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<h4 class="subsection">A.1.8 Calling Octave Functions from Oct-Files</h4>
<p>There is often a need to be able to call another octave function from
within an oct-file, and there are many examples of such within octave
itself. For example the <code>quad</code> function is an oct-file that
calculates the definite integral by quadrature over a user supplied
function.
<p>There are also many ways in which a function might be passed. It might
be passed as one of
<ol type=1 start=1>
<li>Function Handle
<li>Anonymous Function Handle
<li>Inline Function
<li>String
</ol>
<p>The example below demonstrates an example that accepts all four means of
passing a function to an oct-file.
<pre class="example"><pre class="verbatim"> #include <octave/oct.h>
#include <octave/parse.h>
DEFUN_DLD (funcdemo, args, nargout, "Function Demo")
{
int nargin = args.length();
octave_value_list retval;
if (nargin < 2)
print_usage ();
else
{
octave_value_list newargs;
for (octave_idx_type i = nargin - 1; i > 0; i--)
newargs (i - 1) = args(i);
if (args(0).is_function_handle ()
|| args(0).is_inline_function ())
{
octave_function *fcn = args(0).function_value ();
if (! error_state)
retval = feval (fcn, newargs, nargout);
}
else if (args(0).is_string ())
{
std::string fcn = args (0).string_value ();
if (! error_state)
retval = feval (fcn, newargs, nargout);
}
else
error ("funcdemo: expected string,",
" inline or function handle");
}
return retval;
}
</pre>
</pre>
<p>The first argument to this demonstration is the user supplied function
and the following arguments are all passed to the user function.
<pre class="example"> funcdemo (@sin,1)
⇒ 0.84147
funcdemo (@(x) sin(x), 1)
⇒ 0.84147
funcdemo (inline ("sin(x)"), 1)
⇒ 0.84147
funcdemo ("sin",1)
⇒ 0.84147
funcdemo (@atan2, 1, 1)
⇒ 0.78540
</pre>
<p>When the user function is passed as a string, the treatment of the
function is different. In some cases it is necessary to always have the
user supplied function as an <code>octave_function</code> object. In that
case the string argument can be used to create a temporary function like
<pre class="example"> std::octave fcn_name = unique_symbol_name ("__fcn__");
std::string fname = "function y = ";
fname.append (fcn_name);
fname.append ("(x) y = ");
fcn = extract_function (args(0), "funcdemo", fcn_name,
fname, "; endfunction");
...
if (fcn_name.length ())
clear_function (fcn_name);
</pre>
<p>There are two important things to know in this case. The number of input
arguments to the user function is fixed, and in the above is a single
argument, and secondly to avoid leaving the temporary function in the
Octave symbol table it should be cleared after use.
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