This is a list of frequently asked questions (FAQ) for Octave users.
We are always looking for new questions (with answers), better answers, or both. Please send suggestions to bug@octave.org. If you have general questions about Octave, or need help for something that is not covered by the Octave manual or the FAQ, please use the help@octave.org mailing list.
This FAQ is intended to supplement, not replace, the Octave manual. Before posting a question to the help@octave.org mailing list, you should first check to see if the topic is covered in the manual.
Octave is a high-level interactive language, primarily intended for numerical computations that is mostly compatible with Matlab.1
Octave can do arithmetic for real, complex or integer-valued scalars and matrices, solve sets of nonlinear algebraic equations, integrate functions over finite and infinite intervals, and integrate systems of ordinary differential and differential-algebraic equations.
Octave uses the GNU readline library to handle reading and editing input. By default, the line editing commands are similar to the cursor movement commands used by GNU Emacs, and a vi-style line editing interface is also available. At the end of each session, the command history is saved, so that commands entered during previous sessions are not lost.
The Octave distribution includes a 590+ page Texinfo manual. Access to the complete text of the manual is available via the help command at the Octave prompt.
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 was the original author of Octave, starting full-time development in February 1992. He is still the primary maintainer. The community of users/developers has in addition contributed some code and fuels the discussion on the mailing lists help@octave.org (user forum), bug@octave.org (bug reports), maintainers@octave.org (development issues), and octave-dev@lists.sourceforge.net (all things related to the Octave Forge repository of user-contributed functions).
The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system.
GNU is a recursive acronym for “GNU's Not Unix”; it is pronounced guh-noo, approximately like canoe.
The Free Software Foundation (FSF) is the principal organizational sponsor of 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. However, Octave is not and has never been developed by the FSF.
For more information about the GNU project, see www.gnu.org.
In general, you will find the latest version on http://www.octave.org/download.html. It is recommended to use the “testing” version of octave for general use, and the “development” version if you want the latest features.
A list of user-visible changes since the last release is available in the file NEWS. The file ChangeLog in the source distribution contains a more detailed record of changes made since the last release.
Octave runs on various Unices—at least Linux and Solaris, Mac OS X, Windows and anything you can compile it on. Binary distributions exist at least for Debian, Suse, Fedora and RedHat Linuxes (Intel and AMD CPUs, at least), for Mac Os X and Windows' 98, 2000 and XP.
Two and three dimensional plotting is fully supported using gnuplot.
The underlying numerical solvers are currently standard Fortran ones like Lapack, Linpack, Odepack, the Blas, etc., packaged in a library of C++ classes. If possible, the Fortran subroutines are compiled with the system's Fortran compiler, and called directly from the C++ functions. If that's not possible, you can still compile Octave if you have the free Fortran to C translator f2c.
Octave is also free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation.
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 plug-in interface (also known as a .oct file) necessarily links with Octave internals and is considered a derivative work of Octave and therefore 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:
A program 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.
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.
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 so that the free software community can benefit from your work the same as you have benefitted from the work of all the people who have contributed to Octave.
Pointing to http://www.octave.org is good, because that gives people a direct way to find out more. If citation of a URL is not allowed by a publisher, or if you also want to point to a traditional reference, then you can cite the Octave manual:
@BOOK{eaton:2008,
author = "John W. Eaton and David Bateman and Søren Hauberg",
title = "GNU Octave Manual Version 3",
publisher = "Network Theory Limited",
year = "2008",
isbn = "0-9546120-6-X"
}
The 3.0.N series has enough new features to justify a major version number change. The 3.0.N series brings
Here are some features that have been around since 2.1.N
The 3.1.N series is the current development release and will become a 3.2.N release in the future. This series brings the new features
An experimental OpenGL graphics backend to replace the gnuplot
The functions are based on the GraphicsMagick library.
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.
This section refers to Matlab R2008b and Octave 2.1.51.
Functions can be defined by entering code on the command line, a feature not supported by the other leading brand. For example, you may type:
octave:1> function s = hello_string (to_who)
> ## Say hello
> if nargin<1, to_who = "World"; end
> s = ["Hello ",\
> to_who];
> endfunction
octave:2> hello_string ("Moon")
ans = Hello Moon
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.
The double quote, ‘"’, may be used to delimit strings, in addition to the single quote ‘'’. See the previous example. Also, double-quoted strings include backslash interpretation (like C++, C, and Perl) while single quoted are uninterpreted (like Matlab and Perl).
Lines can be continued with a backslash, ‘\’, in addition to three points ‘...’. See the previous example.
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.
Indexing other things than variables is possible, as in:
octave:1> [3 1 4 1 5 9](3)
ans = 4
octave:2> cos([0 pi pi/4 7])(3)
ans = 0.70711
The exclamation mark '!' (aka “Bang!”) is a negation operator, just like the tilde '~':
octave:1> if ! strcmp (program_name, "octave"),
> "It's an error"
> else
> "It works!"
> end
ans = It works!
If you like the ‘++’, ‘+=’ etc operators, rejoice! Octave includes the C-like increment and decrement operators ‘++’ and ‘--’ in both their prefix and postfix forms, in addition to ‘+=’, ‘-=’, ‘*=’, ‘/=’, ‘^=’, ‘.*=’, ‘./=’, and ‘.^=’.
For example, to pre-increment 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 post-increment 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.
Octave supports a limited form of exception handling modelled after the
unwind-protect 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 OnCleanUp
function that was introduced in 2008a.
Octave includes LSODE and DASSL for solving systems of stiff ordinary differential and differential-algebraic equations. These functions are built in to the interpreter.
The Octave distribution includes a 590+ page manual that is also distributed under the terms of the GNU GPL. It is available on the web at http://www.octave.org/docs.html and you will also find there instructions on how to order a paper version.
The complete text of the Octave manual is also available using the GNU Info system via the GNU Emacs, info, or xinfo programs, or by using the ‘help -i’ command to start the GNU info browser directly from the Octave prompt.
If you have problems using this documentation, or find that some topic is not adequately explained, indexed, or cross-referenced, please send a bug report to bug@octave.org.
If you can't find an answer to your question, the help@octave.org mailing list is available for questions related to using, installing, and porting Octave that are not adequately answered by the Octave manual or by this document.
To subscribe to the list, go to www.octave.org/archive.html and follow the link to the subscription page for the list.
Please do not send requests to be added or removed from the mailing list, or other administrative trivia to the list itself.
An archive of old postings to the help-octave mailing list is maintained on http://www.octave.org/archive.html.
You will also find some user advice and code spread over the web. Good starting points are the Octave Wiki http://wiki.octave.org and Octave-Forge http://octave.sourceforge.net
“I think I have found a bug in Octave, but I'm not sure. How do I know, and who should I tell?”
First, see the section on bugs and bug reports in the Octave manual. When you report a bug, make sure to describe the type of computer you are using, the version of the operating system it is running, and the version of Octave that you are using. Also provide enough code so that the Octave maintainers can duplicate your bug.
If you have Octave working at all, the easiest way to do this is to use
the Octave function bug_report. When you execute this function,
Octave will prompt you for a subject and then invoke the editor on a
file that already contains all the configuration information. When you
exit the editor, Octave will mail the bug report for you (in a unix-like
operating system).
If for some reason you cannot use Octave's bug_report function,
mail your bug report to bug@octave.org. Your message needs to
include enough information to allow the maintainers of Octave to fix the
bug. Please read the section on bugs and bug reports in the Octave
manual for a list of things that should be included in every bug report.
Source code is available on the Octave development site, where you are sure to get the latest version.
Since Octave is distrubted under the terms of the GPL, you can get Octave from a friend who has a copy, by anonymous FTP, or by ordering a tape or CD-ROM from the Free Software Foundation (FSF).
The Octave project does not distribute binary packages, but other projects do. For an up-to-date listing of packagers, see:
As of today, Octave binaries are available at least on Debian, RedHat, Suse and Fedora Linuxes, Mac OS X, Windows' 98, 2000 and XP.
Octave currently runs on Unix-like systems, Mac OS X, 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 bug@octave.org.
Octave 3.2 require approximately 800MB of disk storage to unpack and compile from source (considerably less if you don't compile with debugging symbols). Once installed, Octave requires approximately 200MB of disk space (again, considerably less if you don't compile with debugging symbols).
To compile Octave, you will need a recent version of GNU Make. You will also need GCC 3.3 or later, although GCC 4.1 or later is recommended.
You must have GNU Make to compile octave. Octave's Makefiles use features of GNU Make that are not present in other versions of make. GNU Make is very portable and easy to install.
Yes, but development is done primarily with GCC, so you may hit some incompatibilities. Octave is intended to be portable to any standard conforming compiler. If you have difficulties that you think are bugs, please report them to the bug@octave.org mailing list, or ask for help on the help@octave.org mailing list.
This list is probably far too short. Feel free to suggest additional questions (preferably with answers!)
Octave uses the genpath function to recursively add directories
to the list of directories searched for function files. Check the list
of directories with the path command. If the path list is very
long check your use of the genpath function.
There is a known bug in gnuplot 4.2 that can cause an off by one error while piping data to gnuplot. The relevant gnuplot bug report can be found at http://sourceforge.net/tracker/index.php?func=detail&aid=1716556&group_id=2055&atid=102055
If you have obtained your copy of Octave from a distribution please file a bug report requesting that the fix reported in the above bug report be included.
mkoctfile.
Most distributions split Octave into several packages. The script
mkoctfile is then part of a separate package:
aptitude install octave3.0-headers
yum install octave-devel
octave:1> format long
octave:2> pi
pi = 3.14159265358979
octave:3> format short
octave:4> pi
pi = 3.1416
“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 a difference between Octave behavior and Matlab, then you should send a description of this difference (with code illustrating the difference, if possible) to bug@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 at anytime.
You should also look at the page http://octave.sourceforge.net/packages.html and http://octave.sourceforge.net/doc/ that has a function reference that is up to date. You can use this function reference to see the number of octave function that are available and their Matlab compatibility.
The major differences between Octave 3.2.N and Matlab R2008a are:
Octave doesn't yet have nested functions. That is
function y = foo (x)
y = bar(x)
function y = bar (x)
y = ...;
end
end
There was discussion in Octave of having these even prior to Matlab, and the decision was made not to have these in Octave at the time for compatibility. The above written with sub-functions functions would be
function y = foo (x)
y = bar(x)
end
function y = bar (x)
y = ...;
end
Now that Matlab has recently introduced nested functions, Octave will probably have them soon as well. Until then nested functions in Octave are treated as sub-functions with the same scoping rules as sub-functions.
The authors of Octave consider the nested function scoping rules of Matlab to be more problems than they are worth as they introduce diffiult to find bugs as inadvertantly modifying a variable in a nested function that is also used in the parent is particularly easy.
condest or 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.
patch
function is currently limited to 2-D patches, due to an underlying
limitation in gnuplot.
Although Octave can load inline abd function handles saved by Matlab, it can not yet save them.
Finally, Some multi-byte unicode characters aren't yet treated in mat-files.
http://www.cae.wisc.edu/pipermail/octave-maintainers/2007-January/001685.html
for more details
& and | operators in Matlab short-circuit when
included in an if statemant 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
are different 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 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.
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 ([]) != if (all ([]))
because samp ([]) == 1 (because, despite the name, it 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). But, somewhere
along the line, someone decided that if ([]) should be false.
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 a matrix that doesn't actually have
any elements are nonzero. Octave however duplicates this behavior for if
statements containing empty 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 over-determined 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 over-determined matrices, Octave continues to use a minimum norm solution, whereas Matlab uses an approach that is equivalent to
function x = mldivide (A, b)
[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 minimum-norm solution? Can it be nicely bounded, or can it be arbitrarily big? Consider this example:
m = 10;
n = 10000;
A = ones(m, n) + 1e-6 * randn(m,n);
b = ones(m, 1) + 1e-6 * randn(m,1);
norm(A \ b)
while Octave's minimum-norm values are around 3e-2, Matlab's results are 50-times 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;
[dummy,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 over-determined matrices is a better choice that Matlab's
try/catch combined with ‘rethrow (lasterror ())’ in
Matlab, however rethrow and lasterror are only available in Octave 2.9.10 and later.
Note that using try/catch combined with ‘rethrow
(lasterror ())’ can not guarantee that global variables will be
correctly reset, as it won't catch user interrupts with Ctrl-C. 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 Ctrl-C 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 save way of temporarily changing global variables.
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]);
\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 transfered to a Matlab user.