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<a name="Defining-Functions"></a>
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<p>
Next: <a href="Multiple-Return-Values.html#Multiple-Return-Values" accesskey="n" rel="next">Multiple Return Values</a>, Previous: <a href="Introduction-to-Function-and-Script-Files.html#Introduction-to-Function-and-Script-Files" accesskey="p" rel="prev">Introduction to Function and Script Files</a>, Up: <a href="Functions-and-Scripts.html#Functions-and-Scripts" accesskey="u" rel="up">Functions and Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Defining-Functions-1"></a>
<h3 class="section">11.2 Defining Functions</h3>
<a name="index-function-statement"></a>
<a name="index-endfunction-statement"></a>
<p>In its simplest form, the definition of a function named <var>name</var>
looks like this:
</p>
<div class="example">
<pre class="example">function <var>name</var>
<var>body</var>
endfunction
</pre></div>
<p>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.
</p>
<p>The function <var>body</var> consists of Octave statements. It is the
most important part of the definition, because it says what the function
should actually <em>do</em>.
</p>
<p>For example, here is a function that, when executed, will ring the bell
on your terminal (assuming that it is possible to do so):
</p>
<div class="example">
<pre class="example">function wakeup
printf ("\a");
endfunction
</pre></div>
<p>The <code>printf</code> statement (see <a href="Input-and-Output.html#Input-and-Output">Input and Output</a>) simply tells
Octave to print the string <code>"\a"</code>. The special character
‘<samp>\a</samp>’ stands for the alert character (ASCII 7). See <a href="Strings.html#Strings">Strings</a>.
</p>
<p>Once this function is defined, you can ask Octave to evaluate it by
typing the name of the function.
</p>
<p>Normally, you will want to pass some information to the functions you
define. The syntax for passing parameters to a function in Octave is
</p>
<div class="example">
<pre class="example">function <var>name</var> (<var>arg-list</var>)
<var>body</var>
endfunction
</pre></div>
<p>where <var>arg-list</var> 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.
</p>
<p>To print a message along with ringing the bell, you might modify the
<code>wakeup</code> to look like this:
</p>
<div class="example">
<pre class="example">function wakeup (message)
printf ("\a%s\n", message);
endfunction
</pre></div>
<p>Calling this function using a statement like this
</p>
<div class="example">
<pre class="example">wakeup ("Rise and shine!");
</pre></div>
<p>will cause Octave to ring your terminal’s bell and print the message
‘<samp>Rise and shine!</samp>’, followed by a newline character (the ‘<samp>\n</samp>’
in the first argument to the <code>printf</code> statement).
</p>
<p>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:
</p>
<div class="example">
<pre class="example">function <var>ret-var</var> = <var>name</var> (<var>arg-list</var>)
<var>body</var>
endfunction
</pre></div>
<p>The symbol <var>ret-var</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.
</p>
<p>Variables used in the body of a function are local to the
function. Variables named in <var>arg-list</var> and <var>ret-var</var> are also
local to the function. See <a href="Global-Variables.html#Global-Variables">Global Variables</a>, for information about
how to access global variables inside a function.
</p>
<p>For example, here is a function that computes the average of the
elements of a vector:
</p>
<div class="example">
<pre class="example">function retval = avg (v)
retval = sum (v) / length (v);
endfunction
</pre></div>
<p>If we had written <code>avg</code> like this instead,
</p>
<div class="example">
<pre class="example">function retval = avg (v)
if (isvector (v))
retval = sum (v) / length (v);
endif
endfunction
</pre></div>
<p>and then called the function with a matrix instead of a vector as the
argument, Octave would have printed an error message like this:
</p>
<div class="example">
<pre class="example">error: value on right hand side of assignment is undefined
</pre></div>
<p>because the body of the <code>if</code> statement was never executed, and
<code>retval</code> 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</code> could have been
written like this:
</p>
<div class="example">
<pre class="example">function retval = avg (v)
retval = 0;
if (isvector (v))
retval = sum (v) / length (v);
else
error ("avg: expecting vector argument");
endif
endfunction
</pre></div>
<p>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</code>. Each time a function is called, <code>nargin</code> is
automatically initialized to the number of arguments that have actually
been passed to the function. For example, we might rewrite the
<code>avg</code> function like this:
</p>
<div class="example">
<pre class="example">function retval = avg (v)
retval = 0;
if (nargin != 1)
usage ("avg (vector)");
endif
if (isvector (v))
retval = sum (v) / length (v);
else
error ("avg: expecting vector argument");
endif
endfunction
</pre></div>
<p>Although Octave does not automatically report an error if you call a
function with more arguments than expected, doing so probably indicates
that something is wrong. Octave also does not automatically report an
error if a function is called with too few arguments, but any attempt to
use a variable that has not been given a value will result in an error.
To avoid such problems and to provide useful messages, we check for both
possibilities and issue our own error message.
</p>
<a name="XREFnargin"></a><dl>
<dt><a name="index-nargin"></a><em></em> <strong>nargin</strong> <em>()</em></dt>
<dt><a name="index-nargin-1"></a><em></em> <strong>nargin</strong> <em>(<var>fcn</var>)</em></dt>
<dd><p>Report the number of input arguments to a function.
</p>
<p>Called from within a function, return the number of arguments passed to the
function. At the top level, return the number of command line arguments
passed to Octave.
</p>
<p>If called with the optional argument <var>fcn</var>—a function name or
handle—return the declared number of arguments that the function can
accept.
</p>
<p>If the last argument to <var>fcn</var> is <var>varargin</var> the returned value is
negative. For example, the function <code>union</code> for sets is declared as
</p>
<div class="example">
<pre class="example">function [y, ia, ib] = union (a, b, varargin)
and
nargin ("union")
⇒ -3
</pre></div>
<p>Programming Note: <code>nargin</code> does not work on compiled functions
(<samp>.oct</samp> files) such as built-in or dynamically loaded functions.
</p>
<p><strong>See also:</strong> <a href="Multiple-Return-Values.html#XREFnargout">nargout</a>, <a href="Multiple-Return-Values.html#XREFnarginchk">narginchk</a>, <a href="Multiple-Return-Values.html#XREFvarargin">varargin</a>, <a href="#XREFinputname">inputname</a>.
</p></dd></dl>
<a name="XREFinputname"></a><dl>
<dt><a name="index-inputname"></a><em></em> <strong>inputname</strong> <em>(<var>n</var>)</em></dt>
<dt><a name="index-inputname-1"></a><em></em> <strong>inputname</strong> <em>(<var>n</var>, <var>ids_only</var>)</em></dt>
<dd><p>Return the name of the <var>n</var>-th argument to the calling function.
</p>
<p>If the argument is not a simple variable name, return an empty string. As
an example, a reference to a field in a structure such as <code>s.field</code> is
not a simple name and will return <code>""</code>.
</p>
<p><code>inputname</code> is only useful within a function. When used at the command
line it always returns an empty string.
</p>
<p>By default, return an empty string if the <var>n</var>-th argument is not
a valid variable name. If the optional argument <var>ids_only</var> is
false, return the text of the argument even if it is not a valid
variable name.
</p>
<p><strong>See also:</strong> <a href="#XREFnargin">nargin</a>, <a href="Multiple-Return-Values.html#XREFnthargout">nthargout</a>.
</p></dd></dl>
<a name="XREFsilent_005ffunctions"></a><dl>
<dt><a name="index-silent_005ffunctions"></a><em><var>val</var> =</em> <strong>silent_functions</strong> <em>()</em></dt>
<dt><a name="index-silent_005ffunctions-1"></a><em><var>old_val</var> =</em> <strong>silent_functions</strong> <em>(<var>new_val</var>)</em></dt>
<dt><a name="index-silent_005ffunctions-2"></a><em></em> <strong>silent_functions</strong> <em>(<var>new_val</var>, "local")</em></dt>
<dd><p>Query or set the internal variable that controls whether internal
output from a function is suppressed.
</p>
<p>If this option is disabled, Octave will display the results produced by
evaluating expressions within a function body that are not terminated with
a semicolon.
</p>
<p>When called from inside a function with the <code>"local"</code> option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.
</p></dd></dl>
<hr>
<div class="header">
<p>
Next: <a href="Multiple-Return-Values.html#Multiple-Return-Values" accesskey="n" rel="next">Multiple Return Values</a>, Previous: <a href="Introduction-to-Function-and-Script-Files.html#Introduction-to-Function-and-Script-Files" accesskey="p" rel="prev">Introduction to Function and Script Files</a>, Up: <a href="Functions-and-Scripts.html#Functions-and-Scripts" accesskey="u" rel="up">Functions and Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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