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1,211 bytes removed, 09:36, 28 July 2017
Some more refactoring.
* [http://hg.savannah.gnu.org/hgweb/octave/file/83792dd9bcc1/etc/NEWS.3 NEWS.3] for the 3.X.Y series
=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
</syntaxhighlight>
==I have installed a package but still get a "foo undefined" error?== You have probably forgotten to load the package. Use {{Codeline|pkg load package-name}} to load it. Most packages are no longer loaded automatically to avoid surprises. See reasoning on related FAQ [[FAQ#How_do_I_install_all_Octave_packages.3F|how do I install all Octave packages]]. If you want a specific package to be loaded by default at startup, consider adding the {{Codeline|pkg load}} command on your {{path|[[.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 <code>foo</code> will be a deb or rpm called <code>octave-foo</code>, and you should install that instead using <code>apt</code> or <code>yum</code>. If you really need to build Octave packages from source to install them, you'll need {{manual|mkoctfile}}. Most distributions split Octave into several packages. The script {{manual|mkoctfile}} is then part of a separate package: * Debian/Ubuntu: [https://packages.debian.org/stretch/liboctave-dev liboctave-dev] * Fedora: {{Codeline|octave-devel}} == How do I automatically load a package at Octave startup? ==
When Octave starts, it runs the file {{Path|~/.octaverc}} (in your user's home directory). If you want Octave to automatically load a package, simply add a <code>pkg load pkg-name</code> command to it. If the files does not exist, create it.
>> corr
corr corrcoef
 
=Common problems=
==I am running a do not see any output of my script that should produce output during execution but I don't see anything until it has finished?==
By default Octave is set to pass its screen output through a [https://en.wikipedia.org/wiki/Terminal_pager pager ] (usually the default pager is "less") which allowsthings 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 [http://www.octave.org/doc/interpreter/Paging-Screen-Output.html herein 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 {{Codeline|pause}} command at the end of your script. ==How do I get sound input or output in Windows?== Sound input from a sound card and output to a sound card is fully supported in Octave 4.0.0 and newer in all operating systems. Older versions of Octave had very limited audio support that was essentially Linux-specific. If you have problems with the audio I/O functions using Octave 4.0 or a newer version, please report them on the [http://bugs.octave.org bug tracker]. ==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 '''octave-foo''', and you should install that instead using apt or yum.
If you really need A common solution is to build Octave packages from source to install them, you'll need mkoctfile. Most distributions split Octave into several packagesput a {{manual|pause}} command at the end of your script. The script mkoctfile is then part of a separate package:
* Debian/Ubuntu: {{Codeline|octave-headers}} ==How do I get sound input or {{Codeline|liboctave-dev}}output in Windows?==
* FedoraSound 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 [http: {{Codeline|//www.octave.org/doc/interpreter/Audio-devel}}Processing.html audio I/O functions] using Octave 4.0.0 or a newer version, please report them on the [http://bugs.octave.org bug tracker].
==I'm having have problem X using the latest Octave version==
Please be more specific. What is the latest version, according to you? If about what you mean the by "latest released version, be aware that you may still have an older version due to whatever distribution method you're using. There may be a newer version available that you're not aware of due to the distribution method you're using to get Octave, and in most cases, there is a way to get a newer version via your distribution method (see other wiki pages for [[Octave_for_GNU/Linux|GNU/Linux]], [[Octave_for_MacOS_X|Mac OSX]], and [[Octave_for_Windows|Windows]])."?
If you mean the * The latest Mercurial revision, please specify which one that stable version is{{Release}}. "Latest tip" is not informative, because from the time Be aware that you wrote "latest tip" may still have an older version due to the time someone reads that message, "latest tip" might have changed meaning. Also, whatever distribution method you might be standing on a different commit than what "hg tip" saysare using. The tip may be on To get a different branchnewer stable version for your system see for example [[Octave for GNU/Linux|GNU/Linux]], [[Octave for macOS|macOS]], or you might have updated to a different revision different from what "hg tip" says[[Octave for Microsoft Windows|Windows]]).
Instead* If you refer to the latest Mercurial revision, report please specify the [https://www.mercurial-scm.org/wiki/ChangeSetID changeset ID] not the revision number, e.g. the output of "<code>hg summary" </code> or "<code>hg id". Also please use hashes instead or in addition to revision numbers. Revision numbers are just a convenience and only make sense in your local repo, and might not coincide with what someone sees on their own repo</code>. Hashes Changeset IDs are globally unique across all repos.
If your problem truly persists with the "latest version", as indicated then please [http://wwwbugs.gnuoctave.org/software/octave/download.html here], then by all means report a bug ] or ask for helpat[https://lists.gnu.org/mailman/listinfo/help-octave help@octave.org]. Otherwise, but don't be surprised if volunteers are less inclined to help you with a problem that only exists in an older version of Octaveand is already fixed in a newer version.
==Why is Octave's floating-point computation wrong?==
Floating -point 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 are not guaranteed to be accurate, see [https://en.wikipedia.org/wiki/Rounding#Table-maker.27s_dilemma Table-maker's dilemma]. Their results are system-dependent.
This isn't an Octave bug. It happens with any program that uses [httphttps://en.wikipedia.org/wiki/IEEE_754 IEEE 754 floating -point arithmetic]. To be fair, IEEE 754 also specifies decimal floating -point arithmetic, which has never seen wide adoption. The reason why Octave and other programs use using IEEE 754 binary floats floating-point 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 floating -point arithmetic.
Like death and taxes, Another approach to deal with rounding errors are a fact of life. You cannot avoid them. You can only move a rounding error from one part of a computation to another, is interval arithmetic with the [[Interval package]] or you can use more precision and delay symbolic computatons with the rounding error[[Symbolic package]]. One way These approaches are likely to delay the rounding error is to use arbitrary precision arithmeticbe slower, which is inevitably slower as it has to since not all operations can be implemented in software instead of hardware. You may use the vpa function from the symbolic package for this purposeperformed on Hardware like pure floatin-point arithmetic.
Another approach to the problem is interval arithmetic with the [[Interval package]]. Then, the exact result would always be enclosed by two binary floats. Again, this is slower since only the most basic interval arithmetic operations can be performed in hardware. To learn more about floating -point arithmetic, consult the [httphttps://en.wikipedia.org/wiki/Floating_point_arithmetic its Floating-point_arithmetic Wikipedia article] or the classical reference by David Goldberg [http://floatingdocs.oracle.com/cd/E19957-point01/806-gui3568/ncg_goldberg.de/ html What Every Computer Scientist Should Know About Floating -Point Arithmetic]. ==I have installed a package but still get a "foo undefined" error?== You have probably forgotten to load the package. Use {{Codeline|pkg load package-name}} to load it. Most packages are no longer loaded automatically to avoid surprises. See reasoning on related FAQ [[FAQ#How_do_I_install_all_Octave_packages.3F|how do I install all Octave packages]]. If you want a specific package to be loaded by default at startup, consider adding the {{Codeline|pkg load}} command on your {{path|[[.octaverc]]}} file.
==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. {{bug|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://qt-project.org/wiki/Cross-compiling-Mesa-for-Windows.
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==
</blockquote>
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.
==Nested Functions==
Octave has limited support for nested functionssince version 3.8.0. That is
<syntaxhighlight lang="Matlab">function y = foo (x) y = bar(x) function y = bar (x) y = ...; end end</syntaxhighlight>
is equivalent to
<syntaxhighlight lang="Octave">function y = foo (x) y = bar(x) endendfunction  function y = bar (x) y = ...; endendfunction</syntaxhighlight>
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 “un-nests” 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.
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 not yet only partially supported, though work is underway in a branch of the development treesee [[classdef]] for details.
==Differences in core functions==
A large number of the Matlab core functions (ie 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 {{manual|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.
==Just-In-Time compiler==
Matlab includes a "Just-In-Time" compiler. This compiler allows the acceleration of for-loops 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 doesn't have has a [[JIT|not fully functional JIT and so to some might seem slower than Matlabcompiler]]. For this reason you must [[Performance#Vectorization|vectorize your code ]] as much as possible. The MathWorks themselves have a good document discussing vectorization at http://www.mathworks.com/support/tech-notes/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. There have been several aborted attempts at creating an Octave compiler. Should the JIT compiler above ever be implemented, an Octave compiler should be more feasible.
==Graphic handles==
Up to Octave 2.9.9 there was no support for graphic handles in Octave itself. In the 3.2.N versions of Octave and beyond, the The support for graphics handles is converging towards full compatibility. The patch function is currently limited to 2-D patches If you notice any incompatibilities, due to an underlying limitation in gnuplot, but the experimental OpenGL backend is starting to see an implementation of 3-D patchesplease [http://bugs.octave.org report a bug].
==GUI functions==
There are no The support for [http://www.octave.org/doc/interpreter/GUI-Development.html Matlab compatible GUI functions yet] was added in Octave version 3.6.0 and is converging towards full compatibility. This might be an issue if If you intend to exchange Octave code with Matlab users. There are a number of bindings from Octave to {{Forge|tcl-octave|Tcl/Tk}}notice any incompatibilities, please [http://octavizbugs.sourceforgeoctave.net/index.php? VTKorg report a bug] and {{Forge|zenity}} for example, that can be used for a GUI, but these are not Matlab compatible. Work on a Matlab compatible GUI is in an alpha stage in the QtHandles project, which may form part of a future release of Octave.
==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.
==MEX-Files==
Octave includes an [http://www.octave.org/doc/interpreter/Mex_002dFiles.html 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 co-locating 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 {{Codeline|<code>#{}} </code> and {{Codeline|<code>#&#125;}} </code> markers (or {{Codeline| <code>%{}} </code> and {{Codeline|<code>%&#125;}}</code>) are supported by Octave with some limitations. The major limitation is that block comments are not supported within [] or {}.
==Mat-File 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 64-bit 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.
==Profiler==
Thanks to Daniel Kraft's 2011 Google Summer of Code project, [http://www.octave.org/doc/interpreter/Profiling.html Octave has a profiler ] since version 3.6.0. However, at the moment it only produces text output and has its own makeshift interface for hierarchical profiling.
==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.
==Short-circuit {{Codeline|<code>&}} </code> and {{Codeline<code>|&#124;}} </code> operators==
The {{Codeline|<code>&}} </code> and {{Codeline<code>|&#124;}} </code> operators in Matlab short-circuit when included in a condition (e.g. an {{Codeline|if}} or {{Codeline|while}} statement) and not otherwise. In Octave only the {{Codeline|<code>&&}} </code> and {{Codeline<code>||&#124;&#124;}} </code> short circuit. Note that this means that
<syntaxhighlight lang="Octave">if (a | b) ... end</syntaxhighlight>
and
<syntaxhighlight lang="Octave">t = a | b; if (t) ... end</syntaxhighlight> 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 <code>&&</code> and <code>||</code> operators in {{Codeline|if}} statements if possible.
have different semantics in Note that the difference with Matlab. This is really also significant when either argument is a Matlab bug, but there function with side effects or if the first argument is too much code out there that relies on this behavior to change it. Prefer a scalar and the {{Codeline|&#124;&#124;}} and {{Codeline|&&}} operators in {{Codeline|if}} statements if possiblesecond argument is an empty matrix. If you need to use code written for Matlab that depends on this buggy behavior For example, you can enable it since Octave 3.4.0 with note the following command:difference between
do_braindead_shortcircuit_evaluation<syntaxhighlight lang="Octave">t = 1 | []; ## results in [], so...if (t) 1, end ## in if ([]), this is false.</syntaxhighlight>
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 <syntaxhighlight lang= "Octave">if (1 | []; ## results in [], so... if (t) 1, end ## in if ([]), this short circuits so condition is falsetrue.</syntaxhighlight>
andIn the latter case, Octave displays since version 4.0.0 a warning:
if (1 | []) 1, end ## warning: Matlab-style short circuits so condition is true.-circuit operation performed for operator |
Another case that is documented in the Matlab manuals is that
<syntaxhighlight lang="Matlab">t = [1, 1] | [1, 2, 3]; ## error if ([1, 1] | [1, 2, 3]) 1, end ## OK</syntaxhighlight>
Also Matlab requires the operands of {{Codeline|<code>&&}} </code> and {{Codeline<code>||&#124;&#124;}} </code> 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 {{Codeline|if}} statement as being equivalent to {{Codeline|all(X(:))}} when {{Codeline|X}} is a matrix. This is true for all cases EXCEPT empty matrices:
<syntaxhighlight lang="Matlab">if ([0, 1]) == if (all ([0, 1])) ==> i.e., condition is false. if ([1, 1]) == if (all ([1, 1])) ==> i.e., condition is true.</syntaxhighlight>
However,
<syntaxhighlight lang="Matlab">if ([])</syntaxhighlight>
is not the same as
<syntaxhighlight lang="Matlab">if (all ([]))</syntaxhighlight>
because, despite the name, the {{Codelinemanual|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 [httphttps://en.wikipedia.org/wiki/Vacuous_truth vacuous truth]. But, somewhere along the line, someone decided that {{Codeline|if ([])}} should be false. The Mathworks probably thought it just looks wrong to have {{Codeline|[]}} 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 {{Codeline|if}} statements containing empty matrices.
==Solvers for singular, under- and over-determined matrices==
Matlab's solvers as used by the operators {{manual|mldivide (}} <code>\) </code> and {{manual|mrdivide (}} <code>/</)code>, 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
<syntaxhighlight lang="Octave">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</syntaxhighlight>
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:
<syntaxhighlight lang="Octave">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)</syntaxhighlight>
while Octave's minimum-norm values are around about 3e-2, Matlab's results are 50-times larger. For another issue, try this code:
<syntaxhighlight lang="Octave">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)</syntaxhighlight>
It shows that unlike in Octave, {{manual|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 {{manual|mldivide (}} <code>\) </code> and {{manual|mrdivide (}} <code>/</) code> 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 than Matlab's.
==Octave extensions==
Note that using try/catch combined with {{Codeline|rethrow (lasterror ())}} can not guarantee that global variables will be correctly reset, as it won't catch user interrupts with Ctrl-C. For example
<syntaxhighlight lang="Matlab">global a a = 1; try _a = a; a = 2 while true end catch fprintf ('caught interrupt\n'); a = _a; rethrow (lasterror()); end</syntaxhighlight>
compared to
<syntaxhighlight lang="Octave">global a a = 1; unwind_protect _a = a; a = 2 while true end unwind_protect_cleanup fprintf ('caught interrupt\n'); a = _a; end</syntaxhighlight>
Typing Ctrl-C in the first case returns the user directly to the prompt, and the variable ''<code>a'' </code> is not reset to the saved value. In the second case the variable ''<code>a'' </code> 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 {{Codeline|sin(x)(1:10)}} for example is perfectly valid in Octave but not Matlab. To do the same in Matlab you must do {{Codeline|y &#61; sin(x); y &#61; y([1:10]);}}
Functions can be defined by entering code on the command line, a feature not supported by Matlab. For example, you may type:
octave:1<syntaxhighlight lang="Octave">>> 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</syntaxhighlight>
As a natural extension of this, functions can also be defined in script files (m-files whose first non-comment line isn't {{Codeline|function out &#61; foo (...)}})
===Strings delimited by double quotes "===
The double quoteIn 2016, {{Codeline|"}}Matlab introduced String Arrays, may be used to delimit that are initialized by using double quoted strings, and are not implemented in addition to the single quote {{Codeline|'}}. See the previous exampleOctave yet. Also, In Octave double-quoted 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, {{Codeline|\}}, in addition to three points {{Codeline|...}}. See the previous exampleas in Matlab.
===Informative block closing===
Indexing other things than variables is possible, as in:
octave:1<syntaxhighlight lang="Octave">>> [3 1 4 1 5 9](3) ans = 4 octave:2>> cos([0 pi pi/4 7])(3) ans = 0.70711</syntaxhighlight>
In Matlab, it is for example necessary to assign the intermediate result {{Codeline|cos([0 pi pi/4 7])}} to a variable before it can be indexed again.
The exclamation mark {{Codeline|!}} (aka “Bang!”) is a negation operator, just like the tilde {{Codeline|~}}:
octave:1<syntaxhighlight lang="Octave">>> if ! strcmp (program_name, "octave"), > "It's an error" > else > "It works!" > end ans = It works!</syntaxhighlight> 
Note however that Matlab uses the {{Codeline|!}} operator for shell escapes, for which Octave requires using the system command.
In addition to try-catch blocks, Octave supports an alternative form of exception handling modeled after the unwind-protect form of Lisp. The general form of an unwind_protect block looks like this:
<syntaxhighlight lang="Octave">unwind_protect body unwind_protect_cleanup cleanup end_unwind_protect</syntaxhighlight>
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 {{Codelinemanaul|onCleanup}} function that was introduced in 2008a. Octave also has {{Codelinemanual|onCleanup}} since version 3.4.0.
===Built-in ODE and DAE solvers===
===Do-Until loop structure===
Similar to the do-while loop in C and C++, Octave allows a do-until loop which does not exist in Matlab:
<syntaxhighlight lang="Octave">x = 0 do x += 1; until (x == 10)</syntaxhighlight>
===Broadcasting===
Borrowed from [http://stackoverflow.com/q/26948776/3565696 other languages], [http://www.gnuoctave.org/software/octave/doc/v4.0.1interpreter/Broadcasting.html octave broadcasting] allows easy and readable vectorization.
<syntaxhighlight lang="Octave">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 2-dimensional array s = sin(omega .* t); # which can be displayed as pcolor(t, f, s) xlabel("t (s)") ylabel("f (Hz)")</syntaxhighlight>
Note: [https://www.mathworks.com/help/matlab/matlab_prog/compatible-array-sizes-for-basic-operations.html Automatic expansion of dimensions] was added to MATLAB R2016b.
Octave allows to add self-tests to user defined functions. Tests are put after function definition in specially commented block.
<syntaxhighlight lang="Octave">function mult = a(val) mult = val.*2; endfunction %!test %! assert (a(3), 6);</syntaxhighlight>
Such a function can be tested for valid outputs by following code:
octave:1>> 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 {{manual|demo }} for function multinom of package specfun, use:
demo multinom

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