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.
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 (see Formatted Output) and print the resulting message on thestderr
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"); endcalling 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 15If 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 != 1A 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 aserr_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 theregexptranslate
function.
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.
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: help.
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.
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.