@c Copyright (C) 1994, 1995 John W. Eaton
@c This is part of the Octave manual.
@c For copying conditions, see the file gpl.texi.

@node Functions and Scripts, Built-in Variables, Statements, Top
@chapter Functions and Script Files
@cindex defining functions
@cindex user-defined functions
@cindex functions, user-defined
@cindex script files

Complicated Octave programs can often be simplified by defining
functions.  Functions can be defined directly on the command line during
interactive Octave sessions, or in external files, and can be called just
like built-in ones. 

@menu
* Defining Functions::          
* Multiple Return Values::      
* Variable-length Argument Lists::  
* Variable-length Return Lists::  
* Returning From a Function::   
* Function Files::              
* Script Files::                
* Dynamically Linked Functions::  
* Organization of Functions::   
@end menu

@node Defining Functions, Multiple Return Values, Functions and Scripts, Functions and Scripts
@section Defining Functions

In its simplest form, the definition of a function named @var{name}
looks like this:

@example
@group
function @var{name}
  @var{body}
endfunction
@end group
@end example

@noindent
A valid function name is like a valid variable name: a sequence of
letters, digits and underscores, not starting with a digit.  Functions
share the same pool of names as variables.

The function @var{body} consists of Octave statements.  It is the
most important part of the definition, because it says what the function
should actually @emph{do}.

For example, here is a function that, when executed, will ring the bell
on your terminal (assuming that it is possible to do so):

@example
@group
function wakeup
  printf ("\a");
endfunction
@end group
@end example

The @code{printf} statement (@pxref{Input and Output}) simply tells
Octave to print the string @code{"\a"}.  The special character @samp{\a}
stands for the alert character (ASCII 7).  @xref{String Constants}.

Once this function is defined, you can ask Octave to evaluate it by
typing the name of the function.

Normally, you will want to pass some information to the functions you
define.  The syntax for passing parameters to a function in Octave is

@example
@group
function @var{name} (@var{arg-list})
  @var{body}
endfunction
@end group
@end example

@noindent
where @var{arg-list} is a comma-separated list of the function's
arguments.  When the function is called, the argument names are used to
hold the argument values given in the call.  The list of arguments may
be empty, in which case this form is equivalent to the one shown above.

To print a message along with ringing the bell, you might modify the
@code{beep} to look like this:

@example
@group
function wakeup (message)
  printf ("\a%s\n", message);
endfunction
@end group
@end example

Calling this function using a statement like this

@example
wakeup ("Rise and shine!");
@end example

@noindent
will cause Octave to ring your terminal's bell and print the message
@samp{Rise and shine!}, followed by a newline character (the @samp{\n}
in the first argument to the @code{printf} statement).

In most cases, you will also want to get some information back from the
functions you define.  Here is the syntax for writing a function that
returns a single value:

@example
@group
function @var{ret-var} = @var{name} (@var{arg-list})
  @var{body}
endfunction
@end group
@end example

@noindent
The symbol @var{ret-var} is the name of the variable that will hold the
value to be returned by the function.  This variable must be defined
before the end of the function body in order for the function to return
a value.

For example, here is a function that computes the average of the
elements of a vector:

@example
@group
function retval = avg (v)
  retval = sum (v) / length (v);
endfunction
@end group
@end example

If we had written @code{avg} like this instead,

@example
@group
function retval = avg (v)
  if (is_vector (v))
    retval = sum (v) / length (v);
  endif
endfunction
@end group
@end example

@noindent
and then called the function with a matrix instead of a vector as the
argument, Octave would have printed an error message like this:

@example
@group
error: `retval' undefined near line 1 column 10
error: evaluating index expression near line 7, column 1
@end group
@end example

@noindent
because the body of the @code{if} statement was never executed, and
@code{retval} was never defined.  To prevent obscure errors like this,
it is a good idea to always make sure that the return variables will
always have values, and to produce meaningful error messages when
problems are encountered.  For example, @code{avg} could have been
written like this:

@example
@group
function retval = avg (v)
  retval = 0;
  if (is_vector (v))
    retval = sum (v) / length (v);
  else
    error ("avg: expecting vector argument");
  endif
endfunction
@end group
@end example

There is still one additional problem with this function.  What if it is
called without an argument?  Without additional error checking, Octave
will probably print an error message that won't really help you track
down the source of the error.  To allow you to catch errors like this,
Octave provides each function with an automatic variable called
@code{nargin}.  Each time a function is called, @code{nargin} is
automatically initialized to the number of arguments that have actually
been passed to the function.  For example, we might rewrite the
@code{avg} function like this:
@vindex nargout

@example
@group
function retval = avg (v)
  retval = 0;
  if (nargin != 1)
    error ("usage: avg (vector)");
  endif
  if (is_vector (v))
    retval = sum (v) / length (v);
  else
    error ("avg: expecting vector argument");
  endif
endfunction
@end group
@end example

Although Octave does not consider it an error if you call a function
with more arguments than were expected, doing so is probably an error,
so we check for that possibility too, and issue the error message if
either too few or too many arguments have been provided.

The body of a user-defined function can contain a @code{return}
statement.  This statement returns control to the rest of the Octave
program.  A @code{return} statement is assumed at the end of every
function definition.

@node Multiple Return Values, Variable-length Argument Lists, Defining Functions, Functions and Scripts
@section Multiple Return Values

Unlike many other computer languages, Octave allows you to define
functions that return more than one value.  The syntax for defining
functions that return multiple values is

@example
function [@var{ret-list}] = @var{name} (@var{arg-list})
  @var{body}
endfunction
@end example

@noindent
where @var{name}, @var{arg-list}, and @var{body} have the same meaning
as before, and @var{ret-list} is a comma-separated list of variable
names that will hold the values returned from the function.  The list of
return values must have at least one element.  If @var{ret-list} has
only one element, this form of the @code{function} statement is
equivalent to the form described in the previous section.

Here is an example of a function that returns two values, the maximum
element of a vector and the index of its first occurrence in the vector.

@example
@group
function [max, idx] = vmax (v)
  idx = 1;
  max = v (idx);
  for i = 2:length (v)
    if (v (i) > max)
      max = v (i);
      idx = i;
    endif
  endfor
endfunction
@end group
@end example

In this particular case, the two values could have been returned as
elements of a single array, but that is not always possible or
convenient.  The values to be returned may not have compatible
dimensions, and it is often desirable to give the individual return
values distinct names.

In addition to setting @code{nargin} each time a function is called,
Octave also automatically initializes @code{nargout} to the number of
values that are expected to be returned.  This allows you to write
functions that behave differently depending on the number of values that
the user of the function has requested.  The implicit assignment to the
built-in variable @code{ans} does not figure in the count of output
arguments, so the value of @code{nargout} may be zero.

The @code{svd} and @code{lu} functions are examples of built-in
functions that behave differently depending on the value of
@code{nargout}.

It is possible to write functions that only set some return values.  For
example, calling the function

@example
function [x, y, z] = f ()
  x = 1;
  z = 2;
endfunction
@end example

@noindent
as

@example
[a, b, c] = f ()
@end example

@noindent
produces:

@example
a = 1

b = [](0x0)

c = 2
@end example

@node Variable-length Argument Lists, Variable-length Return Lists, Multiple Return Values, Functions and Scripts
@section Variable-length Argument Lists
@cindex Variable-length argument lists
@cindex @code{...}
@findex va_arg
@findex va_start

Octave has a real mechanism for handling functions that take an
unspecified number of arguments, so it is not 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:

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

The ellipsis that marks the variable argument list may only appear once
and must be the last element in the list of arguments.

Calling @code{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 @code{va_start()} if you do not plan
to cycle through the arguments more than once.

The function @code{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 @code{va_arg()} when there are no more arguments
available.

Sometimes it is useful to be able to pass all unnamed arguments to
another function.  The keyword @var{all_va_args} makes this very easy to
do.  For example, given the functions

@example
function f (...)
  while (nargin--)
    disp (va_arg ())
  endwhile
endfunction
function g (...)
  f ("begin", all_va_args, "end")
endfunction
@end example

@noindent
the statement

@example
g (1, 2, 3)
@end example

@noindent
prints

@example
begin
1
2
3
end
@end example

The keyword @code{all_va_args} always stands for the entire list of
optional argument, so it is possible to use it more than once within the
same function without having to call @code{var_start ()}.  It can only
be used within functions that take a variable number of arguments.  It
is an error to use it in other contexts.

@node Variable-length Return Lists, Returning From a Function, Variable-length Argument Lists, Functions and Scripts
@section Variable-length Return Lists
@cindex Variable-length return lists
@cindex @code{...}
@findex vr_val

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:

@example
function [...] = foo (n, x)
  for i = 1:n
    vr_val (i * x);
  endfor
endfunction
@end example

Each time @code{vr_val()} is called, it places the value of its argument
at the end of the list of values to return from the function.  Once
@code{vr_val()} has been called, there is no way to go back to the
beginning of the list and rewrite any of the return values.

As with variable argument lists, the ellipsis that marks the variable
return list may only appear once and must be the last element in the
list of returned values.

@node Returning From a Function, Function Files, Variable-length Return Lists, Functions and Scripts
@section Returning From a Function

The body of a user-defined function can contain a @code{return} statement.
This statement returns control to the rest of the Octave program.  It
looks like this:

@example
return
@end example

Unlike the @code{return} statement in C, Octave's @code{return}
statement cannot be used to return a value from a function.  Instead,
you must assign values to the list of return variables that are part of
the @code{function} statement.  The @code{return} statement simply makes
it easier to exit a function from a deeply nested loop or conditional
statement.

Here is an example of a function that checks to see if any elements of a
vector are nonzero.

@example
@group
function retval = any_nonzero (v)
  retval = 0;
  for i = 1:length (v)
    if (v (i) != 0)
      retval = 1;
      return;
    endif
  endfor
  printf ("no nonzero elements found\n");
endfunction
@end group
@end example

Note that this function could not have been written using the
@code{break} statement to exit the loop once a nonzero value is found
without adding extra logic to avoid printing the message if the vector
does contain a nonzero element.

@node Function Files, Script Files, Returning From a Function, Functions and Scripts
@section Function Files
@cindex function file

Except for simple one-shot programs, it is not practical to have to
define all the functions you need each time you need them.  Instead, you
will normally want to save them in a file so that you can easily edit
them, and save them for use at a later time.

Octave does not require you to load function definitions from files
before using them.  You simply need to put the function definitions in a
place where Octave can find them.

When Octave encounters an identifier that is undefined, it first looks
for variables or functions that are already compiled and currently
listed in its symbol table.  If it fails to find a definition there, it
searches the list of directories specified by the built-in variable
@code{LOADPATH} for files ending in @file{.m} that have the same base
name as the undefined identifier.@footnote{The @samp{.m} suffix was
chosen for compatibility with @sc{Matlab}.}  @xref{User Preferences}
for a description of @code{LOADPATH}.  Once Octave finds a file
with a name that matches, the contents of the file are read.  If it
defines a @emph{single} function, it is compiled and executed.
@xref{Script Files}, for more information about how you can define more
than one function in a single file.

When Octave defines a function from a function file, it saves the full
name of the file it read and the time stamp on the file.  After
that, it checks the time stamp on the file every time it needs the
function.  If the time stamp indicates that the file has changed since
the last time it was read, Octave reads it again.

Checking the time stamp allows you to edit the definition of a function
while Octave is running, and automatically use the new function
definition without having to restart your Octave session.  Checking the
time stamp every time a function is used is rather inefficient, but it
has to be done to ensure that the correct function definition is used.

Octave assumes that function files in the
@file{/usr/local/lib/octave/@value{VERSION}} directory tree will not
change, so it doesn't have to check their time stamps every time the
functions defined in those files are used.  This is normally a very good
assumption and provides a significant improvement in performance for the
function files that are distributed with Octave.

If you know that your own function files will not change while you are
running Octave, you can improve performance by setting the variable
@code{ignore_function_time_stamp} to @code{"all"}, so that Octave will
ignore the time stamps for all function files.  Setting it to
@code{"system"} gives the default behavior.  If you set it to anything
else, Octave will check the time stamps on all function files.

@node Script Files, Dynamically Linked Functions, Function Files, Functions and Scripts
@section Script Files

A script file is a file containing (almost) any sequence of Octave
commands.  It is read and evaluated just as if you had typed each
command at the Octave prompt, and provides a convenient way to perform a
sequence of commands that do not logically belong inside a function.

Unlike a function file, a script file must @emph{not} begin with the
keyword @code{function}.  If it does, Octave will assume that it is a
function file, and that it defines a single function that should be
evaluated as soon as it is defined.

A script file also differs from a function file in that the variables
named in a script file are not local variables, but are in the same
scope as the other variables that are visible on the command line.

Even though a script file may not begin with the @code{function}
keyword, it is possible to define more than one function in a single
script file and load (but not execute) all of them at once.  To do 
this, the first token in the file (ignoring comments and other white
space) must be something other than @code{function}.  If you have no
other statements to evaluate, you can use a statement that has no
effect, like this:

@example
@group
# Prevent Octave from thinking that this
# is a function file:

1;

# Define function one:

function one ()
  ...
@end group
@end example

To have Octave read and compile these functions into an internal form,
you need to make sure that the file is in Octave's @code{LOADPATH}, then
simply type the base name of the file that contains the commands.
(Octave uses the same rules to search for script files as it does to
search for function files.)

If the first token in a file (ignoring comments) is @code{function},
Octave will compile the function and try to execute it, printing a
message warning about any non-whitespace characters that appear after
the function definition.

Note that Octave does not try to lookup the definition of any identifier
until it needs to evaluate it.  This means that Octave will compile the
following statements if they appear in a script file, or are typed at
the command line,

@example
@group
# not a function file:
1;
function foo ()
  do_something ();
endfunction
function do_something ()
  do_something_else ();
endfunction
@end group
@end example

@noindent
even though the function @code{do_something} is not defined before it is
referenced in the function @code{foo}.  This is not an error because the
Octave does not need to resolve all symbols that are referenced by a
function until the function is actually evaluated.

Since Octave doesn't look for definitions until they are needed, the
following code will always print @samp{bar = 3} whether it is typed
directly on the command line, read from a script file, or is part of a
function body, even if there is a function or script file called
@file{bar.m} in Octave's @code{LOADPATH}.

@example
@group
eval ("bar = 3");
bar
@end group
@end example

Code like this appearing within a function body could fool Octave if
definitions were resolved as the function was being compiled.  It would
be virtually impossible to make Octave clever enough to evaluate this
code in a consistent fashion.  The parser would have to be able to
perform the @samp{eval ()} statement at compile time, and that would be
impossible unless all the references in the string to be evaluated could
also be resolved, and requiring that would be too restrictive (the
string might come from user input, or depend on things that are not
known until the function is evaluated).

@node Dynamically Linked Functions, Organization of Functions, Script Files, Functions and Scripts
@section Dynamically Linked Functions

On some systems, Octave can dynamically load and execute functions
written in C++ or other compiled languages.  This currently only works
on systems that have a working version of the GNU dynamic linker,
@code{dld}. Unfortunately, @code{dld} does not work on very many
systems, but someone is working on making @code{dld} use the GNU Binary
File Descriptor library, @code{BFD}, so that may soon change.  In any
case, it should not be too hard to make Octave's dynamic linking
features work on other systems using system-specific dynamic linking
facilities.

Here is an example of how to write a C++ function that Octave can load.

@example
#include <iostream.h>

#include "defun-dld.h"
#include "tree-const.h"

DEFUN_DLD ("hello", Fhello, Shello, -1, -1,
  "hello (...)\n\
\n\
Print greeting followed by the values of all the arguments passed.\n\
Returns all the arguments passed.")
@{
  Octave_object retval;
  cerr << "Hello, world!\n";
  int nargin = args.length ();
  for (int i = 1; i < nargin; i++)
    retval (nargin-i-1) = args(i).eval (1);
  return retval;
@}
@end example

Octave's dynamic linking features currently have the following
limitations.

@itemize @bullet
@item
Dynamic linking only works on systems that support the GNU dynamic
linker, @code{dld}.
@item
Clearing dynamically linked functions doesn't work.

@item
Configuring Octave with @code{--enable-lite-kernel} seems to mostly work
to make nonessential built-in functions dynamically loaded, but there
also seem to be some problems.  For example, fsolve seems to always
return @code{info == 3}.  This is difficult to debug since @code{gdb}
won't seem to allow breakpoints to be set inside dynamically loaded
functions.

@item
Octave uses a lot of memory if the dynamically linked functions are
compiled to include debugging symbols.  This appears to be a limitation
with @code{dld}, and can be avoided by not using @code{-g} to compile
functions that will be linked dynamically.
@end itemize

If you would like to volunteer to help improve Octave's ability to
dynamically link externally compiled functions, please contact
@code{bug-octave@@che.utexas.edu}.

@node Organization of Functions,  , Dynamically Linked Functions, Functions and Scripts
@section Organization of Functions Distributed with Octave

Many of Octave's standard functions are distributed as function files.
They are loosely organized by topic, in subdirectories of
@file{OCTAVE_HOME/lib/octave/VERSION/m}, to make it easier to find
them.

The following is a list of all the function file subdirectories, and the
types of functions you will find there.

@table @file
@item control
Functions for design and simulation of automatic control systems.

@item elfun
Elementary functions.

@item general
Miscellaneous matrix manipulations, like @code{flipud}, @code{rot90},
and @code{triu}, as well as other basic functions, like
@code{is_matrix}, @code{nargchk}, etc.

@item image
Image processing tools.  These functions require the X Window System.

@item linear-algebra
Functions for linear algebra.

@item miscellaneous
Functions that don't really belong anywhere else.

@item plot
A set of functions that implement the @sc{Matlab}-like plotting functions.

@item polynomial
Functions for manipulating polynomials.

@item set
Functions for creating and manipulating sets of unique values.

@item signal
Functions for signal processing applications.

@item specfun
Special functions.

@item special-matrix
Functions that create special matrix forms.

@item startup
Octave's system-wide startup file.

@item statistics
Statistical functions.

@item strings
Miscellaneous string-handling functions.
@end table

@xref{User Preferences} for an explanation of the built-in variable
@code{LOADPATH}, and @ref{Function Files} for a description of the way
Octave resolves undefined variable and function names.

