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<h3 class="section">11.2 Multiple Return Values</h3>
<p>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
<pre class="example"> function [<var>ret-list</var>] = <var>name</var> (<var>arg-list</var>)
<var>body</var>
endfunction
</pre>
<p class="noindent">where <var>name</var>, <var>arg-list</var>, and <var>body</var> have the same meaning
as before, and <var>ret-list</var> 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</var> has
only one element, this form of the <code>function</code> statement is
equivalent to the form described in the previous section.
<p>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.
<pre class="example"> 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
</pre>
<p>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.
<p>It is possible to use the <code>nthargout</code> function to obtain only some
of the return values or several at once in a cell array.
<a href="Cell-Array-Objects.html#Cell-Array-Objects">Cell Array Objects</a>
<!-- nthargout scripts/general/nthargout.m -->
<p><a name="doc_002dnthargout"></a>
<div class="defun">
— Function File: <b>nthargout</b> (<var>n, func, <small class="dots">...</small></var>)<var><a name="index-nthargout-730"></a></var><br>
— Function File: <b>nthargout</b> (<var>n, ntot, func, <small class="dots">...</small></var>)<var><a name="index-nthargout-731"></a></var><br>
<blockquote><p>Return the <var>n</var>th output argument of function given by the
function handle or string <var>func</var>. Any arguments after <var>func</var>
are passed to <var>func</var>. The total number of arguments to call
<var>func</var> with can be passed in <var>ntot</var>; by default <var>ntot</var>
is <var>n</var>. The input <var>n</var> can also be a vector of indices of the
output, in which case the output will be a cell array of the
requested output arguments.
<p>The intended use <code>nthargout</code> is to avoid intermediate variables.
For example, when finding the indices of the maximum entry of a
matrix, the following two compositions of nthargout
<pre class="example"> <var>m</var> = magic (5);
cell2mat (nthargout ([1, 2], @ind2sub, size(<var>m</var>),
nthargout (2, @max, <var>m</var>(:))))
⇒ 5 3
</pre>
<p class="noindent">are completely equivalent to the following lines:
<pre class="example"> <var>m</var> = magic(5);
[~, idx] = max (<var>M</var>(:));
[i, j] = ind2sub (size (<var>m</var>), idx);
[i, j]
⇒ 5 3
</pre>
<p>It can also be helpful to have all output arguments in a single cell
in the following manner:
<pre class="example"> <var>USV</var> = nthargout ([1:3], @svd, hilb (5));
</pre>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnargin.html#doc_002dnargin">nargin</a>, <a href="doc_002dnargout.html#doc_002dnargout">nargout</a>, <a href="doc_002dvarargin.html#doc_002dvarargin">varargin</a>, <a href="doc_002dvarargout.html#doc_002dvarargout">varargout</a>, <a href="doc_002disargout.html#doc_002disargout">isargout</a>.
</p></blockquote></div>
<p>In addition to setting <code>nargin</code> each time a function is called,
Octave also automatically initializes <code>nargout</code> 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</code> does not figure in the count of output
arguments, so the value of <code>nargout</code> may be zero.
<p>The <code>svd</code> and <code>lu</code> functions are examples of built-in
functions that behave differently depending on the value of
<code>nargout</code>.
<p>It is possible to write functions that only set some return values. For
example, calling the function
<pre class="example"> function [x, y, z] = f ()
x = 1;
z = 2;
endfunction
</pre>
<p class="noindent">as
<pre class="example"> [a, b, c] = f ()
</pre>
<p class="noindent">produces:
<pre class="example"> a = 1
b = [](0x0)
c = 2
</pre>
<p class="noindent">along with a warning.
<!-- nargout src/ov-usr-fcn.cc -->
<p><a name="doc_002dnargout"></a>
<div class="defun">
— Built-in Function: <b>nargout</b> ()<var><a name="index-nargout-732"></a></var><br>
— Built-in Function: <b>nargout</b> (<var>fcn_name</var>)<var><a name="index-nargout-733"></a></var><br>
<blockquote><p>Within a function, return the number of values the caller expects to
receive. If called with the optional argument <var>fcn_name</var>, return the
maximum number of values the named function can produce, or -1 if the
function can produce a variable number of values.
<p>For example,
<pre class="example"> f ()
</pre>
<p class="noindent">will cause <code>nargout</code> to return 0 inside the function <code>f</code> and
<pre class="example"> [s, t] = f ()
</pre>
<p class="noindent">will cause <code>nargout</code> to return 2 inside the function
<code>f</code>.
<p>At the top level, <code>nargout</code> is undefined.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnargin.html#doc_002dnargin">nargin</a>, <a href="doc_002dvarargin.html#doc_002dvarargin">varargin</a>, <a href="doc_002disargout.html#doc_002disargout">isargout</a>, <a href="doc_002dvarargout.html#doc_002dvarargout">varargout</a>, <a href="doc_002dnthargout.html#doc_002dnthargout">nthargout</a>.
</p></blockquote></div>
<p>It is good practice at the head of a function to verify that it has been called
correctly. In Octave the following idiom is seen frequently
<pre class="example"> if (nargin < min_#_inputs || nargin > max_#_inputs)
print_usage ();
endif
</pre>
<p class="noindent">which stops the function execution and prints a message about the correct
way to call the function whenever the number of inputs is wrong.
<p>For compatibility with <span class="sc">matlab</span>, <code>nargchk</code>, <code>narginchk</code> and
<code>nargoutchk</code> are available which provide similar error checking.
<!-- nargchk scripts/general/nargchk.m -->
<p><a name="doc_002dnargchk"></a>
<div class="defun">
— Function File: <var>msgstr</var> = <b>nargchk</b> (<var>minargs, maxargs, nargs</var>)<var><a name="index-nargchk-734"></a></var><br>
— Function File: <var>msgstr</var> = <b>nargchk</b> (<var>minargs, maxargs, nargs, "string"</var>)<var><a name="index-nargchk-735"></a></var><br>
— Function File: <var>msgstruct</var> = <b>nargchk</b> (<var>minargs, maxargs, nargs, "struct"</var>)<var><a name="index-nargchk-736"></a></var><br>
<blockquote><p>Return an appropriate error message string (or structure) if the
number of inputs requested is invalid.
<p>This is useful for checking to see that the number of input arguments
supplied to a function is within an acceptable range.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnargoutchk.html#doc_002dnargoutchk">nargoutchk</a>, <a href="doc_002dnarginchk.html#doc_002dnarginchk">narginchk</a>, <a href="doc_002derror.html#doc_002derror">error</a>, <a href="doc_002dnargin.html#doc_002dnargin">nargin</a>, <a href="doc_002dnargout.html#doc_002dnargout">nargout</a>.
</p></blockquote></div>
<!-- narginchk scripts/general/narginchk.m -->
<p><a name="doc_002dnarginchk"></a>
<div class="defun">
— Function File: <b>narginchk</b> (<var>minargs, maxargs</var>)<var><a name="index-narginchk-737"></a></var><br>
<blockquote><p>Check for correct number of arguments or generate an error message if
the number of arguments in the calling function is outside the range
<var>minargs</var> and <var>maxargs</var>. Otherwise, do nothing.
<p>Both <var>minargs</var> and <var>maxargs</var> need to be scalar numeric
values. Zero, Inf and negative values are all allowed, and
<var>minargs</var> and <var>maxargs</var> may be equal.
<p>Note that this function evaluates <code>nargin</code> on the caller.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnargchk.html#doc_002dnargchk">nargchk</a>, <a href="doc_002dnargoutchk.html#doc_002dnargoutchk">nargoutchk</a>, <a href="doc_002derror.html#doc_002derror">error</a>, <a href="doc_002dnargout.html#doc_002dnargout">nargout</a>, <a href="doc_002dnargin.html#doc_002dnargin">nargin</a>.
</p></blockquote></div>
<!-- nargoutchk scripts/general/nargoutchk.m -->
<p><a name="doc_002dnargoutchk"></a>
<div class="defun">
— Function File: <b>nargoutchk</b> (<var>minargs, maxargs</var>)<var><a name="index-nargoutchk-738"></a></var><br>
— Function File: <var>msgstr</var> = <b>nargoutchk</b> (<var>minargs, maxargs, nargs</var>)<var><a name="index-nargoutchk-739"></a></var><br>
— Function File: <var>msgstr</var> = <b>nargoutchk</b> (<var>minargs, maxargs, nargs, "string"</var>)<var><a name="index-nargoutchk-740"></a></var><br>
— Function File: <var>msgstruct</var> = <b>nargoutchk</b> (<var>minargs, maxargs, nargs, "struct"</var>)<var><a name="index-nargoutchk-741"></a></var><br>
<blockquote><p>Check for correct number of output arguments.
<p>On the first form, returns an error unless the number of arguments in its
caller is between the values of <var>minargs</var> and <var>maxargs</var>. It does
nothing otherwise. Note that this function evaluates the value of
<code>nargout</code> on the caller so its value must have not been tampered with.
<p>Both <var>minargs</var> and <var>maxargs</var> need to be a numeric scalar. Zero, Inf
and negative are all valid, and they can have the same value.
<p>For backward compatibility reasons, the other forms return an appropriate
error message string (or structure) if the number of outputs requested is
invalid.
<p>This is useful for checking to see that the number of output
arguments supplied to a function is within an acceptable range.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dnargchk.html#doc_002dnargchk">nargchk</a>, <a href="doc_002dnarginchk.html#doc_002dnarginchk">narginchk</a>, <a href="doc_002derror.html#doc_002derror">error</a>, <a href="doc_002dnargout.html#doc_002dnargout">nargout</a>, <a href="doc_002dnargin.html#doc_002dnargin">nargin</a>.
</p></blockquote></div>
<p><a name="doc_002dvarargin"></a><a name="doc_002dvarargout"></a>
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