(octave.info)Catching Errors

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12.1.2 Catching Errors
----------------------

When an error occurs, it can be detected and handled using the ‘try’
statement as described in Note: 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.

     See also: Note: lasterr, Note: error, Note:
     lastwarn.

 -- : [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: Note: lasterror, Note: error,
     Note: lastwarn.

   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’.

     See also: Note: lasterror, *note lasterr:
     XREFlasterr, Note: error.

 -- : ERR = errno ()
 -- : ERR = errno (VAL)
 -- : ERR = errno (NAME)
     Query or set the system-dependent variable errno.

     When called with no inputs, return the current value of errno.

     When called with a numeric input VAL, set the current value of
     errno to the specified value.  The previous value of errno is
     returned as ERR.

     When called with a character string NAME, return the numeric value
     of errno which corresponds to the specified error code.  If NAME is
     not a recognized error code then -1 is returned.

     See also: Note: errno_list.

 -- : errno_list ()
     Return a structure containing the system-dependent errno values.

     See also: Note: errno.