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<h3 class="section" id="Calling-a-Function-by-its-Name-1"><span>9.1 Calling a Function by its Name<a class="copiable-link" href="#Calling-a-Function-by-its-Name-1"> ¶</a></span></h3>
<p>The <code class="code">feval</code> function allows you to call a function from a string
containing its name. This is useful when writing a function that needs to
call user-supplied functions. The <code class="code">feval</code> function takes the name
of the function to call as its first argument, and the remaining
arguments are given to the function.
</p>
<p>The following example is a simple-minded function using <code class="code">feval</code>
that finds the root of a user-supplied function of one variable using
Newton’s method.
</p>
<div class="example">
<pre class="example-preformatted">function result = newtroot (fname, x)
# usage: newtroot (fname, x)
#
# fname : a string naming a function f(x).
# x : initial guess
delta = tol = sqrt (eps);
maxit = 200;
fx = feval (fname, x);
for i = 1:maxit
if (abs (fx) < tol)
result = x;
return;
else
fx_new = feval (fname, x + delta);
deriv = (fx_new - fx) / delta;
x = x - fx / deriv;
fx = fx_new;
endif
endfor
result = x;
endfunction
</pre></div>
<p>Note that this is only meant to be an example of calling user-supplied
functions and should not be taken too seriously. In addition to using a
more robust algorithm, any serious code would check the number and type
of all the arguments, ensure that the supplied function really was a
function, etc. See <a class="xref" href="Predicates-for-Numeric-Objects.html">Predicates for Numeric Objects</a>,
for a list of predicates for numeric objects, and see <a class="pxref" href="Status-of-Variables.html">Status of Variables</a>, for a description of the <code class="code">exist</code> function.
</p>
<a class="anchor" id="XREFfeval"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-feval"><span><code class="def-type">[<var class="var">y1</var>, <var class="var">y2</var>, …] =</code> <strong class="def-name">feval</strong> <code class="def-code-arguments">('<var class="var">fcn</var>', <var class="var">x1</var>, <var class="var">x2</var>, …)</code><a class="copiable-link" href="#index-feval"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-feval-1"><span><code class="def-type">[<var class="var">y1</var>, <var class="var">y2</var>, …] =</code> <strong class="def-name">feval</strong> <code class="def-code-arguments">(@<var class="var">fcn</var>, <var class="var">x1</var>, <var class="var">x2</var>, …)</code><a class="copiable-link" href="#index-feval-1"> ¶</a></span></dt>
<dd><p>Evaluate the function <var class="var">fcn</var> with inputs <var class="var">x1</var>, <var class="var">x2</var>, <small class="enddots">...</small>
</p>
<p>The function <var class="var">fcn</var> may be specified by name in a string or given as a
function handle. Any arguments after the first are passed as inputs to the
named function. For example,
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">feval ("acos", -1)
⇒ 3.1416
</pre></div></div>
<p>calls the function <code class="code">acos</code> with the argument ‘<samp class="samp">-1</samp>’.
</p>
<p>The function <code class="code">feval</code> can also be used with function handles of any sort
(see <a class="pxref" href="Function-Handles.html">Function Handles</a>). Historically, <code class="code">feval</code> was the only way to
call user-supplied functions in strings, but function handles are now preferred
due to the cleaner syntax they offer. For example,
</p>
<div class="example">
<div class="group"><pre class="example-preformatted"><var class="var">f</var> = @exp;
feval (<var class="var">f</var>, 1)
⇒ 2.7183
<var class="var">f</var> (1)
⇒ 2.7183
</pre></div></div>
<p>are equivalent ways to call the function referred to by <var class="var">f</var>. If it cannot
be predicted beforehand whether <var class="var">f</var> is a function handle, function name in
a string, or inline function then <code class="code">feval</code> can be used instead.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Overloading-and-Autoloading.html#XREFbuiltin">builtin</a>, <a class="ref" href="Evaluation.html#XREFeval">eval</a>, <a class="ref" href="Evaluation-in-a-Different-Context.html#XREFevalin">evalin</a>.
</p></dd></dl>
<p>A similar function <code class="code">run</code> exists for calling user script files, that
are not necessarily on the user path
</p>
<a class="anchor" id="XREFrun"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-run"><span><strong class="def-name">run</strong> <code class="def-code-arguments"><var class="var">script</var></code><a class="copiable-link" href="#index-run"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-run-1"><span><strong class="def-name">run</strong> <code class="def-code-arguments">("<var class="var">script</var>")</code><a class="copiable-link" href="#index-run-1"> ¶</a></span></dt>
<dd><p>Run <var class="var">script</var> in the current workspace.
</p>
<p>Scripts which reside in directories specified in Octave’s load path, and
which end with the extension <samp class="file">.m</samp>, can be run simply by typing
their name. For scripts not located on the load path, use <code class="code">run</code>.
</p>
<p>The filename <var class="var">script</var> can be a bare, fully qualified, or relative
filename and with or without a file extension. If no extension is
specified, Octave will first search for a script with the <samp class="file">.m</samp>
extension before falling back to the script name without an extension.
</p>
<p>Implementation Note: If <var class="var">script</var> includes a path component, then
<code class="code">run</code> first changes the working directory to the directory where
<var class="var">script</var> is found. Next, the script is executed. Finally, <code class="code">run</code>
returns to the original working directory <em class="emph">unless</em> <var class="var">script</var> has
specifically changed directories.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Manipulating-the-Load-Path.html#XREFpath">path</a>, <a class="ref" href="Manipulating-the-Load-Path.html#XREFaddpath">addpath</a>, <a class="ref" href="Script-Files.html#XREFsource">source</a>.
</p></dd></dl>
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