(octave.info)Raising 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.
It is also possible to assign an identification string to an error.
If an error has such an ID the user can catch this error as will be
described in the next section. To assign an ID to an error, simply call
‘error’ with two string arguments, where the first is the identification
string, and the second is the actual error. Note that error IDs are in
the format "NAMESPACE:ERROR-NAME". The namespace "Octave" is used for
Octave’s own errors. Any other string is available as a namespace for
user’s own errors.
-- : error (TEMPLATE, ...)
-- : error (ID, TEMPLATE, ...)
Display an error message and stop m-file execution.
Format the optional arguments under the control of the template
string TEMPLATE using the same rules as the ‘printf’ family of
functions (Note: 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 further
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 find the exact location of the error:
f ()
error: nargin != 1
error: called from:
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 in TEMPLATE (e.g., "\n" => newline) are processed
regardless of whether TEMPLATE has been defined with single quotes,
as long as there are two or more input arguments. To disable
escape sequence expansion use a second backslash before the
sequence (e.g., "\\n") or use the ‘regexptranslate’ function.
See also: Note: warning, *note lasterror:
XREFlasterror.
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 {} 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:
⊣
⊣ -- 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 https://www.octave.org and via the help@octave.org
⊣ mailing list.
-- : print_usage ()
-- : print_usage (NAME)
Print the usage message for the function NAME.
When called with no input arguments the ‘print_usage’ function
displays the usage message of the currently executing function.
See also: Note: help.
-- : beep ()
Produce a beep from the speaker (or visual bell).
This function sends the alarm character "\a" to the terminal.
Depending on the user’s configuration this may produce an audible
beep, a visual bell, or nothing at all.
See also: Note: puts, Note: fputs, Note:
printf, Note: fprintf.
-- : VAL = beep_on_error ()
-- : OLD_VAL = beep_on_error (NEW_VAL)
-- : 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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