Sophie: octave-doc-0:3.2.3-3mdv2010.0 i586

octave-doc-3.2.3-3mdv2010.0.i586.rpm

<html lang="en">
<head>
<title>Calling a Function by its Name - Untitled</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Untitled">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Evaluation.html#Evaluation" title="Evaluation">
<link rel="next" href="Evaluation-in-a-Different-Context.html#Evaluation-in-a-Different-Context" title="Evaluation in a Different Context">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
</head>
<body>
<div class="node">
<a name="Calling-a-Function-by-its-Name"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Evaluation-in-a-Different-Context.html#Evaluation-in-a-Different-Context">Evaluation in a Different Context</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Evaluation.html#Evaluation">Evaluation</a>
<hr>
</div>

<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>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's method.

<pre class="example">     <a name="index-Fordyce_002c-A_002e-P_002e-547"></a><a name="index-newtroot-548"></a>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>
   <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 example,
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.

<!-- parse.cc -->
   <p><a name="doc_002dfeval"></a>

<div class="defun">
&mdash; Built-in Function:  <b>feval</b> (<var>name, <small class="dots">...</small></var>)<var><a name="index-feval-549"></a></var><br>
<blockquote><p>Evaluate the function named <var>name</var>.  Any arguments after the first
are passed on to the named function.  For example,

     <pre class="example">          feval ("acos", -1)
               &rArr; 3.1416
</pre>
        <p class="noindent">calls the function <code>acos</code> with the argument &lsquo;<samp><span class="samp">-1</span></samp>&rsquo;.

        <p>The function <code>feval</code> is necessary in order to be able to write
functions that call user-supplied functions, because Octave does not
have a way to declare a pointer to a function (like C) or to declare a
special kind of variable that can be used to hold the name of a function
(like <code>EXTERNAL</code> in Fortran).  Instead, you must refer to functions
by name, and use <code>feval</code> to call them. 
</p></blockquote></div>

   <p>A similar function <code>run</code> exists for calling user script files, that
are not necessarily on the user path

<!-- ./miscellaneous/run.m -->
   <p><a name="doc_002drun"></a>

<div class="defun">
&mdash; Function File:  <b>run</b> (<var>f</var>)<var><a name="index-run-550"></a></var><br>
&mdash; Command:  <b>run</b><var> f<a name="index-run-551"></a></var><br>
<blockquote><p>Run scripts in the current workspace that are not necessarily on the
path.  If <var>f</var> is the script to run, including its path, then <code>run</code>
change the directory to the directory where <var>f</var> is found.  <code>run</code>
then executes the script, and returns to the original directory. 
<!-- Texinfo @sp should work but in practice produces ugly results for HTML. -->
<!-- A simple blank line produces the correct behavior. -->
<!-- @sp 1 -->

     <p class="noindent"><strong>See also:</strong> <a href="doc_002dsystem.html#doc_002dsystem">system</a>. 
</p></blockquote></div>

   </body></html>