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ΓòÉΓòÉΓòÉ 1. Title page ΓòÉΓòÉΓòÉ

                                   Octave FAQ

                     Frequently asked questions about Octave

                                February 14, 1998

                                  John W. Eaton


ΓòÉΓòÉΓòÉ 2. Preface ΓòÉΓòÉΓòÉ

This is a list of frequently asked questions (FAQ) for Octave users.

Some information in this FAQ was written for earlier versions of Octave and may
now be obsolete.

I'm looking for new questions (with answers), better answers, or both.  Please
send suggestions to [e-mail]. 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 [e-mail] mailing list.

This FAQ is intended to supplement, not replace, the Octave manual. Before
posting a question to the help-octave mailing list, you should first check to
see if the topic is covered in the manual.

 What is Octave?
 Version 2.0
 Octave Features
 Documentation
 Getting Octave
 Installation
 Common problems
 Getting additional help
 Bug reports
 MATLAB compatibility
 Index


ΓòÉΓòÉΓòÉ 3. What is Octave? ΓòÉΓòÉΓòÉ

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 and complex 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 200+ page Texinfo manual.  Access to the
complete text of the manual is available via the help command at the Octave
prompt.

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.


ΓòÉΓòÉΓòÉ 4. What's new in version 2.0 of Octave ΓòÉΓòÉΓòÉ

Version 2.0.10 of Octave was released February 6, 1998.  Most bugs reported
since version 2.0 was release have been fixed, and a number of new features
have been added.  Octave is now much more compatible with Matlab.

A list of user-visible changes in recent versions of Octave may be found in the
file NEWS, distributed in both source and binary releases of Octave.


ΓòÉΓòÉΓòÉ 5. What features are unique to Octave? ΓòÉΓòÉΓòÉ

 Command and variable name completion
 Command history
 Data structures
 Short-circuit boolean operators
 Increment and decrement operators
 Unwind-protect
 Variable-length argument lists
 Variable-length return lists
 Built-in ODE and DAE solvers


ΓòÉΓòÉΓòÉ 5.1. Command and variable name completion ΓòÉΓòÉΓòÉ

Typing a TAB character (ASCII code 9) on the command line causes Octave to
attempt to complete variable, function, and file names.  Octave uses the text
before the cursor as the initial portion of the name to complete.

For example, if you type `fu' followed by TAB at the Octave prompt, Octave will
complete the rest of the name `function' on the command line (unless you have
other variables or functions defined that begin with the characters `fu').  If
there is more than one possible completion, Octave will ring the terminal bell
to let you know that your initial sequence of characters is not enough to
specify a unique name. To complete the name, you may either edit the initial
character sequence (usually adding more characters until completion is
possible) or type another TAB to cause Octave to display the list of possible
completions.


ΓòÉΓòÉΓòÉ 5.2. Command history ΓòÉΓòÉΓòÉ

When running interactively, Octave saves the commands you type in an internal
buffer so that you can recall and edit them.  Emacs and vi editing modes are
available with Emacs keybindings enabled by default.

When Octave exits, the current command history is saved to the file
`~/.octave_hist', and each time Octave starts, it inserts the contents of the
`~/.octave_hist' file in the history list so that it is easy to begin working
where you left off.


ΓòÉΓòÉΓòÉ 5.3. Data structures ΓòÉΓòÉΓòÉ

Octave includes a limited amount of support for organizing data in structures.
The current implementation uses an associative array with indices limited to
strings, but the syntax is more like C-style structures.  Here are some
examples of using data structures in Octave.

     Elements of structures can be of any value type.

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

              1  2
              3  4

            octave:4> x.c
            x.c = string

     Structures may be copied.

            octave:1> y = x
            y =
            {
              a = 1
              b =

                1  2
                3  4

              c = string
              s =

                0.00000  0.00000  0.00000
                0.00000  5.46499  0.00000
                0.00000  0.00000  0.36597

              u =

                -0.40455  -0.91451
                -0.91451   0.40455

              v =

                -0.57605   0.81742
                -0.81742  -0.57605
            }

     Structure elements may reference other structures.

            octave:1> x.b.d = 3
            x.b.d = 3
            octave:2> x.b
            ans =
            {
              d = 3
            }
            octave:3> x.b.d
            ans = 3

     Functions can return structures.

            octave:1> function y = f (x)
            > y.re = real (x);
            > y.im = imag (x);
            > endfunction

            octave:2> f (rand + rand*I);
            ans =
            {
              im = 0.18033
              re = 0.19069
            }

     Function return lists can include structure elements, and they may be
      indexed like any other variable.

            octave:1> [x.u, x.s(2:3,2:3), x.v] = svd ([1, 2; 3, 4]);
            octave:2> x
            x =
            {
              s =

                0.00000  0.00000  0.00000
                0.00000  5.46499  0.00000
                0.00000  0.00000  0.36597

              u =

                -0.40455  -0.91451
                -0.91451   0.40455

              v =

                -0.57605   0.81742
                -0.81742  -0.57605
            }

     You can also use the function is_struct to determine whether a given
      value is a data structure.  For example

            is_struct (x)

      returns 1 if the value of the variable x is a data structure.

 This feature should be considered experimental, but you should expect it to
 work.  Suggestions for ways to improve it are welcome.


ΓòÉΓòÉΓòÉ 5.4. Short-circuit boolean operators ΓòÉΓòÉΓòÉ

Octave's `&&' and `||' logical operators are evaluated in a short-circuit
fashion (like the corresponding operators in the C language) and work
differently than the element by element operators `&' and `|'.


ΓòÉΓòÉΓòÉ 5.5. Increment and decrement operators ΓòÉΓòÉΓòÉ

Octave includes the C-like increment and decrement operators `++' and `--' in
both their prefix and postfix forms.

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.

It is not currently possible to increment index expressions.  For example, you
might expect that the expression v(4)++ would increment the fourth element of
the vector v, but instead it results in a parse error.  This problem may be
fixed in a future release of Octave.


ΓòÉΓòÉΓòÉ 5.6. Unwind-protect ΓòÉΓòÉΓòÉ

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.


ΓòÉΓòÉΓòÉ 5.7. Variable-length argument lists ΓòÉΓòÉΓòÉ

Octave has a real mechanism for handling functions that take an unspecified
number of arguments, so it is no longer necessary to place an upper bound on
the number of optional arguments that a function can accept.

Here is an example of a function that uses the new syntax to print a header
followed by an unspecified number of values:

function foo (heading, ...)
  disp (heading);
  va_start ();
  while (--nargin)
    disp (va_arg ());
  endwhile
endfunction

Calling va_start() positions an internal pointer to the first unnamed argument
and allows you to cycle through the arguments more than once.  It is not
necessary to call va_start() if you do not plan to cycle through the arguments
more than once.

The function va_arg() returns the value of the next available argument and
moves the internal pointer to the next argument.  It is an error to call
va_arg() when there are no more arguments available.

It is also possible to use the keyword all_va_args to pass all unnamed
arguments to another function.


ΓòÉΓòÉΓòÉ 5.8. Variable-length return lists ΓòÉΓòÉΓòÉ

Octave also has a real mechanism for handling functions that return an
unspecified number of values, so it is no longer necessary to place an upper
bound on the number of outputs that a function can produce.

Here is an example of a function that uses the new syntax to produce `N'
values:

function [...] = foo (n)
  for i = 1:n
    vr_val (i);
  endfor
endfunction


ΓòÉΓòÉΓòÉ 5.9. Built-in ODE and DAE solvers ΓòÉΓòÉΓòÉ

Octave includes LSODE and DASSL for solving systems of stiff ordinary
differential and differential-algebraic equations.  These functions are built
in to the interpreter.


ΓòÉΓòÉΓòÉ 6. What documentation exists for Octave? ΓòÉΓòÉΓòÉ

The Octave distribution includes a 220+ page manual that is also distributed
under the terms of the GNU GPL.

The Octave manual is intended to be a complete reference for Octave, but it is
not a finished document.  If you have problems using it, or find that some
topic is not adequately explained, indexed, or cross-referenced, please send a
bug report to [e-mail].

Because the Octave manual is written using Texinfo, the complete text of the
Octave manual is also available on line 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.

It is also possible to use your favorite WWW browser to read the Octave manual
by converting the Texinfo source to HTML using the texi2html program.


ΓòÉΓòÉΓòÉ 7. Obtaining Source Code ΓòÉΓòÉΓòÉ

 Octave for Unix
 Octave for other platforms
 latest versions


ΓòÉΓòÉΓòÉ 7.1. How do I get a copy of Octave for Unix? ΓòÉΓòÉΓòÉ

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).

Octave was not developed by the FSF, but the FSF does distribute Octave, and
the developers of Octave support the efforts of the FSF by encouraging users of
Octave to order Octave on CD-ROM directly from the FSF.

The FSF is a nonprofit organization that distributes software and manuals to
raise funds for more GNU development.  Buying a CD-ROM from the FSF contributes
directly to paying staff to develop GNU software. CD-ROMs cost $240 if an
organization is buying, or $60 if an individual is buying.

For more information about ordering from the FSF, contact [e-mail], phone
[phone] or anonymous ftp the file `/pub/gnu/GNUinfo/ORDERS' from
ftp.gnu.org.

If you are on the Internet, you can copy the latest distribution version of
Octave from the file `/pub/octave/octave-M.N.tar.gz', on the host
`ftp.che.wisc.edu'.  This tar file has been compressed with GNU gzip, so be
sure to use binary mode for the transfer.  `M' and `N' stand for version
numbers; look at a listing of the directory through ftp to see what version is
available.  After you unpack the distribution, be sure to look at the files
`README' and `INSTALL'.

Binaries for several popular systems are also available.  If you would like
help out by making binaries available for other systems, please contact
[e-mail].

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.


ΓòÉΓòÉΓòÉ 7.2. How do I get a copy of Octave for (some other platform)? ΓòÉΓòÉΓòÉ

Octave currently runs on Unix-like systems, OS/2, and Windows NT/95 (using the
gnu-win32 tools from Cygnus Support).  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 [e-mail].


ΓòÉΓòÉΓòÉ 7.3. What is the latest version of Octave ΓòÉΓòÉΓòÉ

The latest version of Octave is 2.0.10, released February 6, 1998.


ΓòÉΓòÉΓòÉ 8. Installation Issues and Problems ΓòÉΓòÉΓòÉ

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).

 What else do I need?
 Other C++ compilers?


ΓòÉΓòÉΓòÉ 8.1. What else do I need? ΓòÉΓòÉΓòÉ

To compile Octave, you will need a recent version of GNU Make.  You will also
need g++ 2.7.2 or later.  Version 2.8.0 or egcs 1.0.x should work.  Later
versions may work, but C++ is still evolving, so don't be too surprised if you
run into some trouble.

It is no longer necessary to have libg++, but you do need to have the GNU
implementation of libstdc++.  If you are using g++ 2.7.2, libstdc++ is
distributed along with libg++, but for later versions, libstdc++ is distributed
separately.  For egcs, libstdc++ is included with the compiler distribution.

<em>You must have gnu make to compile octave</em>.  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.


ΓòÉΓòÉΓòÉ 8.2. Can I compile Octave with another C++ compiler? ΓòÉΓòÉΓòÉ

Currently, Octave can only be compiled with the GNU C++ compiler.  It would be
nice to make it possible to compile Octave with other C++ compilers, but the
maintainers do not have sufficient time to devote to this.  If you are
interested in working to make Octave portable to other compilers, please
contact [e-mail].


ΓòÉΓòÉΓòÉ 9. Common problems ΓòÉΓòÉΓòÉ

This list is probably far too short.  Feel free to suggest additional questions
(preferably with answers!)

     Octave takes a long time to find symbols.

      Octave is probably spending this time recursively searching directories
      for function files.  Check the value of your LOADPATH.  For those
      elements that end in `//', do any name a very large directory tree? Does
      it contain directories that have a mixture of files and directories?  In
      order for the recursive directory searching code to work efficiently,
      directories that are to be searched recursively should have either
      function files only, or subdirectories only, but not a mixture of both.
      Check to make sure that Octave's standard set of function files is
      installed this way.


ΓòÉΓòÉΓòÉ 10. Getting additional help ΓòÉΓòÉΓòÉ

The mailing list

[e-mail]

is available for questions related to using, installing, and porting Octave
that are not adequately answered by the Octave manual or by this document.

If you would like to join the discussion and receive all messages sent to the
list, please send a short note to

[e-mail]
            ^^^^^^^

*Please do not* send requests to be added or removed from the 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
ftp.che.wisc.edu in the directory `/pub/octave/MAILING-LISTS'.


ΓòÉΓòÉΓòÉ 11. I think I have found a bug in Octave. ΓòÉΓòÉΓòÉ

``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. The Octave
manual is included in the Octave distribution.

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.

If for some reason you cannot use Octave's bug_report function, mail your bug
report to "[e-mail]".  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.


ΓòÉΓòÉΓòÉ 12. Porting programs from Matlab to Octave ΓòÉΓòÉΓòÉ

``I wrote some code for Matlab, and I want to get it running under Octave.  Is
there anything I should watch out for?''

The differences between Octave and Matlab typically fall into one of three
categories:

   1. Irrelevant.

   2. Known differences, perhaps configurable with a user preference variable.

   3. Unknown differences.

 The first category, irrelevant differences, do not affect computations and
 most likely do not affect the execution of function files.

 The differences of the second category are usually because the authors of
 Octave decided on a better (subjective) implementation that the way Matlab
 does it, and so introduced ``user preference variables'' so that you can
 customize Octave's behavior to be either Matlab-compatible or to use Octave's
 new features.  To make Octave more Matlab-compatible, put the following
 statements in your `~/.octaverc' file, or use the command line option
 `--traditional', which implies all of these settings.  Note that this list may
 not be complete, because some new variables may have been introduced since
 this document was last updated.

    PS1 = ">> "
    PS2 = ""
    beep_on_error = 1.0
    default_eval_print_flag = 0.0
    default_save_format = "mat-binary"
    define_all_return_values = 1.0
    do_fortran_indexing = 1.0
    empty_list_elements_ok = 1.0
    fixed_point_format = 1.0
    implicit_num_to_str_ok = 1.0
    implicit_str_to_num_ok = 1.0
    ok_to_lose_imaginary_part = 1.0
    page_screen_output = 0.0
    prefer_column_vectors = 0.0
    prefer_zero_one_indexing = 1.0
    print_empty_dimensions = 0.0
    treat_neg_dim_as_zero = 1.0
    warn_function_name_clash = 0.0
    whitespace_in_literal_matrix = "traditional"

 Some other known differences are:

     The Octave plotting functions are mostly compatible with the ones from
      Matlab 3.x, but not from Matlab 4.x.

 The third category of differences is (hopefully) shrinking.  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 [e-mail].

 An archive of old postings to the Octave mailing lists is maintained on
 ftp.che.wisc.edu in the directory `/pub/octave/MAILING-LISTS'.


ΓòÉΓòÉΓòÉ 13. Concept Index ΓòÉΓòÉΓòÉ

Sorry, no cp index


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

Matlab is a registered trademark of The MathWorks,