When an error occurs, it can be detected and handled using the
try statement as described in The try Statement.
As an example, the following piece of code counts the number of errors
that occurs during a
number_of_errors = 0; for n = 1:100 try … catch number_of_errors++; end_try_catch endfor
The above example treats all errors the same. In many situations it
can however be necessary to discriminate between errors, and take
different actions depending on the error. The
function returns a structure containing information about the last
error that occurred. As an example, the code above could be changed
to count the number of errors related to the ‘*’ operator.
number_of_errors = 0; for n = 1:100 try … catch msg = lasterror.message; if (strfind (msg, "operator *")) number_of_errors++; endif end_try_catch endfor
Alternatively, the output of the
lasterror function can be found
in a variable indicated immediately after the
catch keyword, as
in the example below showing how to redirect an error as a warning:
try … catch err warning(err.identifier, err.message); … end_try_catch
Query or set the last error message structure.
When called without arguments, return a structure containing the last error message and other information related to this error. The elements of the structure are:
The text of the last error message
The message identifier of this error message
A structure containing information on where the message occurred. This may be an empty structure if the information cannot be obtained. The fields of the structure are:
The name of the file where the error occurred
The name of function in which the error occurred
The line number at which the error occurred
An optional field with the column number at which the error occurred
The last error structure may be set by passing a scalar structure, err, as input. Any fields of err that match those above are set while any unspecified fields are initialized with default values.
lasterror is called with the argument
fields are set to their default values.
See also: lasterr, error, lastwarn.
Query or set the last error message.
When called without input arguments, return the last error message and message identifier.
With one argument, set the last error message to msg.
With two arguments, also set the last message identifier.
See also: lasterror, error, lastwarn.
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 shown in the next example. To assign an ID to an error,
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.
The next example counts indexing errors. The errors are caught using the
field identifier of the structure returned by the function
number_of_errors = 0; for n = 1:100 try … catch id = lasterror.identifier; if (strcmp (id, "Octave:invalid-indexing")) number_of_errors++; endif end_try_catch endfor
The functions distributed with Octave can issue one of the following errors.
Indicates the error was generated by an operation that cannot be executed in
the scope from which it was called. For example, the function
print_usage () when called from the Octave prompt raises this error.
Indicates that a function was called with invalid input arguments.
Indicates that a function was called in an incorrect way, e.g., wrong number of input arguments.
Indicates that a data-type was indexed incorrectly, e.g., real-value index for arrays, nonexistent field of a structure.
Indicates that memory couldn’t be allocated.
Indicates a call to a function that is not defined. The function may exist but Octave is unable to find it in the search path.
When an error has been handled it is possible to raise it again. This
can be useful when an error needs to be detected, but the program should
still abort. This is possible using the
rethrow function. The
previous example can now be changed to count the number of errors
related to the ‘*’ operator, but still abort if another kind of
number_of_errors = 0; for n = 1:100 try … catch msg = lasterror.message; if (strfind (msg, "operator *")) number_of_errors++; else rethrow (lasterror); endif end_try_catch endfor
Reissue a previous error as defined by err.
err is a structure that must contain at least the
"identifier" fields. err can also contain a field
"stack" that gives information on the assumed location of the
error. Typically err is returned from
See also: lasterror, lasterr, error.
Return the current value of the system-dependent variable errno, set its value to val and return the previous value, or return the named error code given name as a character string, or -1 if name is not found.
See also: errno_list.
Return a structure containing the system-dependent errno values.
See also: errno.