Next: Catching Errors, Up: Handling Errors

#### 12.1.1 Raising Errors

The most common use of errors is for checking input arguments to functions. The following example calls the `error` function if the function `f` is called without any input arguments.

         function f (arg1)
           if (nargin == 0)
             error("not enough input arguments");
           endif
         endfunction

When the `error` function is called, it prints the given message and returns to the Octave prompt. This means that no code following a call to `error` will be executed.

— Built-in Function: error (template, ...)
— Built-in Function: error (id, template, ...)

> Format the optional arguments under the control of the template string template using the same rules as the `printf` family of functions (see Formatted Output) and print the resulting message on the `stderr` stream. The message is prefixed by the character string ``error:` '.

> Calling `error` also sets Octave's internal error state such that control will return to the top level without evaluating any more commands. This is useful for aborting from functions or scripts.

> If the error message does not end with a new line character, Octave will print a traceback of all the function calls leading to the error. For example, given the following function definitions:

>               function f () g (); end
>               function g () h (); end
>               function h () nargin == 1 || error ("nargin != 1"); end

> calling the function `f` will result in a list of messages that can help you to quickly locate the exact location of the error:

>               f ()
>               error: nargin != 1
>               error: evaluating index expression near line 1, column 30
>               error: evaluating binary operator `||' near line 1, column 27
>               error: called from `h'
>               error: called from `g'
>               error: called from `f'

> If the error message ends in a new line character, Octave will print the message but will not display any traceback messages as it returns control to the top level. For example, modifying the error message in the previous example to end in a new line causes Octave to only print a single message:

>               function h () nargin == 1 || error ("nargin != 1\n"); end
>               f ()
>               error: nargin != 1

Since it is common to use errors when there is something wrong with the input to a function, Octave supports functions to simplify such code. When the `print_usage` function is called, it reads the help text of the function calling `print_usage`, and presents a useful error. If the help text is written in Texinfo it is possible to present an error message that only contains the function prototypes as described by the `@deftypefn` parts of the help text. When the help text isn't written in Texinfo, the error message contains the entire help message.

Consider the following function.

         ## -*- texinfo -*-
         ## @deftypefn {Function File} f (@var{arg1})
         ## Function help text goes here...
         ## @end deftypefn
         function f (arg1)
           if (nargin == 0)
             print_usage ();
           endif
         endfunction

When it is called with no input arguments it produces the following error.

         f ()
              -| Invalid call to f.  Correct usage is:
              -|
              -|  -- Function File: f (ARG1)
              -|
              -|
              -|
              -| error: evaluating if command near line 6, column 3
              -| error: called from `f' in file `/home/jwe/octave/f.m'

— Loadable Function: print_usage ()

> Print the usage message for the currently executing function. The `print_usage` function is only intended to work inside a user-defined function.

> See also: help.

— Built-in Function: usage (msg)

> Print the message msg, prefixed by the string ``usage:` ', and set Octave's internal error state such that control will return to the top level without evaluating any more commands. This is useful for aborting from functions.

> After `usage` is evaluated, Octave will print a traceback of all the function calls leading to the usage message.

> You should use this function for reporting problems errors that result from an improper call to a function, such as calling a function with an incorrect number of arguments, or with arguments of the wrong type. For example, most functions distributed with Octave begin with code like this

>               if (nargin != 2)
>                 usage ("foo (a, b)");
>               endif

> to check for the proper number of arguments.

— Function File: beep ()

> Produce a beep from the speaker (or visual bell).

> See also: puts, fputs, printf, fprintf.

— Built-in Function: val = beep_on_error ()
— Built-in Function: old_val = beep_on_error (new_val)

> Query or set the internal variable that controls whether Octave will try to ring the terminal bell before printing an error message.