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<a name="Calling-a-Function-by-its-Name"></a>
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<a name="Calling-a-Function-by-its-Name-1"></a>
<h3 class="section">9.1 Calling a Function by its Name</h3>

<p>The <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>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>feval</code>
that finds the root of a user-supplied function of one variable using
Newton&rsquo;s method.
</p>
<div class="example">
<pre class="example">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) &lt; 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 href="Predicates-for-Numeric-Objects.html#Predicates-for-Numeric-Objects">Predicates for Numeric Objects</a>,
for a list of predicates for numeric objects, and see <a href="Status-of-Variables.html#Status-of-Variables">Status of Variables</a>, for a description of the <code>exist</code> function.
</p>
<a name="XREFfeval"></a><dl>
<dt><a name="index-feval"></a>Built-in Function: <em></em> <strong>feval</strong> <em>(<var>name</var>, &hellip;)</em></dt>
<dd><p>Evaluate the function named <var>name</var>.  Any arguments after the first
are passed as inputs to the named function.  For example,
</p>
<div class="example">
<pre class="example">feval (&quot;acos&quot;, -1)
     &rArr; 3.1416
</pre></div>

<p>calls the function <code>acos</code> with the argument &lsquo;<samp>-1</samp>&rsquo;.
</p>
<p>The function <code>feval</code> can also be used with function handles of
any sort (see <a href="Function-Handles.html#Function-Handles">Function Handles</a>).  Historically, <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">
<pre class="example"><var>f</var> = @exp;
feval (<var>f</var>, 1)
    &rArr; 2.7183
<var>f</var> (1)
    &rArr; 2.7183
</pre></div>

<p>are equivalent ways to call the function referred to by <var>f</var>.  If it
cannot be predicted beforehand whether <var>f</var> is a function handle,
function name in a string, or inline function then <code>feval</code> can be used
instead.
</p></dd></dl>


<p>A similar function <code>run</code> exists for calling user script files, that
are not necessarily on the user path
</p>
<a name="XREFrun"></a><dl>
<dt><a name="index-run"></a>Command: <em></em> <strong>run</strong> <em><var>script</var></em></dt>
<dt><a name="index-run-1"></a>Function File: <em></em> <strong>run</strong> <em>(&quot;<var>script</var>&quot;)</em></dt>
<dd><p>Run <var>script</var> in the current workspace.
</p>
<p>Scripts which reside in directories specified in Octave&rsquo;s load
path, and which end with the extension <samp>&quot;.m&quot;</samp>, can be run simply by
typing their name.  For scripts not located on the load path, use <code>run</code>.
</p>
<p>The file name <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>&quot;.m&quot;</samp> extension before
falling back to the script name without an extension.
</p>
<p>Implementation Note: If <var>script</var> includes a path component, then
<code>run</code> first changes the directory to the directory where <var>script</var>
is found.  <code>run</code> then executes the script, and returns to the original
directory. 
</p>
<p><strong>See also:</strong> <a href="Manipulating-the-Load-Path.html#XREFpath">path</a>, <a href="Manipulating-the-Load-Path.html#XREFaddpath">addpath</a>, <a href="Script-Files.html#XREFsource">source</a>.
</p></dd></dl>


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