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<title> Octave -- a high-level language for numerical computations </title>
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<h1>A Brief Introduction to Octave</h1>

<ul>
<li><a href="readme.html#Overview">Overview</a></li>
<li><a href="readme.html#Language Features">Language Features</a></li>
<li><a href="readme.html#Distribution Terms">Distribution Terms</a></li>
<li><a href="readme.html#Availability">Availability</a></li>
<li><a href="readme.html#Installation and Bugs">Installation and Bugs</a></li>
<li><a href="readme.html#Documentation">Documentation</a></li>
<li><a href="readme.html#Implementation">Implementation</a></li>
</ul>
<hr>

<h2><a name="Overview">Overview</a></h2>
<p>
Octave is a high-level language, primarily intended for numerical
computations.  It provides a convenient command line interface for
solving linear and nonlinear problems numerically, and for performing
other numerical experiments.  It may also be used as a batch-oriented
language.
</p>

<h2><a name="Language Features">Language Features</a></h2>
<p>
The best way to introduce Octave's language is probably to show a few
simple examples.
</p>

<p>
If you are new to Octave, I recommend that you try these examples to
begin learning Octave by using it.  Lines marked with
<tt>octave:13></tt> are lines you type, ending each with a carriage
return.  Octave will respond with an answer, or by displaying a graph.
</p>

<h3>Creating a Matrix</h3>

<p>
To create a new matrix and store it in a variable so that it you can
refer to it later, type the command

<pre>
    octave:1> a = [ 1, 1, 2; 3, 5, 8; 13, 21, 34 ]
</pre>

Octave will respond by printing the matrix in neatly aligned columns.
Ending a command with a semicolon tells Octave to not print the result
of a command.  For example

<pre>
    octave:2> b = rand (3, 2);
</pre>

will create a 3 row, 2 column matrix with each element set to a random
value between zero and one.
</p>

<p>
To display the value of any variable, simply type the name of the
variable.  For example, to display the value stored in the matrix
<tt>b</tt>, type the command

<pre>
    octave:3> b
</pre>

<h3>Matrix Arithmetic</h3>

<p>
Octave has a convenient operator notation for performing matrix
arithmetic.  For example, to multiply the matrix <tt>a</tt> by a
scalar value, type the command

<pre>
    octave:4> 2 * a
</pre>

<p>
To multiply the two matrices <tt>a</tt> and <tt>b</tt>, type the
command

<pre>
    octave:5> a * b
</pre>

<p>
To form the matrix product <tt>transpose (a) * a</tt>, type the command

<pre>
    octave:6> a' * a
</pre>
</p>

<h3>Solving Linear Equations</h3>

<p>
To solve the set of linear equations <tt>Ax = b</tt> use the left
division operator, <tt>\</tt>:

<pre>
    octave:7> a \ b
</pre>

This is conceptually equivalent to <tt>inv (A) * b</tt>, but avoids
computing the inverse of a matrix directly.
</p>

<p>
If the coefficient matrix is singular, Octave will print a warning
message and compute a minimum norm solution.
</p>

<h3>Integrating Differential Equations</h3>

<p>
Octave has built-in functions for solving nonlinear differential
equations of the form

<pre>
    dx
    -- = f (x, t)
    dt
</pre>

with the initial condition

<pre>
    x(t = t0) = x0
</pre>
</p>

<p>
For Octave to integrate equations of this form, you must first provide a
definition of the function <tt> f (x, t)</tt>.  This is
straightforward, and may be accomplished by entering the function body
directly on the command line.  For example, the following commands
define the right hand side function for an interesting pair of
nonlinear differential equations.  Note that while you are entering a
function, Octave responds with a different prompt, to indicate that it
is waiting for you to complete your input.

<pre>
    octave:8> function xdot = f (x, t)
    >
    >  r = 0.25;
    >  k = 1.4;
    >  a = 1.5;
    >  b = 0.16;
    >  c = 0.9;
    >  d = 0.8;
    >
    >  xdot(1) = r*x(1)*(1 - x(1)/k) - a*x(1)*x(2)/(1 + b*x(1));
    >  xdot(2) = c*a*x(1)*x(2)/(1 + b*x(1)) - d*x(2);
    >
    > endfunction
</pre>
</p>

<p>
Given the initial condition

<pre>
    x0 = [1; 2];
</pre>

and the set of output times as a column vector (note that the first
output time corresponds to the initial condition given above)

<pre>
    t = linspace (0, 50, 200)';
</pre>

it is easy to integrate the set of differential equations:

<pre>
    x = lsode ("f", x0, t);
</pre>

<p>
The function <tt>lsode</tt> uses the Livermore Solver for Ordinary
Differential Equations, described in A. C. Hindmarsh, <em>ODEPACK, a
Systematized Collection of ODE Solvers</em>, in: Scientific Computing,
R. S.  Stepleman et al. (Eds.), North-Holland, Amsterdam, 1983, pages
55-64.
</p>

<h3>Producing Graphical Output</h3>

<p>
To display the solution of the previous example graphically, use the
command

<pre>
    plot (t, x)
</pre>
</p>

<p>
If you are using the X Window System, Octave will automatically create
a separate window to display the plot.  If you are using a terminal that
supports some other graphics commands, you will need to tell Octave what
kind of terminal you have.  Type the command
</p>

<pre>
    gset term
</pre>

to see a list of the supported terminal types.  Octave uses
<tt>gnuplot</tt> to display graphics, and can display graphics on any
terminal that is supported by <tt>gnuplot</tt>.

<p>
To capture the output of the plot command in a file rather than sending
the output directly to your terminal, you can use a set of commands like
this

<pre>
    gset term postscript
    gset output "foo.ps"
    replot
</pre>

This will work for other types of output devices as well.  Octave's
<tt>gset</tt> command is really just piped to the <tt>gnuplot</tt>
subprocess, so that once you have a plot on the screen that you like,
you should be able to do something like this to create an output file
suitable for your graphics printer.
</p>

<p>
Or, you can eliminate the intermediate file by using commands like this

<pre>
    gset term postscript
    gset output "|lpr -Pname_of_your_graphics_printer"
    replot
</pre>

<h3>Editing What You Have Typed</h3>

<p>
At the Octave prompt, you can recall, edit, and reissue previous
commands using Emacs- or vi-style editing commands.  The default
keybindings use Emacs-style commands.
</p>

<h3>Getting Help</h3>

<p>
Octave has an extensive help facility.  The same documentation that is
available in printed form is also available from the Octave prompt,
because both forms of the documentation are created from the same input
file.
</p>

<p>
In order to get good help you first need to know the name of the command
that you want to use.  This name of the function may not always be
obvious, but a good place to start is to just type <tt>help</tt>.
This will show you all the operators, reserved words, functions,
built-in variables, and function files.  You can then get more
help on anything that is listed by simply including the name as an
argument to help.  For example,

<pre>
    help plot
</pre>

will display the help text for the <tt>plot</tt> function.
</p>

<p>
The complete text of the manual is availabe from Octave's command line
using the command <tt>help -i</tt>.  Because it is written in Texinfo,
it is also possible to put
<a href="http://www.che.wisc.edu/cgi-bin/info2www?(octave)">the manual
on the WWW</a>.
</p>

<h2><a name="Distribution Terms">Distribution Terms</a></h2>
<p>
Octave is free software; you can redistribute it and/or modify it
under the terms of the
<a href="http://www.che.wisc.edu/cgi-bin/info2www?(octave)Copying">GNU
General Public License</a> as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any
later version.
</p>

<p>
Octave is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the file COPYING for more
details.
</p>

<h2><a name="Availability">Availability</a></h2>
<p>
The latest released version of Octave is always available via
anonymous ftp from
<a href="ftp://ftp.che.wisc.edu/pub/octave">ftp://ftp.che.wisc.edu/pub/octave</a>.
Complete source and binaries for several popular systems are available.
</p>

<h2><a name="Installation and Bugs">Installation and Bugs</a></h2>
<p>
Octave requires approximately 125MB of disk storage to unpack and
compile from source (significantly less if you don't compile with
debugging symbols or create shared libraries).  Once installed, Octave
requires approximately 65MB of disk space (again, considerably less if
you don't build shared libraries or the binaries and libraries do not
include debugging symbols).
</p>

<p>
In order to build Octave, you will need a current version of g++,
libstdc++, and GNU make.
</p>

<p>
<b>You must have GNU Make to compile Octave</b>.  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.
</p>

<p>
See the notes in the files INSTALL and INSTALL.OCTAVE for more
specific installation instructions, including directions for
installing Octave from a binary distribution.
</p>

<p>
The file BUGS contains a recommended procedure for reporting bugs, as
well as a list of known problems and possible fixes.
</p>

<h2><a name="Documentation">Documentation</a></h2>

<p>
Octave's manual has been revised for version 2.0, but it is lagging a
bit behind the development of the software.  In particular, there is
currently no complete documentation of the C++ class libraries or the
support for dynamic linking and user-defined data types.  If you
notice ommissions or inconsistencies, please report them as bugs to
bug@@octave.org.  Specific suggestions for ways to
improve Octave and its documentation are always welcome.
</p>

<h2><a name="Implementation">Implementation</a></h2>
<p>
Octave is being developed with the Free Software Foundation's make,
bison (a replacement for YACC), flex (a replacement for lex), gcc/g++,
and libstdc++ on an Intel Pentium II system running Linux/GNU.  It
should be possible to install it on any machine that runs GCC/G++.  It
may also be possible to install it using other implementations of
these tools, but it will most certainly require much more work.  Do
yourself a favor and get the GNU development tools, either via
anonymous ftp from ftp.gnu.org or by writing the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301, USA.
</p>

<p>
The underlying numerical solvers are currently standard Fortran ones
like Lapack, Linpack, Odepack, the Blas, etc., packaged in a library
of C++ classes (see the files in the libcruft and liboctave
subdirectories).  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, they are translated with f2c and
compiled with a C compiler.  Better performance is usually achieved if
the intermediate translation to C is avoided.
</p>

<p>
The library of C++ classes may also be useful by itself.
</p>

<hr>
<p>
Back to the <a href="http://www.che.wisc.edu/octave">Octave home page</a>.
</p>
<hr>
<p>
<a href="http://www.che.wisc.edu/~jwe">John W. Eaton</a><br>
<a href="mailto:jwe@bevo.che.wisc.edu"><i>jwe@bevo.che.wisc.edu</i></a><br>
University of Wisconsin<br>
Department of Chemical Engineering<br>
Madison WI 53719
</p>
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author	John W. Eaton <jwe@octave.org>
date	Fri, 18 Jul 2008 17:42:48 -0400
parents	93c65f2a5668
children	66fdc831c580