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#### 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 newline 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: called from:
      error:   error at line -1, column -1
      error:   h at line 1, column 27
      error:   g at line 1, column 15
      error:   f at line 1, column 15

  If the error message ends in a newline 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 newline causes Octave to only print a single message:

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

  A null string ("") input to `error` will be ignored and the code will continue running as if the statement were a NOP. This is for compatibility with MATLAB. It also makes it possible to write code such as

      err_msg = "";
      if (CONDITION 1)
        err_msg = "CONDITION 1 found";
      elseif (CONDITION2)
        err_msg = "CONDITION 2 found";
      …
      endif
      error (err_msg);

  which will only stop execution if an error has been found.

  Implementation Note: For compatibility with MATLAB, escape sequences (e.g., "\n" => newline) are processed in template regardless of whether template has been defined within single quotes as long as there are two or more input arguments. Use a second backslash to stop interpolation of the escape sequence (e.g., "\\n") or use the `regexptranslate` function.

  See also: warning, lasterror.

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 ()

    -|  error: Invalid call to f.  Correct usage is:
    -|
    -|   -- Function File: f (ARG1)
    -|
    -|
    -|  Additional help for built-in functions and operators is
    -|  available in the online version of the manual.  Use the command
    -|  `doc <topic>' to search the manual index.
    -|
    -|  Help and information about Octave is also available on the WWW
    -|  at http://www.octave.org and via the help@octave.org
    -|  mailing list.

Function File: print_usage ()
Function File: print_usage (name)

  Print the usage message for a function. When called with no input arguments the `print_usage` function displays the usage message of the currently executing function.

  See also: help.

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)
Built-in Function: beep_on_error (new_val, "local")

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

  When called from inside a function with the `"local"` option, the variable is changed locally for the function and any subroutines it calls. The original variable value is restored when exiting the function.

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