Difference between revisions of "FAQ"
(→Why is Octave's floatingpoint computation wrong?: Grammatical errors & spelling mistakes) 

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Floatingpoint arithmetic is an approximation '''in binary''' to arithmetic on real or complex numbers. Just like you cannot represent 1/3 exactly in decimal arithmetic (0.333333... is only a rough approximation to 1/3), you cannot represent some fractions like <math>1/10</math> exactly in base 2. In binary, the representation to one tenth is <math>0.0\overline{0011}_b</math> where the bar indicates that it repeats infinitely (like how <math>1/6 = 0.1\overline{6}_d</math> in decimal). Because this infinite repetition cannot be represented exactly with a finite number of digits, rounding errors occur for values that appear to be exact in decimal but are in fact approximations in binary, such as for example how 0.3  0.2  0.1 is not equal to zero.  Floatingpoint arithmetic is an approximation '''in binary''' to arithmetic on real or complex numbers. Just like you cannot represent 1/3 exactly in decimal arithmetic (0.333333... is only a rough approximation to 1/3), you cannot represent some fractions like <math>1/10</math> exactly in base 2. In binary, the representation to one tenth is <math>0.0\overline{0011}_b</math> where the bar indicates that it repeats infinitely (like how <math>1/6 = 0.1\overline{6}_d</math> in decimal). Because this infinite repetition cannot be represented exactly with a finite number of digits, rounding errors occur for values that appear to be exact in decimal but are in fact approximations in binary, such as for example how 0.3  0.2  0.1 is not equal to zero.  
−  In addition, some advanced operations are computed by approximation and  +  In addition, some advanced operations are computed by approximation and there is no guarantee for them to be accurate, see [https://en.wikipedia.org/wiki/Rounding#Tablemaker.27s_dilemma Tablemaker's dilemma] for further references . Their results are systemdependent. 
−  This isn't  +  This isn't a bug that is limited to GNUOctave & it happens with any program that uses [https://en.wikipedia.org/wiki/IEEE_754 IEEE 754 floatingpoint arithmetic]. To be fair, IEEE 754 also specifies decimal floatingpoint arithmetic, which has never seen wide adoption. The reason why Octave and other programs using IEEE 754 binary floatingpoint numbers is that they are ''fast'', because they are implemented in hardware or system libraries. Unless you are using very exotic hardware, Octave will use your computer's processor for basic floatingpoint arithmetic. 
−  Another approach to deal with rounding errors is interval arithmetic with the [[Interval package]] or symbolic  +  Another approach to deal with rounding errors is interval arithmetic with the [[Interval package]] or symbolic computations with the [[Symbolic package]]. These approaches are likely to be slower, since not all operations can be performed on Hardware like pure floatingpoint arithmetic. 
−  To learn more about floatingpoint arithmetic, consult  +  To learn more about floatingpoint arithmetic, consult this [https://en.wikipedia.org/wiki/Floatingpoint_arithmetic Wikipedia article] or the classical reference by David Goldberg [http://docs.oracle.com/cd/E1995701/8063568/ncg_goldberg.html What Every Computer Scientist Should Know About FloatingPoint Arithmetic]. 
==Missing lines when printing under Windows with OpenGL toolkit and Intel integrated GPU==  ==Missing lines when printing under Windows with OpenGL toolkit and Intel integrated GPU== 
Revision as of 05:17, 8 May 2020
This is a list of frequently asked questions (FAQ) for GNU Octave users.
We are always looking for new questions (with answers), better answers, or both. Feel free to edit this page with your changes.
Where do I get additional help?
If you can't find an answer to your question in this FAQ, wiki, or in the manual (PDF) you can:
 Search for an answer in our mailing list archives
 Contact our user community using our help@octave.org mailing list (feel free to subscribe to this mailing list for the latest updates and discussions)
 Contact our user community using our IRC chat room
#octave
in Freenode
General
What is Octave?
GNU Octave is a highlevel interpreted language, primarily intended for numerical computations. It provides capabilities for the numerical solution of linear and nonlinear problems, and for performing other numerical experiments. It also provides extensive graphics capabilities for data visualization and manipulation. GNU Octave is normally used through its interactive interface (CLI and GUI), but it can also be used to write noninteractive programs. The GNU Octave language is quite similar to Matlab so that most programs are easily portable.
The GNU Octave distribution includes a 1000+ page Texinfo manual. Access to the complete text of the manual is available via the doc
command at the GNU Octave prompt.
What is Octave Forge?
Octave Forge is a collection of packages for GNU Octave, something similar to the Matlab toolboxes. When talking about the two projects at the same time, GNU Octave is usually referred to as Octave core (or just "core"). Octave Forge also serves as a test bed for code that may eventually end up in the core, and distributes binaries for systems with a lack of developers tools (mainly Windows).
Who uses Octave?
A huge number of people ranging from students to researchers involved in various fields such as statistics,Machine Learning, data analytics, etc . Universities use it for research and teaching, companies of all sizes for development and individuals for certain private purposes. See Who Uses Octave? for more clarity .
Who develops Octave?
Discussions about writing the software that would eventually become Octave started in about 1988 with James B. Rawlings and John W. Eaton at the University of Texas. John W. Eaton is the original author of Octave, starting fulltime development in February 1992. He is still the primary maintainer. The community of users and developers has in addition contributed some code and fuels the discussion on the mailing lists help@octave.org (user forum), maintainers@octave.org (development issues). Since 2011, GNU Octave regularly participates in Summer of Code events as a mentor organisation. As part of those events, several students contributed to Octave by helping in removal of bugs and development of new features.
Why "Octave"?
Octave's name has nothing to do with music. It is named after Octave Levenspiel, a former professor of John who was famous for his ability to do quick backoftheenvelope calculations. You can hear John pronounce the name "Octave" a few times in this video. We hope that GNU Octave will help perform computations with the same ease as Dr. Levenspiel.
Why "GNU" Octave?
The GNU Project was launched in 1984 to develop a complete Unixlike operating system which is free software: the GNU system. GNU is a recursive acronym for "GNU's Not Unix"; it is pronounced g'noo.
The Free Software Foundation (FSF) is the principal organisation that has sponsored the GNU Project.
Octave became GNU Octave in 1997 (beginning with version 2.0.6). This meant agreeing to consider Octave a part of the GNU Project and support the efforts of the FSF. A big part of this effort is to adhere to the GNU coding standards and to benefit from GNU's infrastructure (e.g. code hosting and bug tracking). Additionally, Octave receives sponsorship from the FSF's Working Together fund. However, Octave is not and has never been developed by the FSF.
How can I cite Octave?
Octave is free software and does not legally bind you to cite it. However, we have invested a lot of time and effort in creating GNU Octave, and we would appreciate if you would cite if you used. To cite GNU Octave in publications use:
John W. Eaton, David Bateman, Søren Hauberg, Rik Wehbring (2021). GNU Octave version 6.4.0 manual: a highlevel interactive language for numerical computations. URL https://www.gnu.org/software/octave/doc/v6.4.0/
A BibTeX entry for LaTeX users is:
@manual{, title = {{GNU Octave} version 6.4.0 manual: a highlevel interactive language for numerical computations}, author = {John W. Eaton and David Bateman and S{\o}ren Hauberg and Rik Wehbring}, year = {2021}, url = {https://www.gnu.org/software/octave/doc/v6.4.0/}, }
Run citation
at the Octave prompt for details on how to best cite the Octave version you are running. Certain Octave packages also have recommended citations in which case use citation package_name
.
Note that there are two reasons for citing the software used. One is giving recognition to the work done by others which we have already addressed to . The other is giving details on the system used so that experiments can be replicated. For this, you should cite the version of Octave and all packages used (you can get this information using the ver
command), as well as any details of your setup as part of your Methods. In addition, you should make your source available as well . See How to cite and describe software for more details and an in depth discussion for the same .
What documentation exists for Octave?
Besides this wiki, the GNU Octave distribution includes a 1000+ page Texinfo manual (PDF). Access to the complete text of the manual is available via the doc
command at the GNU Octave prompt. If you have problems using this manual, or find that some topic is not adequately explained, indexed, or crossreferenced, please report it on http://bugs.octave.org.
How can I report a bug in Octave?
Please read our website http://www.octave.org/bugs.html.
Common problems
Octave does not start
The following steps have been the solution to several bug reports and help requests. Please try them before asking for further support. If nothing below helps, please give us the following information:
 Operating system: e.g. Windows 10 (version 1909) or Ubuntu 20.04
 GNU Octave version: e.g. Version 6.4.0
 Installation method: e.g. Downloaded and installed "octave6.4.0w64installer.exe" from https://www.octave.org/download.html
MS Windows
 After upgrade the GUI does not open / shuts down immediately.
 Solution: Delete the folder C:\Users\YOUR_USER_NAME\.config\octave
 Missing/conflicting files.
 Solution: Remove/Uninstall all existing Octave versions. Restart the system. Install GNU Octave again.
 Permission errors.
 Solution 1: Consult your malware detection (a.k.a. AntiVirus) software, if files are blocked.
 Solution 2: Did you install Octave on a networkdrive? Do you have the execution permissions?
I do not see any output of my script until it has finished?
By default Octave is set to pass its screen output through a pager (usually the default pager is "less") which allows things such as navigating through the output with arrow keys or searching for text or regular expressions within the output. The pager only displays the output after it's finished receiving it, so when it is active you'll not be able to see anything until your script has terminated. To change this behavior temporarily or permanently you may want to use one of the options described in the manual.
When I try plotting from a script, why am I not seeing anything?
If you are running an Octave script that includes a plotting command, the script and Octave may terminate immediately. So the plot window does show up, but immediately closes when Octave finishes execution. Alternatively, if using fltk, the plot window needs a readline loop to show up (the time when Octave is sitting around doing nothing waiting for interactive input).
A common solution is to put a pause
command at the end of your script.
How do I get sound input or output in MS Windows?
Sound input from a sound card and output to a sound card is fully supported since Octave 4.0.0 for all platforms. If you have problems with the audio I/O functions using Octave 4.0.0 or a newer version, please report them on the bug tracker.
I have problem X using the latest Octave version
Please be more specific about what you mean by "latest version"?
 The latest stable version is 6.4.0. Be aware that you may still have an older version due to whatever distribution method you are using. To get a newer stable version for your system see the following as in accordance to your Operating system: GNU/Linux, macOS, or Windows.
 If you refer to the latest Mercurial revision, please specify the changeset ID not the revision number, e.g. the output of
hg summary
orhg id
. Changeset IDs are globally unique across all repos.
If your problem still persists with the "latest version", then please report a bug or ask for help at help@octave.org. Otherwise, don't be surprised if volunteers are less inclined to help you with a problem that only exists in an older version of Octave and is already fixed in a newer version.
Why is Octave's floatingpoint computation wrong?
Floatingpoint arithmetic is an approximation in binary to arithmetic on real or complex numbers. Just like you cannot represent 1/3 exactly in decimal arithmetic (0.333333... is only a rough approximation to 1/3), you cannot represent some fractions like exactly in base 2. In binary, the representation to one tenth is where the bar indicates that it repeats infinitely (like how in decimal). Because this infinite repetition cannot be represented exactly with a finite number of digits, rounding errors occur for values that appear to be exact in decimal but are in fact approximations in binary, such as for example how 0.3  0.2  0.1 is not equal to zero.
In addition, some advanced operations are computed by approximation and there is no guarantee for them to be accurate, see Tablemaker's dilemma for further references . Their results are systemdependent.
This isn't a bug that is limited to GNUOctave & it happens with any program that uses IEEE 754 floatingpoint arithmetic. To be fair, IEEE 754 also specifies decimal floatingpoint arithmetic, which has never seen wide adoption. The reason why Octave and other programs using IEEE 754 binary floatingpoint numbers is that they are fast, because they are implemented in hardware or system libraries. Unless you are using very exotic hardware, Octave will use your computer's processor for basic floatingpoint arithmetic.
Another approach to deal with rounding errors is interval arithmetic with the Interval package or symbolic computations with the Symbolic package. These approaches are likely to be slower, since not all operations can be performed on Hardware like pure floatingpoint arithmetic.
To learn more about floatingpoint arithmetic, consult this Wikipedia article or the classical reference by David Goldberg What Every Computer Scientist Should Know About FloatingPoint Arithmetic.
Missing lines when printing under Windows with OpenGL toolkit and Intel integrated GPU
Some windows users with integrated Intel GPUs have reported missing lines when printing with an OpenGL toolkit like FLTK or Qt. #42534
Users with this kind of problem should try to install/update their Intel OpenGL drivers for Windows or consider installing Mesa drivers from http://qtproject.org/wiki/CrosscompilingMesaforWindows .
See also https://www.opengl.org/wiki/FAQ#Why_is_my_GL_version_only_1.4_or_lower.3F .
Plot hangs and makes the GUI unresponsive
If the Qt graphics toolkit is used and "plot" is used for the first time, the fontconfig scanner searches the font directory to build a font cache. This can take up to 3min on slow CPUs. See #45458
Error message about invalid call to script or invalid use of script in index expression
If Octave shows an error message about invalid call to script .../close.m
or invalid use of of script .../close.m in index expression
, it means that you have created a script called close.m that is overriding the builtin Octave function close
. Octave functions and scripts share the samem global namespace. It is best to avoid creating your own scripts or functions that have the same name as an Octave function.
Licensing issues
If I write code using Octave do I have to release it under the GPL?
The answer depends on precisely how the code is written and how it works:
 Code written entirely in the scripting language of Octave (interpreted code in .m files) may be released under the terms of whatever license you choose.
 Code written using Octave's native code interface (also known as a .oct file) necessarily links with Octave internals and is considered a derivative work of Octave. Therefore it must be released under terms that are compatible with the GPL.
 Code written using Octave's implementation of the Matlab MEX interface may be released under the terms of whatever license you choose, provided that the following conditions are met:
 The MEX file may not use any bindings that are specific to Octave, it has to use the MEX interface only. In other words, it should be possible in principle to use the MEX file with other programs that implement the MEX interface (e.g., Matlab). For example including an Octave header file or calling an Octave function within the MEX file, that is not related with Octave's implementation of the MEX interface make the MEX file a derivative work of Octave and has therefore to be released under terms that are compatible with the GPL.
 The MEX file may not be distributed together with Octave in such a way that they effectively create a single work. For example, you should not distribute the MEX file and Octave together in a single package such that Octave automatically loads and runs the MEX file when it starts up. There are other possible ways to effectively create a single work; this is just one example.
 Code that embeds the Octave interpreter (e.g., by calling the
octave_main
function), or that calls functions from Octave's libraries (e.g., liboctinterp, liboctave, or libcruft) is considered a derivative work of Octave and therefore must be released under terms that are compatible with the GPL.
Will you change the license of the Octave libraries for me?
No. Instead of asking us to change the licensing terms for Octave, we recommend that you release your program under terms that are compatible with the GPL. This way the free software community can benefit from your work the same as you have benefited from the work of all the people who have contributed to Octave.
Should I favor the MEX interface to avoid the GPL?
No. The original reason for implementing the MEX interface for Octave was to allow Octave to run free software that uses MEX files (the particular goal was to run sundialsTB in Octave). The intent was to liberate that software from Matlab and increase the amount of free software available to Octave users, not to enable people to write proprietary code for Octave. For the good of the community, we strongly encourage users of Octave to release the code they write for Octave under terms that are compatible with the GPL.
Why can't I use code from File Exchange in Octave?
According to the Matlab Central Terms of Use (Last updated: 10Aug2016), all submitted code is licensed under the BSD license by default (cf. § 5 Terms of Use), but it is clearly stated that:
Content submitted to File Exchange may only be used with MathWorks products.—§ 2(a)(iii) Terms of Use
That does not apply to GNU Octave, therefore the usage is in general prohibited. It should suffice — although interpretations of this vary — to contact the author directly to send you the code personally (maybe released under a free license), or download the code from the author's own website, if available. Some examples of letters/email sent to authors for that purpose.
Installation
How can I install Octave on Windows?
How can I install Octave on macOS?
 See: Octave for macOS
How can I install Octave on GNU/Linux?
 See: Octave for GNU/Linux
How can I install Octave on Android / what is this Octave app in the Google Play store?
There is an unofficial Octave app available for Android in the Google Play store. Please see Octave for Android for more information.
How can I install Octave on platform X?
Octave currently runs on GNU/Linux, macOS, and Windows. It should be possible to make Octave work on other systems as well. If you are interested in porting Octave to other systems, please contact the maintainers development mailing list maintainers@octave.org.
What Octave version should I use?
For general use, it is recommended to use the latest stable version of Octave (currently 6.4.0), available from http://www.octave.org/download.html. For development and bleedingedge features one can obtain the development source code from the Mercurial repository https://hg.savannah.gnu.org/hgweb/octave/graph/.
The used version of Octave is available via the ver
command and a list of uservisible changes since the last release is available via the news
command at the GNU Octave prompt.
On what platforms does Octave run?
Octave runs on any platform you can compile it on. Binary distributions exist for GNU/Linux, macOS, and Windows. To work fully functional, Octave requires the used platform to support the underlying numerical libraries like BLAS, LAPACK, SuiteSparse, etc., and for plotting OpenGL or gnuplot.
How can I obtain Octave's source code?
The latest version of the Octave source code (and older versions) is available from:
Since Octave is distributed under the terms of the GPL, you can get Octave from a friend who has a copy.
How can I build Octave from the source code?
To use Octave it is usually not required to build it from it's source code. Binary distributions exist for GNU/Linux, macOS, and Windows.
If you have reasons to build Octave from the source code, see Building for more information.
What do I need to build Octave from the source code?
For a list of build dependencies see Building.
Do I need GCC to build Octave from the source code?
No. The development is done primarily with GCC, so you may hit some incompatibilities. Octave is intended to be portable to any standard conforming compiler (for example clang is know to work as well). If you have difficulties that you think are bugs, please report them to the bug tracker, or ask for help on the help@octave.org mailing list.
What's new in Octave?
Each new Octave release introduces many new features. A complete list of user visible changes can be seen by running news
at the Octave prompt. The following changes are a distilled list of the major changes:
What's new in the next version of Octave?
See the NEWS file on the development branch.
What's new in version series 4.0.X of Octave
See the complete uservisible changes on the NEWS file.
 First official release of the GUI.
 Release of official windows binaries.
 Experimental support for classdef.
 OpenGL graphics with Qt widgets.
 Several functions for reading, writing, and recording of audio.
What's new in version series 3.8.X of Octave
See the complete uservisible changes on the NEWS file.
 Experimental GUI interface.
 OpenGL graphics with fltk widgets.
 Support for nested functions.
 Support for java packages in Octave core.
 Reading and writing of image files vastly extended.
What's new in version series 3.6.X of Octave
See the complete uservisible changes on the NEWS file.
 Perl compatible regular expressions
 A profiler has been added.
 Broadcasting enabled for all builtin binary elementwise operators.
 Performance of all mfile string functions has been improved.
What's new in version series 3.4.X of Octave
See the complete uservisible changes on the NEWS file.
 Many improvements to native OpenGL plotting
 ARPACK now distributed with Octave
 Indexing optimizations
 FTP objects
 Function handles aware of overloaded functions
 bsxfun optimized for basic arithmetic functions
 Matlabstyle ignoring of output arguments using
~
 Many optimizations of the accumarray function
 Sparse matrix indexing has been rewritten for speed
 The pkg command now accepts a forge option to pull packages directly from Octave Forge
What's new in version series 3.2.X of Octave
See the complete uservisible changes on the NEWS file.
 Single precision type
 Experimental OpenGL graphics toolkit to replace gnuplot
 Object orient programming via @class named directories
 64bit compilation support
 gzipped files and stream and consequently support of Matlab v7 files
 a fully compatible MEX interface
 imwrite and imread (based on the GraphicsMagick library)
 Lazy transpose: Special treatment in the parser of things like "a' * b", where the transpose is never explicitly formed but a flag is rather passed to the underlying LAPACK code.
Older releases
For full details on older releases, see:
Packages and Octave Forge
How do I install or load all Octave Forge packages?
Do not do it! Really, there is no reason to do this. Octave has many packages for different needs and is unlikely that you need all of them. You either have a small set of required packages, in which case you know them by name; or you want them all "just because", in which case you don't really need them.
The common misconception is that the more packages one has installed and loaded, the more complete and powerful its Octave installation will be. However, in the same way one would never install all perl modules, ruby gems, python packages, and C++ libraries (because it simply makes no sense), one should not install all Octave packages.
Packages should be installed and loaded selectively. Note that some packages are meant to shadow core functions changing the way Octave works, and that different packages can have different functions with the same name leading to unpredictable results.
If you really really really want to do load all packages, you can with the following:
## WARNING: loading all packages is probably not the solution you are looking for.
cellfun (@(x) pkg ("load", x.name), pkg ("list"));
I have installed a package but still get a "foo undefined" error?
You have probably forgotten to load the package. Use pkg load packagename
to load it. Most packages are no longer loaded automatically to avoid surprises. See reasoning on related FAQ how do I install all Octave packages. If you want a specific package to be loaded by default at startup, consider adding the pkg load
command on your .octaverc file.
I cannot install a package. Octave complains about a missing mkoctfile.
You should normally use your distribution's packages. For Debian and Fedora, Octave package foo
will be a deb or rpm called octavefoo
, and you should install that instead using apt
or yum
.
If you really need to build Octave packages from source to install them, you'll need mkoctfile
. Most distributions split Octave into several packages. The script mkoctfile
is then part of a separate package:
 Debian/Ubuntu: liboctavedev
 Fedora:
octavedevel
How do I automatically load a package at Octave startup?
When Octave starts, it runs the file ~/.octaverc (in your user's home directory). If you want Octave to automatically load a package, simply add a pkg load pkgname
command to it. If the files does not exist, create it.
If you do this, remember that other people may not have Octave configured to load packages at startup. Therefore, if you write code for others, remember that your programs still need to load the packages they require.
Octave usage
How do I execute an Octave script?
First of all, make sure you understand the difference between script files and function files. If you want to execute a function defined in a file, just call the function like any other Octave function: foo(arg1, arg2);
To execute a script from within Octave, just type its name without the .m
extension. Thus, if you have a script called foo.m
, just type foo
from within the Octave command prompt to execute it. You have to make sure that the script is in your current working directory or in Octave's load path. Type pwd
to get the current working directory or type path
to see which paths belong to Octave's load path. The current working directory is referred to as "." in the output of path
.
If the script name has characters that are not valid for an Octave identifier, or if you do not want to use addpath
to add the script's location to the current path, you can use the run
function instead:
run ("Script Name With Spaces.m")
run ("/opt/local/foo.m")
An alternative is to run the script from outside Octave by calling Octave from your operating system shell. Unlike calling the script from inside Octave, this also allows you to pass arguments from the shell into the script, which the script can access using the argv
command:
$ octave thescript.m arg1 arg2
In a Unix environment, if the script has a shebang (e.g. #!/usr/bin/octave
) and executable permissions, you can call it like any other Unix program with arguments:
$ ./thescript arg1 arg2
If you call the script from the shell and it's plotting, please note how to plot when running a script from the shell.
How do I close a figure?
To close the current figure type close
in the Octave command prompt.
How do I set the number of displayed decimals?
You can control the number of displayed decimals using the format
command:
>> format long
>> pi
pi = 3.14159265358979
>> format short
>> pi
pi = 3.1416
How do I call an Octave function from C++?
Please read the manual https://www.gnu.org/software/octave/doc/interpreter/CallingOctaveFunctionsfromOct_002dFiles.html.
How do I change color/line definition in gnuplot postscript?
Here is a awk script to get a rainbow color map
#!/bin/awk f BEGIN { split("0 4 6 7 5 3 1 2 8", rainbow, " "); split("7 3 1 0 2 4 6 5 8", invraim, " "); } $1 ~ /\/LT[08]/ { n = substr($1, 4, 1); if (n == 0) lt = "{ PL [] 0.9 0.1 0.1 DL } def"; else if (n == 1) lt = "{ PL [4 dl 2 dl] 0.1 .75 0.1 DL } def"; else if (n == 2) lt = "{ PL [2 dl 3 dl] 0.1 0.1 0.9 DL } def"; else if (n == 3) lt = "{ PL [1 dl 1.5 dl] 0.9 0 0.8 DL } def"; else if (n == 4) lt = "{ PL [5 dl 2 dl 1 dl 2 dl] 0.1 0.8 0.8 DL } def"; else if (n == 5) lt = "{ PL [4 dl 3 dl 1 dl 3 dl] 0.9 0.8 0.2 DL } def"; else if (n == 6) lt = "{ PL [2 dl 2 dl 2 dl 4 dl] 0.5 0.3 0.1 DL } def"; else if (n == 7) lt = "{ PL [2 dl 2 dl 2 dl 2 dl 2 dl 4 dl] 1 0.4 0 DL } def"; else if (n == 8) lt = "{ PL [2 dl 2 dl 2 dl 2 dl 2 dl 2 dl 2 dl 4 dl] 0.5 0.5 0.5 DL } def"; $0 = sprintf("/LT%d %s", rainbow[n+1], lt); ##$0 = sprintf("/LT%x %s", invraim[n+1], lt); ##$0 = sprintf("/LT%x %s", n, lt); } { print; }
How do I tell if a file exists?
One can use the function exist
to tell if a regular file, say foo.txt
exist in Octave's load path, or the current directory:
>> exist ("foo.txt", "file") # 2, if file exists, 0 otherwise
ans = 2
How do I create a plot without a window popping up (plot to a file directly)?
figure (1, "visible", "off");
plot (sin (1:100));
print deps "/tmp/sin.eps"
One can set that behavior as default:
set (0, "defaultfigurevisible", "off");
How do I increase Octave's precision?
Octave's default numerical type is IEEE 754 binary64 , a.k.a. "double" or "hardware floats". This type has a precision of 53 bits or about 16 decimal digits. It is supported by each modern computer hardware, so it is really fast. This type is assumed throughout for Octave's calculations. If more precision was required, one can obtain a "few bits more" by using integer types, e.g. uint64
, but in general one cannot expect more precision from any fast numerical software. Just to visualize "how big" those numerical limits are, consider the following table:
intmax ("uint64")

18,446,744,073,709,551,615

2^641

intmax ("int64")

9,223,372,036,854,775,807

2^631

flintmax ("double")

9,007,199,254,740,992

2^53

flintmax ("single")

16,777,216

2^24

When working with other types than "double" in Octave, one has to make sure, that the first operand is converted to the desired type:
>> uint64 (2^53 + 1)
ans = 9007199254740992
>> uint64 (2^53) + 1
ans = 9007199254740993
Notice the difference, in the first line the addition within the brackets is performed using double precision, therefore the result gets "truncated" to the maximum possible value without a warning. The third line uses throughout uint64 precision.
Consider carefully if your problem really needs more precision. Often if you're running out of precision the problem lies fundamentally in your methods being numerically unstable, thus more precision will not help you here.
If you absolutely must have more precision, you're at present better off using a CAS instead of Octave. However, CAS or symbolic computations must be implemented in software which makes it much slower than hardware floats. An example of such a CAS is Sage or have a look at Octave's Symbolic package.
How do I run a Matlab Pfile in Octave?
You can't. Matlab Pfiles (files with a .p
file extension), also known as Pcode, are obfuscated files that cannot be run outside of Matlab itself. The original source Matlab mfiles that were used to generate these Pfiles should be used in Octave instead.
There are no plans to support running Pfiles produced by Matlab in Octave.
How does Octave solve linear systems?
In addition to consulting Octave's source for the precise details, you can read the Octave manual for a complete highlevel description of the algorithm that Octave uses to decide how to solve a particular linear system, e.g. how the backslash operator A \ x
will be interpreted. Sections Techniques Used for Linear Algebra and Linear Algebra on Sparse Matrices from the manual describe this procedure.
How do I do X?
You are probably looking for the function lookfor
. This function searches the help text of all functions for a specific string and returns a list of functions. Note that by default it will only search the first line of the help text (check help lookfor
at the octave prompt for more). The following example helps to find the function to calculate correlation coefficient in a matrix:
>> lookfor correlation corr Compute matrix of correlation coefficients. corrcoef Compute a matrix of correlation coefficients. spearman Compute Spearman's rank correlation coefficient RHO.
Also, there's a high chance that the function name has a suggestive name, and so you can try autocompletion to get some hints. For the previous example, typing corr
at the octave prompt followed by pressing the Tab ↹Key twice would suggest the following:
>> corr corr corrcoef
Differences between Octave and Matlab
People often ask
I wrote some code for Matlab, and I want to get it running under Octave. Is there anything I should watch out for?
or alternatively
I wrote some code in Octave, and want to share it with Matlab users. Is there anything I should watch out for?
which is not quite the same thing. There are still a number of differences between Octave and Matlab, however in general differences between the two are considered as bugs. Octave might consider that the bug is in Matlab and do nothing about it, but generally functionality is almost identical. If you find an important functional difference between Octave behavior and Matlab, then you should send a description of this difference (with code illustrating the difference, if possible) to http://bugs.octave.org.
Furthermore, Octave adds a few syntactical extensions to Matlab that might cause some issues when exchanging files between Matlab and Octave users.
As both Octave and Matlab are under constant development, the information in this section is subject to change.
You should also look at the pages http://octave.sourceforge.net/packages.php and http://octave.sourceforge.net/docs.html that have a function reference that is up to date. You can use this function reference to see the number of octave functions that are available and their Matlab compatibility.
Graphical Curve Fitting Tool
Currently, Octave lacks a graphical curve fitting tool such as Matlab's curvefit tool. [1]
You can use polyfit or the packages signal or optim to achieve these task but these are not graphical tools.
Nested Functions
Octave has limited support for nested functions since version 3.8.0. That is
function y = foo (x)
y = bar(x)
function y = bar (x)
y = ...;
end
end
is equivalent to
function y = foo (x)
y = bar(x)
endfunction
function y = bar (x)
y = ...;
endfunction
The main difference with Matlab is a matter of scope. While nested functions have access to the parent function's scope in Matlab, no such thing is available in Octave, due to how Octave essentially “unnests” nested functions.
The authors of Octave consider the nested function scoping rules of Matlab to be more problems than they are worth as they introduce difficult to find bugs as inadvertently modifying a variable in a nested function that is also used in the parent is particularly easy for those not attentive to detail.
Differences in core syntax
There are a few core Matlab syntaxes that are not accepted by Octave, these being
 Some limitations on the use of function handles. The major difference is related to nested function scoping rules (as above) and their use with function handles.
 Some limitations of variable argument lists on the LHS of an expression, though the most common types are accepted.
 Matlab classdef object oriented programming is only partially supported, see classdef for details.
Differences in core functions
A large number of the Matlab core functions (i.e. those that are in the core and not a toolbox) are implemented, and certainly all of the commonly used ones. There are a few functions that aren't implemented, usually to do with specific missing Octave functionality (GUI, DLL, Java, ActiveX, DDE, web, and serial functions). Some of the core functions have limitations that aren't in the Matlab version. For example the sprandn
function can not force a particular condition number for the matrix like Matlab can. Another example is that testing and the runtests function work differently in Matlab and Octave.
JustInTime compiler
Matlab includes a "JustInTime" compiler. This compiler allows the acceleration of forloops in Matlab to almost native performance with certain restrictions. The JIT must know the return type of all functions called in the loops and so you can't include user functions in the loop of JIT optimized loops. Octave has a not fully functional JIT compiler. For this reason you must vectorize your code as much as possible. The MathWorks themselves have a good document discussing vectorization at http://www.mathworks.com/support/technotes/1100/1109.html.
Compiler
On a related point, there is no Octave compiler, and so you can't convert your Octave code into a binary for additional speed or distribution.
Graphic handles
The support for graphics handles is converging towards full compatibility. If you notice any incompatibilities, please report a bug.
GUI functions
The support for Matlab compatible GUI functions was added in Octave version 3.6.0 and is converging towards full compatibility. If you notice any incompatibilities, please report a bug.
Simulink
Octave itself includes no Simulink support. Typically the simulink models lag research and are less flexible, so shouldn't really be used in a research environment. However, some Matlab users that try to use Octave complain about this lack.
MEXFiles
Octave includes an API to the Matlab MEX interface. However, as MEX is an API to the internals of Matlab and the internals of Octave differ from Matlab, there is necessarily a manipulation of the data to convert from a MEX interface to the Octave equivalent. This is notable for all complex matrices, where Matlab stores complex arrays as real and imaginary parts, whereas Octave respects the C99/C++ standards of colocating the real/imag parts in memory. Also due to the way Matlab allows access to the arrays passed through a pointer, the MEX interface might require copies of arrays (even non complex ones).
Block comments
Block comments denoted by #{
and #}
markers (or %{
and %}
) are supported by Octave with some limitations. The major limitation is that block comments are not supported within [] or {}.
MatFile format
There are some differences in the mat v5 file format accepted by Octave. Matlab recently introduced the "V7.3" save option which is an HDF5 format which is particularly useful for 64bit platforms where the standard Matlab format can not correctly save variables. Octave accepts HDF5 files, but is not yet compatible with the "v7.3" versions produced by Matlab.
Although Octave can load inline function handles saved by Matlab, it can not yet save them.
Finally, some multibyte Unicode characters aren't yet treated in matfiles.
Profiler
Thanks to Daniel Kraft's 2011 Google Summer of Code project, Octave has a profiler since version 3.6.0.
Toolboxes
Octave is a community project and so the toolboxes that exist are donated by those interested in them through Octave Forge. These might be lacking in certain functionality relative to the Matlab toolboxes, and might not exactly duplicate the Matlab functionality or interface.
Shortcircuit &
and 
operators
The &
and 
operators in Matlab shortcircuit when included in a condition (e.g. an if
or while
statement) and not otherwise. In Octave only the &&
and 
short circuit. Note that this means that
if (a  b)
...
end
and
t = a  b;
if (t)
...
end
have different semantics in Matlab. This is really a Matlab bug, but there is too much code out there that relies on this behavior to change it. Prefer the &&
and 
operators in if
statements if possible.
Note that the difference with Matlab is also significant when either argument is a function with side effects or if the first argument is a scalar and the second argument is an empty matrix. For example, note the difference between
t = 1  []; ## results in [], so...
if (t) 1, end ## in if ([]), this is false.
and
if (1  []) 1, end ## short circuits so condition is true.
In the latter case, Octave displays since version 4.0.0 a warning:
warning: Matlabstyle shortcircuit operation performed for operator 
Another case that is documented in the Matlab manuals is that
t = [1, 1]  [1, 2, 3]; ## error
if ([1, 1]  [1, 2, 3]) 1, end ## OK
Also Matlab requires the operands of &&
and 
to be scalar values but Octave does not (it just applies the rule that for an operand to be considered true, every element of the object must be nonzero or logically true).
Finally, note the inconsistence of thinking of the condition of an if
statement as being equivalent to all(X(:))
when X
is a matrix. This is true for all cases EXCEPT empty matrices:
if ([0, 1]) == if (all ([0, 1])) ==> i.e., condition is false.
if ([1, 1]) == if (all ([1, 1])) ==> i.e., condition is true.
However,
if ([])
is not the same as
if (all ([]))
because, despite the name, the all
is really returning true if none of the elements of the matrix are zero, and since there are no elements, well, none of them are zero. This is an example of vacuous truth. But, somewhere along the line, someone decided that if ([])
should be false. The Mathworks probably thought it just looks wrong to have []
be true in this context even if you can use logical gymnastics to convince yourself that "all" the elements of an empty matrix are nonzero. Octave however duplicates this behavior for if
statements containing empty matrices.
Solvers for singular, under and overdetermined matrices
Matlab's solvers as used by the operators mldivide
\
and mrdivide
/
, use a different approach than Octave's in the case of singular, under, or overdetermined matrices. In the case of a singular matrix, Matlab returns the result given by the LU decomposition, even though the underlying solver has flagged the result as erroneous. Octave has made the choice of falling back to a minimum norm solution of matrices that have been flagged as singular which arguably is a better result for these cases.
In the case of under or overdetermined matrices, Octave continues to use a minimum norm solution, whereas Matlab uses an approach that is equivalent to
function x = mldivide (A, b)
m = rows (A);
[Q, R, E] = qr (A);
x = [A \ b, E(:, 1:m) * (R(:, 1:m) \ (Q' * b))]
end
While this approach is certainly faster and uses less memory than Octave's minimum norm approach, this approach seems to be inferior in other ways.
A numerical question arises: how big can the null space component become, relative to the minimumnorm solution? Can it be nicely bounded, or can it be arbitrarily big? Consider this example:
m = 10;
n = 10000;
A = ones (m, n) + 1e6 * randn (m, n);
b = ones (m, 1) + 1e6 * randn (m, 1);
norm (A \ b)
while Octave's minimumnorm values are about 3e2, Matlab's results are 50times larger. For another issue, try this code:
m = 5;
n = 100;
j = floor (m * rand (1, n)) + 1;
b = ones (m, 1);
A = zeros (m, n);
A(sub2ind(size(A),j,1:n)) = 1;
x = A \ b;
[~,p] = sort (rand (1, n));
y = A(:,p) \ b;
norm (x(p)  y)
It shows that unlike in Octave, mldivide
in Matlab is not invariant with respect to column permutations. If there are multiple columns of the same norm, permuting columns of the matrix gets you different result than permuting the solution vector. This will surprise many users.
Since the mldivide
\
and mrdivide
/
operators are often part of a more complex expression, where there is no room to react to warnings or flags, it should prefer intelligence (robustness) to speed, and so the Octave developers are firmly of the opinion that Octave's approach for singular, under and overdetermined matrices is a better choice than Matlab's.
Octave extensions
The extensions in Octave over MATLAB syntax are very useful, but might cause issues when sharing with Matlab users. A list of the major extensions that should be avoided to be compatible with Matlab are:
Comments in Octave can be marked with #
. This allows POSIX systems to have the first line as #! octave q
and mark the script itself executable. MATLAB doesn't have this feature due to the absence of comments starting with #
".
Code blocks like if, for, while, etc can be terminated with block specific terminations like endif. MATLAB doesn't have this and all blocks must be terminated with end.
Octave has a lisplike unwind_protect
block that allows blocks of code that terminate in an error to ensure that the variables that are touched are restored. You can do something similar with try/catch combined with rethrow (lasterror ())
in Matlab, however rethrow and lasterror are only available in Octave 2.9.10 and later. MATLAB 2008a also introduced OnCleanUp
that is similar to unwind_protect
, except that the object created by this function has to be explicitly cleared in order for the cleanup code to run.
Note that using try/catch combined with rethrow (lasterror ())
cannot guarantee that global variables will be correctly reset, as it won't catch user interrupts with CtrlC. For example
global a
a = 1;
try
_a = a;
a = 2
while true
end
catch
fprintf ('caught interrupt\n');
a = _a;
rethrow (lasterror());
end
compared to
global a
a = 1;
unwind_protect
_a = a;
a = 2
while true
end
unwind_protect_cleanup
fprintf ('caught interrupt\n');
a = _a;
end
Typing CtrlC in the first case returns the user directly to the prompt, and the variable a
is not reset to the saved value. In the second case the variable a
is reset correctly. Therefore Matlab gives no safe way of temporarily changing global variables.
Indexing can be applied to all objects in Octave and not just variables. Therefore sin(x)(1:10)
for example is perfectly valid in Octave but not Matlab. To do the same in Matlab you must do y = sin(x); y = y([1:10]);
Octave has the operators ++
, –
, =
, +=
, *=
, etc. As MATLAB doesn't, if you are sharing code these should be avoided.
Character strings in Octave can be denoted with double or single quotes. There is a subtle difference between the two in that escaped characters like \n
(newline), \t
(tab), etc are interpreted in double quoted strings but not single quoted strings. This difference is important on Windows platforms where the \
character is used in path names, and so single quoted strings should be used in paths. MATLAB doesn't have double quoted strings and so they should be avoided if the code will be transferred to a MATLAB user.
What features are unique to Octave?
Although most of the Octave language will be familiar to Matlab users, it has some unique features of its own.
Functions defined on the commandline
Functions can be defined by entering code on the command line, a feature not supported by Matlab. For example, you may type:
>> function s = hello_string (to_who)
> ## Say hello
> if nargin<1, to_who = "World"; end
> s = ["Hello ",\
> to_who];
> endfunction
>> hello_string ("Moon")
ans = Hello Moon
As a natural extension of this, functions can also be defined in script files (mfiles whose first noncomment line isn't function out = foo (...)
)
Note: MATLAB R2016b added the ability to define functions in script files.
Comments with #
The pound character, #
, may be used to start comments, in addition to %
. See the previous example. The major advantage of this is that as #
is also a comment character for unix script files, any file that starts with a string like #! /usr/bin/octave q
will be treated as an octave script and be executed by octave.
Strings delimited by double quotes "
In 2016, Matlab introduced String Arrays, that are initialized by using double quoted strings, and are not implemented in Octave yet. In Octave doublequoted strings include backslash interpretation (like C++, C, and Perl) while single quoted are uninterpreted (like Matlab and Perl).
Line continuation by backslash
Lines can be continued with a backslash, \
, in addition to three points ...
as in Matlab.
Informative block closing
You may close function, for, while, if, ... blocks with endfunction, endfor, endwhile, ... keywords in addition to using end. As with Matlab, the end (or endfunction) keyword that marks the end of a function defined in a .m file is optional.
Coherent syntax
Indexing other things than variables is possible, as in:
>> [3 1 4 1 5 9](3)
ans = 4
>> cos([0 pi pi/4 7])(3)
ans = 0.70711
In Matlab, it is for example necessary to assign the intermediate result cos([0 pi pi/4 7])
to a variable before it can be indexed again.
Exclamation mark as not operator
The exclamation mark !
(aka “Bang!”) is a negation operator, just like the tilde ~
:
>> if ! strcmp (program_name, "octave"),
> "It's an error"
> else
> "It works!"
> end
ans = It works!
Note however that Matlab uses the !
operator for shell escapes, for which Octave requires using the system command.
Increment and decrement operators
If you like the ++
, +=
etc operators, rejoice! Octave includes the Clike increment and decrement operators ++
and 
in both their prefix and postfix forms, in addition to +=
, =
, *=
, /=
, ^=
,.+=
,.=
,.*=
, ./=
and .^=
.
For example, to preincrement the variable x, you would write ++x
. This would add one to x and then return the new value of x as the result of the expression. It is exactly the same as the expression x = x + 1
.
To postincrement a variable x, you would write x++
. This adds one to the variable x, but returns the value that x had prior to incrementing it. For example, if x is equal to 2, the result of the expression x++ is 2, and the new value of x is 3.
For matrix and vector arguments, the increment and decrement operators work on each element of the operand.
Unwindprotect
In addition to trycatch blocks, Octave supports an alternative form of exception handling modeled after the unwindprotect form of Lisp. The general form of an unwind_protect block looks like this:
unwind_protect
body
unwind_protect_cleanup
cleanup
end_unwind_protect
Where body and cleanup are both optional and may contain any Octave expressions or commands. The statements in cleanup are guaranteed to be executed regardless of how control exits body.
The unwind_protect statement is often used to reliably restore the values of global variables that need to be temporarily changed.
Matlab can be made to do something similar with their Template:Manaul function that was introduced in 2008a. Octave also has onCleanup
since version 3.4.0.
Builtin ODE and DAE solvers
Octave includes LSODE, DASSL and DASPK for solving systems of stiff ordinary differential and differentialalgebraic equations. These functions are built in to the interpreter.
DoUntil loop structure
Similar to the dowhile loop in C and C++, Octave allows a dountil loop which does not exist in Matlab:
x = 0
do
x += 1;
until (x == 10)
Broadcasting
Borrowed from other languages, octave broadcasting allows easy and readable vectorization.
f = (1:0.1:2);
# put angular frequencies on the first dimension to prepare broadcasting
omega = 2 * pi * f(:);
# time is already on the second dimension (row vector)
t = 0:0.02:2;
# the resulting s will be a 2dimensional array
s = sin(omega .* t);
# which can be displayed as
pcolor(t, f, s)
xlabel("t (s)")
ylabel("f (Hz)")
Note: Automatic expansion of dimensions was added to MATLAB R2016b.
Documentation strings
Octave allows extensive formatting of the help string of functions using Texinfo. The effect on the online documentation is relatively small, but makes the help string of functions conform to the help of Octave’s own functions. However, the effect on the appearance of printed or online documentation will be greatly improved.
Test functions
Octave allows to add selftests to user defined functions. Tests are put after function definition in specially commented block.
function mult = a(val)
mult = val.*2;
endfunction
%!test
%! assert (a(3), 6);
Such a function can be tested for valid outputs by following code:
>> test a PASSES 1 out of 1 test
Demonstration Functions
Example code block can be part of function file in a similar manner as test functions. For example to run demo
for function multinom of package specfun, use:
demo multinom
Powerful assert
Function assert have extended input possibilities.
GUI
Does Octave have a GUI?
Yes! It was officially released with Octave 4.0.0. It was also available since version 3.8.0 as an experimental feature (use the forcegui
option to start Octave).
Why did you create yet another GUI instead of making one that already exists better?
The previously existing GUIs were not part of Octave itself. The integration within Octave was rather bad, as all of them treated Octave as a foreign black box and used pipes for communication. This approach is bound to fail with each new version of Octave, as any fix would only be temporary. For historical reasons and to honor the approaches, a short list of previous GUIs for Octave:
 QtOctave was a great, beautiful, and very useful tool which is now abandoned and incompatible with newer versions of Octave. We are thankful to its developers to make it free software so we could reuse large chunks of it for what is now the Octave GUI.
 Quint was a project for an Octave GUI that actually tried to do it right. Eventually it was merged into the Octave repository and is no longer a separate project. Also, many bits from QtOctave were reused in the GUI.
 Xoctave, which is proprietary and commercial.
 GUI Octave, which was proprietary and is no longer available.
Graphics: backends and toolkits
What are the supported graphics backends?
 OpenGL via the graphics toolkits qt (current default) and fltk
 gnuplot via the gnuplot graphics toolkit
How do I change my graphics toolkit?
There are three commands to deal with graphics toolkits:
available_graphics_toolkits

lists all available graphics toolkits 
graphics_toolkit

displays the currently used graphics toolkit 
graphics_toolkit ("qt/fltk/gnuplot")

sets the graphics toolkit to either of qt, fltk, or gnuplot, if available 
Why did you replace gnuplot with an OpenGL backend?
The development of Octave is committed to being both compatible with Matlab and adding additional features. Toward those ends, the developers decided to introduce a native OpenGL backend that supports Matlab handle graphics and its uicontrols. Starting with the 3.8 release, Octave uses OpenGL graphics by default (with FLTK widgets in Octave 3.8 and Qt widgets in Octave 4.0 and later).
Are there any plans to remove the gnuplot backend?
No. There are no plans to remove the gnuplot backend. It will be available as long as our users find it useful.
How can I implement a new graphics backend/toolkit?
This is one of those times where the best documentation is to read the existing code. We have three different toolkits in Octave now, so there are some examples to draw from.
Development
When will feature X be released or implemented?
When it's ready, sooner if you help. You can send us patches if you can implement feature X yourself. If you can't, some developers may be convinced to work on your specific problem for some money.
How can I get involved in Octave development?
Be around. Be social. Participate in our mailing lists help@octave.org and maintainers@octave.org. Find things about Octave you don't like, and start thinking about how to fix them. Many people who now contribute to Octave first spent several years helping in the mailing list before they started to delve into the code. A good way to learn Octave is to understand the problems other people are having with it, so being helpful in the mailing lists not only helps Octave as a whole, but it also prepares you to be a better Octave contributor.
If you feel ready to dive right into the code, read the Developers wiki page or start here. But do not send an email to the mailing lists listing your skills and offering to help. We won't just suggest things for you to do. We lack volunteers and we do need your help, but because of that, we also lack the time to provide good guidance and mentoring. If there is a specific shortterm project you would like to work on, say so, and just do it. Then ask for help or advice when you're doing it. It is a lot more important that you do something that you're actually interested on than something we suggested because it only matches your skills.
We also need help with this wiki and the manual. These are also important tasks. The documentation is easy to patch, and the help text for individual functions even more so. Editing this wiki is even easier.
Accurate bug reporting is also very useful. Find and report bugs, making an attempt to diagnose them. Eventually, you will also know how to fix them. If you want to help with bug reports or patches, subscribe to the bug tracker mailing list. You'll get updates on all bug activity, and you can jump in when you see something you can help with.
Look at our projects, short projects, and Summer of Code  Getting Started if you need specific inspiration for coding tasks that we would like to get done.