# GNU Octave

Developer(s) John W. Eaton 1988 https://octave.sourceforge.io/ C++ Linux, UNIX, Cygwin, Windows, Android[1] GNU General Public License http://www.gnu.org/software/octave/

GNU Octave is software featuring a high-level programming language, primarily intended for numerical analysis|numerical computations. Octave helps in solving linear and nonlinear problems numerically, and for performing other numerical experiments using a language that is mostly compatible with MATLAB. It may also be used as a Batch processing|batch-oriented language. Since it is part of the GNU Project, it is free software under the terms of the GNU General Public License.

Octave is one of the major free alternatives to MATLAB, others being FreeMat and Scilab, however, puts less emphasis on (bidirectional) syntactic compatibility with MATLAB than Octave does.

## History

Octave was originally conceived (in about 1988) to be companion software for an undergraduate-level textbook on chemical reactor design being written by James B. Rawlings of the University of Wisconsin-Madison and John G. Ekerdt of the University of Texas. We originally envisioned some very specialized tools for the solution of chemical reactor design problems. Later, after seeing the limitations of that approach, we opted to attempt to build a much more flexible tool.

There were still some people who said that we should just be using Fortran instead, because it is the computer language of engineering, but every time we had tried that, the students spent far too much time trying to figure out why their Fortran code failed and not enough time learning about chemical engineering. We believed that with an interactive environment like Octave, most students would be able to pick up the basics quickly, and begin using it confidently in just a few hours.

Full-time development began in the Spring of 1992. The first alpha release was January 4, 1993, and version 1.0 was released February 17,

Since then, Octave has been through several major revisions, is included with Debian GNU/Linux, openSUSE, and many other GNU/Linux distributions. Octave was reviewed in the in the July, 1997 issue of the Linux Journal. Clearly, Octave is now much more than just another courseware package with limited utility beyond the classroom. Although our initial goals were somewhat vague, we knew that we wanted to create something that would enable students to solve realistic problems, and that they could use for many things other than chemical reactor design problems. Today, thousands of people worldwide are using Octave in teaching, research, and commercial applications.

Just about everyone thinks that the name Octave has something to do with music, but it is actually the name of one of the author’s former professors who wrote a famous textbook on chemical reaction engineering, and who was also well known for his ability to do quick “back of the envelope” calculations. We hope that this software will make it possible for many people to do more ambitious computations just as easily.

Everyone is encouraged to share this software with others under the terms of the GNU General Public License (GPL). You are also encouraged to help make Octave more useful by writing and contributing additional functions for it, and by reporting any problems you may have.

The program is named after Octave Levenspiel, a former professor of the principal author. Levenspiel is known for his ability to perform quick back-of-the-envelope calculations.[2]

## Developments

In addition to use on desktops for personal scientific computing, Octave is used in academia and industry. For example, Octave was used on a massive parallel computer at Pittsburgh supercomputing center to find vulnerabilities related to guessing social security numbers.[3]

## Technical details

• Octave is written in C++ using the C++ standard library.
• Octave uses an interpreter to execute the Octave scripting language.
• Octave is extensible using dynamically loadable modules.
• Octave interpreter has an OpenGL-based graphics engine to create plots, graphs and charts and to save or print them. Alternatively, gnuplot can be used for the same purpose.
• Octave versions 3.8.0 and later include a Graphical User Interface (GUI) in addition to the traditional Command Line Interface (CLI).

## Octave, the language

The Octave language is an interpreted programming language. It is a structured programming language (similar to C) and supports many common C standard library functions, and also certain UNIX system calls and functions.[4] However, it does not support passing arguments by reference.[5]

Octave programs consist of a list of function calls or a script. The syntax is matrix-based and provides various functions for matrix operations. It supports various data structures and allows object-oriented programming.[6]

Its syntax is very similar to MATLAB, and careful programming of a script will allow it to run on both Octave and MATLAB.[7]

Because Octave is made available under the GNU General Public License, it may be freely changed, copied and used.[2] The program runs on Microsoft Windows and most Unix and Unix-like operating systems, including OS X.[8]

### Command and variable name completion

Typing a TAB character on the command line causes Octave to attempt to complete variable, function, and file names (similar to Bash's tab completion). Octave uses the text before the cursor as the initial portion of the name to complete. [9]

### Command history

When running interactively, Octave saves the commands typed in an internal buffer so that they can be recalled and edited.

### Data structures

Octave includes a limited amount of support for organizing data in structures. In this example, we see a structure "x" with elements "a", "b", and "c", (an integer, an array, and a string, respectively):

octave:1> x.a = 1; x.b = [1, 2; 3, 4]; x.c = "string";
octave:2> x.a
ans =  1
octave:3> x.b
ans =

1   2
3   4

octave:4> x.c
ans = string
octave:5> x
x =
{
a =  1
b =

1   2
3   4

c = string
}


### Short-circuit boolean operators

Octave's '&&' and '||' logical operators are evaluated in a Short-circuit evaluation|short-circuit fashion (like the corresponding operators in the C language), in contrast to the element-by-element operators '&' and '|'.

### Increment and decrement operators

Template:Main article Octave includes the C-like increment and decrement operators '++' and '--' in both their prefix and postfix forms. Octave also does augmented assignment, e.g. 'x += 5'.

### Unwind-protect

Octave supports a limited form of exception handling modelled after the 'unwind_protect' of Lisp. The general form of an unwind_protect block looks like this:

unwind_protect
body
unwind_protect_cleanup
cleanup
end_unwind_protect


As a general rule, GNU Octave recognizes as termination of a given 'block' either the keyword 'end' (which is compatible with the MATLAB language) or a more specific keyword 'end_block'. As a consequence, an 'unwind_protect' block can be terminated either with the keyword 'end_unwind_protect' as in the example, or with the more portable keyword 'end'.

The cleanup part of the block is always executed. In case an exception is raised by the body part, cleanup is executed immediately before propagating the exception outside the block 'unwind_protect'.

GNU Octave also supports another form of exception handling (compatible with the MATLAB language):

try
body
catch
exception_handling
end


This latter form differs from an 'unwind_protect' block in two ways. First, exception_handling is only executed when an exception is raised by body. Second, after the execution of exception_handling the exception is not propagated outside the block (unless a 'rethrow( lasterror )' statement is purposely inserted within the exception_handling code).

### Variable-length argument lists

Octave has a mechanism for handling functions that take an unspecified number of arguments without explicit upper limit. To specify a list of zero or more arguments, use the special argument varargin as the last (or only) argument in the list.

function s = plus (varargin)
if (nargin==0)
s = 0;
else
s = varargin{1} + plus (varargin{2:nargin});
end
end


### Variable-length return lists

A function can be set up to return any number of values by using the special return value varargout. For example:

function varargout = multiassign (data)
for k=1:nargout
varargout{k} = data(:,k);
end
end


### C++ integration

It is also possible to execute Octave code directly in a C++ program. For example, here is a code snippet for calling rand([10,1]):

#include <octave/oct.h>
...
ColumnVector NumRands(2);
NumRands(0) = 10;
NumRands(1) = 1;
octave_value_list f_arg, f_ret;
f_arg(0) = octave_value(NumRands);
f_ret = feval("rand", f_arg, 1);
Matrix unis(f_ret(0).matrix_value());


C and C++ code can be integrated into GNU Octave by creating oct files, or using the Matlab compatible MEX files.

## MATLAB compatibility

Octave has been built with MATLAB compatibility in mind, and shares many features with MATLAB:

1. Matrices as fundamental data type.
2. Built-in support for complex numbers.
3. Powerful built-in math functions and extensive function libraries.
4. Extensibility in the form of user-defined functions.

In fact, Octave treats incompatibility with MATLAB as a bug;[10] therefore, it can be considered a software clone, which doesn't infringe software copyright as per Lotus v. Borland court case.

MATLAB scripts from the MathWorks' FileExchange repository are compatible with Octave, but can't be used legally due the Terms of use.[11] While often provided and uploaded by users under an Octave compatible and proper Open source BSD license, the fileexchange's Terms of use prohibit any usage beside MathWorks proprietary MATLAB.[12][13]

### Syntax compatibility

There are a few purposeful, albeit minor, syntax additions:

1. Comment lines can be prefixed with the # character as well as the % character;
2. Various C-based operators ++, --, +=, *=, /= are supported;
3. Elements can be referenced without creating a new variable by cascaded indexing, e.g. [1:10](3);
4. Strings can be defined with the " character as well as the ' character;
5. When the variable type is single, Octave calculates the "mean" in the single-domain (Matlab in double-domain) which is faster but gives less accurate results;
6. Blocks can also be terminated with more specific Control structure keywords, i.e., endif, endfor, endwhile, etc.;
7. Functions can be defined within scripts and at the Octave prompt;
8. All operators perform automatic broadcasting or singleton expansion.
9. Presence of a do-until loop (similar to do-while in C).

### Function compatibility

Many of the numerous MATLAB functions are available in GNU Octave, some of them are accessible through packages via Octave-forge, but not all MATLAB functions are available in GNU Octave. List of unavailable functions exists in Octave, and developers are seeking for help to implement them. Looking for function __unimplemented.m__, leads to the list of unimplemented functions.

Unimplemented functions are also categorized in Image, Mapping, Optimization, Signal, and Statistics packages.

When an unimplemented function is called the following error message is shown:

  octave:1> quad2d

contribute missing functionality.
warning: called from
__unimplemented__ at line 523 column 5
error: 'quad2d' undefined near line 1 column 1


## User interfaces

Until version 3.8, Octave did not come with a graphical user interface (GUI)/integrated development environment (IDE) by default. However, an official graphical interface based on Qt has now been migrated to the main source repository and is available with Octave 3.8, but not as the default interface.[14] It has become the default interface with the release of Octave 4.0.[15] Several 3rd-party graphical front-ends have been developed.

## References

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