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<title>Calling a Function by its Name - Untitled</title>
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<p>
<a name="Calling-a-Function-by-its-Name"></a>
Next: <a rel="next" accesskey="n" href="Evaluation-in-a-Different-Context.html#Evaluation-in-a-Different-Context">Evaluation in a Different Context</a>,
Up: <a rel="up" accesskey="u" href="Evaluation.html#Evaluation">Evaluation</a>
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<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) < 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.
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<p><a name="doc_002dfeval"></a>
<div class="defun">
— 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)
3.1416
</pre>
<p class="noindent">calls the function <code>acos</code> with the argument ‘<samp><span class="samp">-1</span></samp>’.
<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">
— Function File: <b>run</b> (<var>f</var>)<var><a name="index-run-550"></a></var><br>
— 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.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dsystem.html#doc_002dsystem">system</a>.
</p></blockquote></div>
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