octave: Catching Errors
12.1.2 Catching Errors
----------------------
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
‘for’ loop.
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 ‘lasterror’ 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
-- : LASTERR = lasterror ()
-- : lasterror (ERR)
-- : lasterror ("reset")
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:
‘message’
The text of the last error message
‘identifier’
The message identifier of this error message
‘stack’
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:
‘file’
The name of the file where the error occurred
‘name’
The name of function in which the error occurred
‘line’
The line number at which the error occurred
‘column’
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.
If ‘lasterror’ is called with the argument "reset", all fields are
set to their default values.
DONTPRINTYET See also: lasterr XREFlasterr, error XREFerror, *noteDONTPRINTYET See also: lasterr XREFlasterr, error XREFerror,
lastwarn XREFlastwarn.
-- : [MSG, MSGID] = lasterr ()
-- : lasterr (MSG)
-- : lasterr (MSG, MSGID)
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 XREFlasterror, error XREFerror,
lastwarn XREFlastwarn.
The next example counts indexing errors. The errors are caught using
the field identifier of the structure returned by the function
‘lasterror’.
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.
‘Octave:invalid-context’
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.
‘Octave:invalid-input-arg’
Indicates that a function was called with invalid input arguments.
‘Octave:invalid-fun-call’
Indicates that a function was called in an incorrect way, e.g.,
wrong number of input arguments.
‘Octave:invalid-indexing’
Indicates that a data-type was indexed incorrectly, e.g.,
real-value index for arrays, nonexistent field of a structure.
‘Octave:bad-alloc’
Indicates that memory couldn’t be allocated.
‘Octave:undefined-function’
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 error
occurs.
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
-- : rethrow (ERR)
Reissue a previous error as defined by ERR.
ERR is a structure that must contain at least the "message" and
"identifier" fields. ERR can also contain a field "stack" that
gives information on the assumed location of the error. Typically
ERR is returned from ‘lasterror’.
DONTPRINTYET See also: lasterror XREFlasterror, *notelasterr:
DONTPRINTYET See also: lasterror XREFlasterror, lasterr
XREFlasterr, error XREFerror.
-- : ERR = errno ()
-- : ERR = errno (VAL)
-- : ERR = errno (NAME)
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 XREFerrno_list.
-- : errno_list ()
Return a structure containing the system-dependent errno values.
See also: errno XREFerrno.
Info Catalog
octave: Raising Errors
octave: Handling Errors
octave: Recovering From Errors