richie Cotton's guide: http://4dpiecharts.com/matlab-code-style-guide/

Richard Johnson's guide: http://www.mathworks.com/matlabcentral/fileexchange/2529

David Schwartz's guide: http://www.cs.cornell.edu/courses/cs99/2002fa/matlabstyle.html

**Where are the official Mathworks Tech Notes for MATLAB and Simulink?**Edit

There aren't many of them but they can be found off the Mathworks Support web page here: Tech Notes and How-to Guides

**Where are the official Mathworks Bug Reports for MATLAB and Simulink?**Edit

They can be found off the Mathworks Support web page

**What toolboxes or other m-files does my m-file use?**Edit

First of all, you can find out what toolboxes are installed on your computer by issuing the "ver" command in the MATLAB command window. To find out what toolboxes are required by any specific m-file, you can:

- Select "
**Tools->Show Dependency Report**" from the main MATLAB pulldown menu. To get the Tools menu to show up, you must have focus in the Editor window (not the Command window). In other words, you must have clicked last in the Editor window to make it the active window.

- You can use "fdep" from the File Exchange. This sometimes finds dependent files that the dependency report doesn't find.

- You can use "depfun" from the File Exchange.

## SimulinkEdit

### How do I insert a Simulink model into an MS Office Document?Edit

### What are best practices for modeling communications systems with Simulink?Edit

## MiscellaneousEdit

**Can you program up the algorithm in this article for me and explain it to me?**Edit

You may find that the algorithm in the scientific article is interesting and may be useful to you, but you may not understand it, or you may not know how to program it up in MATLAB, so you ask if anyone in the Answers forum or newsgroup will program it up for you. Unfortunately, the newsgroup and Answers volunteers generally don't have the time to read some scientific article, understand it themselves, then explain it to you, code it up for you, and finally hand over the code to you.There are just too many papers - we can't even find the time to read the papers that *we* find interesting, much less other ones. If you really need it done, here are some options for you:

- Ask the authors of the paper for the source code
- Write it yourself
- Hire The Mathworks to write it for you [3]
- Hire a consultant or university to write it for you.

**How do I run MATLAB in batch mode?**Edit

On the PC, you can use the `/r`

flag to start your m-file immediately upon starting **MATLAB**. Using this in combination with the Windows Task Manager, you can set a **MATLAB** job to run when you want it to run, even if you are not at the computer. Please note that you will need to exit **MATLAB** at the end of the m-file. Alternately, if you only want a single file to run, you can name it startup.m and place it in your **MATLAB** directory.

For UNIX, you can use the `at`

or `cron`

commands to execute a script which runs **MATLAB**. In the script, use this general syntax:

matlab < MyMFile

If you want to record output, either use the SAVE command inside your m-file or redirect the output of the **MATLAB** session to a file using this syntax:

matlab < MyMFile > MyOutputFile

This syntax only works for scripts. If you need to run a function, create a wrapper script that calls the function.