#### A.1.11 Input Parameter Checking in Oct-Files

As oct-files are compiled functions they open up the possibility of crashing Octave through careless function calls or memory faults. It is quite important that each and every function have a sufficient level of parameter checking to ensure that Octave behaves well.

The minimum requirement, as previously discussed, is to check the number of input arguments before using them to avoid referencing a nonexistent argument. However, in some cases this might not be sufficient as the underlying code imposes further constraints. For example, an external function call might be undefined if the input arguments are not integers, or if one of the arguments is zero, or if the input is complex and a real value was expected. Therefore, oct-files often need additional input parameter checking.

There are several functions within Octave that can be useful for the purposes of parameter checking. These include the methods of the octave_value class like `is_real_matrix`, `is_numeric_type`, etc. Often, with a knowledge of the Octave m-file language, you can guess at what the corresponding C++ routine will. In addition there are some more specialized input validation functions of which a few are demonstrated below.

    #include <octave/oct.h>

    DEFUN_DLD (paramdemo, args, nargout, "Parameter Check Demo")
    {
      octave_value retval;
      int nargin = args.length ();

      if (nargin != 1)
        print_usage ();
      else if (nargout != 0)
        error ("paramdemo: OUTPUT argument required");
      else
        {
          NDArray m = args(0).array_value ();
          double min_val = -10.0;
          double max_val = 10.0;
          octave_stdout << "Properties of input array:\n";
          if (m.any_element_is_negative ())
            octave_stdout << "  includes negative values\n";
          if (m.any_element_is_inf_or_nan ())
            octave_stdout << "  includes Inf or NaN values\n";
          if (m.any_element_not_one_or_zero ())
            octave_stdout << "  includes other values than 1 and 0\n";
          if (m.all_elements_are_int_or_inf_or_nan ())
            octave_stdout << "  includes only int, Inf or NaN values\n";
          if (m.all_integers (min_val, max_val))
            octave_stdout << "  includes only integers in [-10,10]\n";
        }
      return retval;
    }

An example of its use is:

    paramdemo ([1, 2, NaN, Inf])
    ⇒ Properties of input array:
         includes Inf or NaN values
         includes other values than 1 and 0
         includes only int, Inf or NaN values